1 File System Interface

This chapter describes the GNU C library’s functions for manipulating files. Unlike the input and output functions described in @ref{I/O on Streams} and @ref{Low-Level I/O}, these functions are concerned with operating on the files themselves, rather than on their contents.

Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.

1.1 Working Directory

Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (@pxref{File Name Resolution}).

When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database. You can find any user’s home directory using the getpwuid or getpwnam functions; see @ref{User Database}.

Users can change the working directory using shell commands like cd. The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory.

Prototypes for these functions are declared in the header file ‘unistd.h’.

Function: char * getcwd (char *buffer, size_t size)

The getcwd function returns an absolute file name representing the current working directory, storing it in the character array buffer that you provide. The size argument is how you tell the system the allocation size of buffer.

The GNU library version of this function also permits you to specify a null pointer for the buffer argument. Then getcwd allocates a buffer automatically, as with malloc (@pxref{Unconstrained Allocation}). If the size is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result.

The return value is buffer on success and a null pointer on failure. The following errno error conditions are defined for this function:

EINVAL

The size argument is zero and buffer is not a null pointer.

ERANGE

The size argument is less than the length of the working directory name. You need to allocate a bigger array and try again.

EACCES

Permission to read or search a component of the file name was denied.

Here is an example showing how you could implement the behavior of GNU’s getcwd (NULL, 0) using only the standard behavior of getcwd:

char *
gnu_getcwd ()
{
  int size = 100;
  char *buffer = (char *) xmalloc (size);

  while (1)
    {
      char *value = getcwd (buffer, size);
      if (value != 0)
        return buffer;
      size *= 2;
      free (buffer);
      buffer = (char *) xmalloc (size);
    }
}

@xref{Malloc Examples}, for information about xmalloc, which is not a library function but is a customary name used in most GNU software.

Function: char * getwd (char *buffer)

This is similar to getcwd. The GNU library provides getwd for backwards compatibility with BSD. The buffer should be a pointer to an array at least PATH_MAX bytes long.

Function: int chdir (const char *filename)

This function is used to set the process’s working directory to filename.

The normal, successful return value from chdir is 0. A value of -1 is returned to indicate an error. The errno error conditions defined for this function are the usual file name syntax errors (@pxref{File Name Errors}), plus ENOTDIR if the file filename is not a directory.

1.2 Accessing Directories

The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.

The opendir function opens a directory stream whose elements are directory entries. You use the readdir function on the directory stream to retrieve these entries, represented as struct dirent objects. The name of the file for each entry is stored in the d_name member of this structure. There are obvious parallels here to the stream facilities for ordinary files, described in @ref{I/O on Streams}.

1.2.1 Format of a Directory Entry

This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file ‘dirent.h’.

Data Type: struct dirent

This is a structure type used to return information about directory entries. It contains the following fields:

char *d_name

This is the null-terminated file name component. This is the only field you can count on in all POSIX systems.

ino_t d_fileno

This is the file serial number. For BSD compatibility, you can also refer to this member as d_ino.

size_t d_namlen

This is the length of the file name, not including the terminating null character.

This structure may contain additional members in the future.

When a file has multiple names, each name has its own directory entry. The only way you can tell that the directory entries belong to a single file is that they have the same value for the d_fileno field.

File attributes such as size, modification times, and the like are part of the file itself, not any particular directory entry. See section File Attributes.

1.2.2 Opening a Directory Stream

This section describes how to open a directory stream. All the symbols are declared in the header file ‘dirent.h’.

Data Type: DIR

The DIR data type represents a directory stream.

You shouldn’t ever allocate objects of the struct dirent or DIR data types, since the directory access functions do that for you. Instead, you refer to these objects using the pointers returned by the following functions.

Function: DIR * opendir (const char *dirname)

The opendir function opens and returns a directory stream for reading the directory whose file name is dirname. The stream has type DIR *.

If unsuccessful, opendir returns a null pointer. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCES

Read permission is denied for the directory named by dirname.

EMFILE

The process has too many files open.

ENFILE

The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.)

The DIR type is typically implemented using a file descriptor, and the opendir function in terms of the open function. @xref{Low-Level I/O}. Directory streams and the underlying file descriptors are closed on exec (@pxref{Executing a File}).

1.2.3 Reading and Closing a Directory Stream

This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file ‘dirent.h’.

Function: struct dirent * readdir (DIR *dirstream)

This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call.

Portability Note: On some systems, readdir may not return entries for ‘.’ and ‘..’, even though these are always valid file names in any directory. @xref{File Name Resolution}.

If there are no more entries in the directory or an error is detected, readdir returns a null pointer. The following errno error conditions are defined for this function:

EBADF

The dirstream argument is not valid.

Function: int closedir (DIR *dirstream)

This function closes the directory stream dirstream. It returns 0 on success and -1 on failure.

The following errno error conditions are defined for this function:

EBADF

The dirstream argument is not valid.

1.2.4 Simple Program to List a Directory

Here’s a simple program that prints the names of the files in the current working directory:

The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see @ref{Array Sort Function}.

1.2.5 Random Access in a Directory Stream

This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file ‘dirent.h’.

Function: void rewinddir (DIR *dirstream)

The rewinddir function is used to reinitialize the directory stream dirstream, so that if you call readdir it returns information about the first entry in the directory again. This function also notices if files have been added or removed to the directory since it was opened with opendir. (Entries for these files might or might not be returned by readdir if they were added or removed since you last called opendir or rewinddir.)

Function: off_t telldir (DIR *dirstream)

The telldir function returns the file position of the directory stream dirstream. You can use this value with seekdir to restore the directory stream to that position.

Function: void seekdir (DIR *dirstream, off_t pos)

The seekdir function sets the file position of the directory stream dirstream to pos. The value pos must be the result of a previous call to telldir on this particular stream; closing and reopening the directory can invalidate values returned by telldir.

1.3 Hard Links

In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.

To add a name to a file, use the link function. (The new name is also called a hard link to the file.) Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file’s existing name or names.

One file can have names in several directories, so the the organization of the file system is not a strict hierarchy or tree.

Since a particular file exists within a single file system, all its names must be in directories in that file system. link reports an error if you try to make a hard link to the file from another file system.

The prototype for the link function is declared in the header file ‘unistd.h’.

Function: int link (const char *oldname, const char *newname)

The link function makes a new link to the existing file named by oldname, under the new name newname.

This function returns a value of 0 if it is successful and -1 on failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}) for both oldname and newname, the following errno error conditions are defined for this function:

EACCES

The directory in which the new link is to be written is not writable.

EEXIST

There is already a file named newname. If you want to replace this link with a new link, you must remove the old link explicitly first.

EMLINK

There are already too many links to the file named by oldname. (The maximum number of links to a file is LINK_MAX; see @ref{Limits for Files}.)

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOENT

The file named by oldname doesn’t exist. You can’t make a link to a file that doesn’t exist.

ENOSPC

The directory or file system that would contain the new link is “full” and cannot be extended.

EPERM

Some implementations only allow privileged users to make links to directories, and others prohibit this operation entirely. This error is used to report the problem.

EROFS

The directory containing the new link can’t be modified because it’s on a read-only file system.

EXDEV

The directory specified in newname is on a different file system than the existing file.

1.4 Symbolic Links

The GNU system supports soft links or symbolic links. This is a kind of “file” that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.

The reason symbolic links work the way they do is that special things happen when you try to open the link. The open function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead. The stat function likewise operates on the file that the symbolic link points to, instead of on the link itself. So does link, the function that makes a hard link.

By contrast, other operations such as deleting or renaming the file operate on the link itself. The functions readlink and lstat also refrain from following symbolic links, because their purpose is to obtain information about the link.

Prototypes for the functions listed in this section are in ‘unistd.h’.

Function: int symlink (const char *oldname, const char *newname)

The symlink function makes a symbolic link to oldname named newname.

The normal return value from symlink is 0. A return value of -1 indicates an error. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EEXIST

There is already an existing file named newname.

EROFS

The file newname would exist on a read-only file system.

ENOSPC

The directory or file system cannot be extended to make the new link.

EIO

A hardware error occurred while reading or writing data on the disk.

Function: int readlink (const char *filename, char *buffer, size_t size)

The readlink function gets the value of the symbolic link filename. The file name that the link points to is copied into buffer. This file name string is not null-terminated; readlink normally returns the number of characters copied. The size argument specifies the maximum number of characters to copy, usually the allocation size of buffer.

If the return value equals size, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call readlink again. Here is an example:

char *
readlink_malloc (char *filename)
{
  int size = 100;

  while (1)
    {
      char *buffer = (char *) xmalloc (size);
      int nchars = readlink (filename, buffer, size);
      if (nchars < size)
        return buffer;
      free (buffer);
      size *= 2;
    }
}

A value of -1 is returned in case of error. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EINVAL

The named file is not a symbolic link.

EIO

A hardware error occurred while reading or writing data on the disk.

1.5 Deleting Files

You can delete a file with the functions unlink or remove. (These names are synonymous.)

Deletion actually deletes a file name. If this is the file’s only name, then the file is deleted as well. If the file has other names as well (see section Hard Links), it remains accessible under its other names.

Function: int unlink (const char *filename)

The unlink function deletes the file name filename. If this is a file’s sole name, the file itself is also deleted. (Actually, if any process has the file open when this happens, deletion is postponed until all processes have closed the file.)

The function unlink is declared in the header file ‘unistd.h’.

This function returns 0 on successful completion, and -1 on error. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCESS

Write permission is denied for the directory from which the file is to be removed.

EBUSY

This error indicates that the file is being used by the system in such a way that it can’t be unlinked. Examples of situations where you might see this error are if the file name specifies the root directory or a mount point for a file system.

ENOENT

The file name to be deleted doesn’t exist.

EPERM

On some systems, unlink cannot be used to delete the name of a directory, or can only be used this way by a privileged user. To avoid such problems, use rmdir to delete directories.

EROFS

The directory in which the file name is to be deleted is on a read-only file system, and can’t be modified.

Function: int remove (const char *filename)

The remove function is another name for unlink. remove is the ANSI C name, whereas unlink is the POSIX.1 name. The name remove is declared in ‘stdio.h’.

Function: int rmdir (const char *filename)

The rmdir function deletes a directory. The directory must be empty before it can be removed; in other words, it can only contain entries for ‘.’ and ‘..’.

In most other respects, rmdir behaves like unlink. There are two additional errno error conditions defined for rmdir:

EEXIST

ENOTEMPTY

The directory to be deleted is not empty.

These two error codes are synonymous; some systems use one, and some use the other.

The prototype for this function is declared in the header file ‘unistd.h’.

1.6 Renaming Files

The rename function is used to change a file’s name.

Function: int rename (const char *oldname, const char *newname)

The rename function renames the file name oldname with newname. The file formerly accessible under the name oldname is afterward accessible as newname instead. (If the file had any other names aside from oldname, it continues to have those names.)

The directory containing the name newname must be on the same file system as the file (as indicated by the name oldname).

One special case for rename is when oldname and newname are two names for the same file. The consistent way to handle this case is to delete oldname. However, POSIX says that in this case rename does nothing and reports success—which is inconsistent. We don’t know what your operating system will do. The GNU system, when completed, will probably do the right thing (delete oldname) unless you explicitly request strict POSIX compatibility “even when it hurts”.

If the oldname is not a directory, then any existing file named newname is removed during the renaming operation. However, if newname is the name of a directory, rename fails in this case.

If the oldname is a directory, then either newname must not exist or it must name a directory that is empty. In the latter case, the existing directory named newname is deleted first. The name newname must not specify a subdirectory of the directory oldname which is being renamed.

One useful feature of rename is that the meaning of the name newname changes “atomically” from any previously existing file by that name to its new meaning (the file that was called oldname). There is no instant at which newname is nonexistent “in between” the old meaning and the new meaning.

If rename fails, it returns -1. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCES

One of the directories containing newname or oldname refuses write permission; or newname and oldname are directories and write permission is refused for one of them.

EBUSY

A directory named by oldname or newname is being used by the system in a way that prevents the renaming from working. This includes directories that are mount points for filesystems, and directories that are the current working directories of processes.

EEXIST

The directory newname isn’t empty.

ENOTEMPTY

The directory newname isn’t empty.

EINVAL

The oldname is a directory that contains newname.

EISDIR

The newname names a directory, but the oldname doesn’t.

EMLINK

The parent directory of newname would have too many links.

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOENT

The file named by oldname doesn’t exist.

ENOSPC

The directory that would contain newname has no room for another entry, and there is no space left in the file system to expand it.

EROFS

The operation would involve writing to a directory on a read-only file system.

EXDEV

The two file names newname and oldnames are on different file systems.

1.7 Creating Directories

Directories are created with the mkdir function. (There is also a shell command mkdir which does the same thing.)

Function: int mkdir (const char *filename, mode_t mode)

The mkdir function creates a new, empty directory whose name is filename.

The argument mode specifies the file permissions for the new directory file. See section The Mode Bits for Access Permission, for more information about this.

A return value of 0 indicates successful completion, and -1 indicates failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCES

Write permission is denied for the parent directory in which the new directory is to be added.

EEXIST

A file named filename already exists.

EMLINK

The parent directory has too many links.

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOSPC

The file system doesn’t have enough room to create the new directory.

EROFS

The parent directory of the directory being created is on a read-only file system, and cannot be modified.

To use this function, your program should include the header file ‘sys/stat.h’.

1.8 File Attributes

When you issue an ‘ls -l’ shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, and the like. This kind of information is called the file attributes; it is associated with the file itself and not a particular one of its names.

This section contains information about how you can inquire about and modify these attributes of files.

1.8.1 What the File Attribute Values Mean

When you read the attributes of a file, they come back in a structure called struct stat. This section describes the names of the attributes, their data types, and what they mean. For the functions to read the attributes of a file, see Reading the Attributes of a File.

The header file ‘sys/stat.h’ declares all the symbols defined in this section.

Data Type: struct stat

The stat structure type is used to return information about the attributes of a file. It contains at least the following members:

mode_t st_mode

Specifies the mode of the file. This includes file type information (see section Testing the Type of a File) and the file permission bits (see section The Mode Bits for Access Permission).

ino_t st_ino

The file serial number, which distinguishes this file from all other files on the same device.

dev_t st_dev

Identifies the device containing the file. The st_ino and st_dev, taken together, uniquely identify the file.

nlink_t st_nlink

The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded. Symbolic links are not counted in the total.

uid_t st_uid

The user ID of the file’s owner. See section File Owner.

gid_t st_gid

The group ID of the file. See section File Owner.

off_t st_size

This specifies the size of a regular file in bytes. For files that are really devices and the like, this field isn’t usually meaningful.

time_t st_atime

This is the last access time for the file. See section File Times.

unsigned long int st_atime_usec

This is the fractional part of the last access time for the file. See section File Times.

time_t st_mtime

This is the time of the last modification to the contents of the file. See section File Times.

unsigned long int st_mtime_usec

This is the fractional part of the time of last modification to the contents of the file. See section File Times.

time_t st_ctime

This is the time of the last modification to the attributes of the file. See section File Times.

unsigned long int st_ctime_usec

This is the fractional part of the time of last modification to the attributes of the file. See section File Times.

unsigned int st_nblocks

This is the amount of disk space that the file occupies, measured in units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse—it may have “holes” which contain zeros but do not actually take up space on the disk.

You can tell (approximately) whether a file is sparse by comparing this value with st_size, like this:

(st.st_blocks * 512 < st.st_size)

This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.

unsigned int st_blksize

The optimal block size for reading of writing this file. You might use this size for allocating the buffer space for reading of writing the file.

Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file ‘sys/types.h’ as well as in ‘sys/stat.h’. Here is a list of them.

Data Type: mode_t

This is an integer data type used to represent file modes. In the GNU system, this is equivalent to unsigned int.

Data Type: ino_t

This is an arithmetic data type used to represent file serial numbers. (In Unix jargon, these are sometimes called inode numbers.) In the GNU system, this type is equivalent to unsigned long int.

Data Type: dev_t

This is an arithmetic data type used to represent file device numbers. In the GNU system, this is equivalent to int.

Data Type: nlink_t

This is an arithmetic data type used to represent file link counts. In the GNU system, this is equivalent to unsigned short int.

1.8.2 Reading the Attributes of a File

To examine the attributes of files, use the functions stat, fstat and lstat. They return the attribute information in a struct stat object. All three functions are declared in the header file ‘sys/stat.h’.

Function: int stat (const char *filename, struct stat *buf)

The stat function returns information about the attributes of the file named by filename in the structure pointed at by buf.

If filename is the name of a symbolic link, the attributes you get describe the file that the link points to. If the link points to a nonexistent file name, then stat fails, reporting a nonexistent file.

The return value is 0 if the operation is successful, and -1 on failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}, the following errno error conditions are defined for this function:

ENOENT

The file named by filename doesn’t exist.

Function: int fstat (int filedes, struct stat *buf)

The fstat function is like stat, except that it takes an open file descriptor as an argument instead of a file name. @xref{Low-Level I/O}.

Like stat, fstat returns 0 on success and -1 on failure. The following errno error conditions are defined for fstat:

EBADF

The filedes argument is not a valid file descriptor.

Function: int lstat (const char *filename, struct stat *buf)

The lstat function is like stat, except that it does not follow symbolic links. If filename is the name of a symbolic link, lstat returns information about the link itself; otherwise, lstat works like stat. See section Symbolic Links.

1.8.3 Testing the Type of a File

The file mode, stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the type code, which you can use to tell whether the file is a directory, whether it is a socket, and so on. For information about the access permission, The Mode Bits for Access Permission.

There are two predefined ways you can access the file type portion of the file mode. First of all, for each type of file, there is a predicate macro which examines a file mode value and returns true or false—is the file of that type, or not. Secondly, you can mask out the rest of the file mode to get just a file type code. You can compare this against various constants for the supported file types.

All of the symbols listed in this section are defined in the header file ‘sys/stat.h’.

The following predicate macros test the type of a file, given the value m which is the st_mode field returned by stat on that file:

Macro: int S_ISDIR (mode_t m)

This macro returns nonzero if the file is a directory.

Macro: int S_ISCHR (mode_t m)

This macro returns nonzero if the file is a character special file (a device like a terminal).

Macro: int S_ISBLK (mode_t m)

This macro returns nonzero if the file is a block special file (a device like a disk).

Macro: int S_ISREG (mode_t m)

This macro returns nonzero if the file is a regular file.

Macro: int S_ISFIFO (mode_t m)

This macro returns nonzero if the file is a FIFO special file, or a pipe. @xref{Pipes and FIFOs}.

Macro: int S_ISLNK (mode_t m)

This macro returns nonzero if the file is a symbolic link. See section Symbolic Links.

Macro: int S_ISSOCK (mode_t m)

This macro returns nonzero if the file is a socket. @xref{Sockets}.

An alterate non-POSIX method of testing the file type is supported for compatibility with BSD. The mode can be bitwise ANDed with S_IFMT to extract the file type code, and compared to the appropriate type code constant. For example,

S_ISCHR (mode)

is equivalent to:

((mode & S_IFMT) == S_IFCHR)

Macro: int S_IFMT

This is a bit mask used to extract the file type code portion of a mode value.

These are the symbolic names for the different file type codes:

S_IFDIR

This macro represents the value of the file type code for a directory file.

S_IFCHR

This macro represents the value of the file type code for a character-oriented device file.

S_IFBLK

This macro represents the value of the file type code for a block-oriented device file.

S_IFREG

This macro represents the value of the file type code for a regular file.

S_IFLNK

This macro represents the value of the file type code for a symbolic link.

S_IFSOCK

This macro represents the value of the file type code for a socket.

S_IFIFO

This macro represents the value of the file type code for a FIFO or pipe.

1.8.4 File Owner

Every file has an owner which is one of the registered user names defined on the system. Each file also has a group, which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.

The file owner and group play a role in determining access because the file has one set of access permission bits for the user that is the owner, another set that apply to users who belong to the file’s group, and a third set of bits that apply to everyone else. See section How Your Access to a File is Decided, for the details of how access is decided based on this data.

When a file is created, its owner is set from the effective user ID of the process that creates it (@pxref{Process Persona}). The file’s group ID may be set from either effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rule, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior, no matter what kind of system you run it on.

You can change the owner and/or group owner of an existing file using the chown function. This is the primitive for the chown and chgrp shell commands.

The prototype for this function is declared in ‘unistd.h’.

Function: int chown (const char *filename, uid_t owner, gid_t group)

The chown function changes the owner of the file filename to owner, and its group owner to group.

Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID bits of the file’s permissions. (This is because those bits may not be appropriate for the new owner.) The other file permission bits are not changed.

The return value is 0 on success and -1 on failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EPERM

This process lacks permission to make the requested change.

Only privileged users or the file’s owner can change the file’s group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on.

@xref{Options for Files}, for information about the _POSIX_CHOWN_RESTRICTED macro.

EROFS

The file is on a read-only file system.

Function: int fchown (int filedes, int owner, int group)

This is like chown, except that it changes the owner of the file with open file descriptor filedes.

The return value from fchown is 0 on success and -1 on failure. The following errno error codes are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The filedes argument corresponds to a pipe or socket, not an ordinary file.

EPERM

This process lacks permission to make the requested change. For details, see chmod, above.

EROFS

The file resides on a read-only file system.

1.8.5 The Mode Bits for Access Permission

The file mode, stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the access permission bits, which control who can read or write the file. See section Testing the Type of a File, for information about the file type code.

All of the symbols listed in this section are defined in the header file ‘sys/stat.h’.

These symbolic constants are defined for the file mode bits that control access permission for the file:

S_IRUSR

S_IREAD

Read permission bit for the owner of the file. On many systems, this bit is 0400. S_IREAD is an obsolete synonym provided for BSD compatibility.

S_IWUSR

S_IWRITE

Write permission bit for the owner of the file. Usually 0200. S_IWRITE is an obsolete synonym provided for BSD compatibility.

S_IXUSR

S_IEXEC

Execute (for ordinary files) or search (for directories) permission bit for the owner of the file. Usually 0100. S_IEXEC is an obsolete synonym provided for BSD compatibility.

S_IRWXU

This is equivalent to ‘(S_IRUSR | S_IWUSR | S_IXUSR)’.

S_IRGRP

Read permission bit for the group owner of the file. Usually 040.

S_IWGRP

Write permission bit for the group owner of the file. Usually 020.

S_IXGRP

Execute or search permission bit for the group owner of the file. Usually 010.

S_IRWXG

This is equivalent to ‘(S_IRGRP | S_IWGRP | S_IXGRP)’.

S_IROTH

Read permission bit for other users. Usually 04.

S_IWOTH

Write permission bit for other users. Usually 02.

S_IXOTH

Execute or search permission bit for other users. Usually 01.

S_IRWXO

This is equivalent to ‘(S_IROTH | S_IWOTH | S_IXOTH)’.

S_ISUID

This is the set-user-ID on execute bit, usually 04000. @xref{How Change Persona}.

S_ISGID

This is the set-group-ID on execute bit, usually 02000. @xref{How Change Persona}.

S_ISVTX

This is the sticky bit, usually 01000.

On an executable file, it modifies the swapping policies of the system. Normally, when a program terminates, its pages in core are immediately freed and reused. If the sticky bit is set on the executable file, the system keeps the pages in core for a while as if the program were still running. This is advantageous for a program that is likely to be run many times in succession.

On a directory, the sticky bit gives permission to delete a file in the directory if you can write the contents of that file. Ordinarily, a user either can delete all the files in the directory or cannot delete any of them (based on whether the user has write permission for the directory). The sticky bit makes it possible to control deletion for individual files.

The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.

Warning: Writing explicit numbers for file permissions is bad practice. It is not only nonportable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean, use the symbolic names.

1.8.6 How Your Access to a File is Decided

Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process, and its supplementary group IDs, together with the file’s owner, group and permission bits. These concepts are discussed in detail in @ref{Process Persona}.

If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding “user” (or “owner”) bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the “group” bits. Otherwise, permissions are controlled by the “other” bits.

Privileged users, like ‘root’, can access any file, regardless of its file permission bits. As a special case, for a file to be executable even for a privileged user, at least one of its execute bits must be set.

1.8.7 Assigning File Permissions

The primitive functions for creating files (for example, open or mkdir) take a mode argument, which specifies the file permissions for the newly created file. But the specified mode is modified by the process’s file creation mask, or umask, before it is used.

The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the “other” access bits in the mask, then newly created files are not accessible at all to processes in the “other” category, even if the mode argument specified to the creation function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.

Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user’s own file creation mask.

To change the permission of an existing file given its name, call chmod. This function ignores the file creation mask; it uses exactly the specified permission bits.

In normal use, the file creation mask is initialized in the user’s login shell (using the umask shell command), and inherited by all subprocesses. Application programs normally don’t need to worry about the file creation mask. It will do automatically what it is supposed to do.

When your program should create a file and bypass the umask for its access permissions, the easiest way to do this is to use fchmod after opening the file, rather than changing the umask.

In fact, changing the umask is usually done only by shells. They use the umask function.

The functions in this section are declared in ‘sys/stat.h’.

Function: mode_t umask (mode_t mask)

The umask function sets the file creation mask of the current process to mask, and returns the previous value of the file creation mask.

Here is an example showing how to read the mask with umask without changing it permanently:

mode_t
read_umask (void)
{
  mask = umask (0);
  umask (mask);
}

However, it is better to use getumask if you just want to read the mask value, because that is reentrant (at least if you use the GNU operating system).

Function: mode_t getumask (void)

Return the current value of the file creation mask for the current process. This function is a GNU extension.

Function: int chmod (const char *filename, mode_t mode)

The chmod function sets the access permission bits for the file named by filename to mode.

If the filename names a symbolic link, chmod changes the permission of the file pointed to by the link, not those of the link itself. There is actually no way to set the mode of a link, which is always -1.

This function returns 0 if successful and -1 if not. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

ENOENT

The named file doesn’t exist.

EPERM

This process does not have permission to change the access permission of this file. Only the file’s owner (as judged by the effective user ID of the process) or a privileged user can change them.

EROFS

The file resides on a read-only file system.

Function: int fchmod (int filedes, int mode)

This is like chmod, except that it changes the permissions of the file currently open via descriptor filedes.

The return value from fchmod is 0 on success and -1 on failure. The following errno error codes are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The filedes argument corresponds to a pipe or socket, or something else that doesn’t really have access permissions.

EPERM

This process does not have permission to change the access permission of this file. Only the file’s owner (as judged by the effective user ID of the process) or a privileged user can change them.

EROFS

The file resides on a read-only file system.

1.8.8 Testing Permission to Access a File

When a program runs as a privileged user, this permits it to access files off-limits to ordinary users—for example, to modify ‘/etc/passwd’. Programs designed to be run by ordinary users but access such files use the setuid bit feature so that they always run with root as the effective user ID.

Such a program may also access files specified by the user, files which conceptually are being accessed explicitly by the user. Since the program runs as root, it has permission to access whatever file the user specifies—but usually the desired behavior is to permit only those files which the user could ordinarily access.

The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.

To do this, use the function access, which checks for access permission based on the process’s real user ID rather than the effective user ID. (The setuid feature does not alter the real user ID, so it reflects the user who actually ran the program.)

There is another way you could check this access, which is easy to describe, but very hard to use. This is to examine the file mode bits and mimic the system’s own access computation. This method is undesirable because many systems have additional access control features; your program cannot portably mimic them, and you would not want to try to keep track of the diverse features that different systems have. Using access is simple and automatically does whatever is appropriate for the system you are using.

The symbols in this section are declared in ‘unistd.h’.

Function: int access (const char *filename, int how)

The access function checks to see whether the file named by filename can be accessed in the way specified by the how argument. The how argument either can be the bitwise OR of the flags R_OK, W_OK, X_OK, or the existence test F_OK.

This function uses the real user and group ID’s of the calling process, rather than the effective ID’s, to check for access permission. As a result, if you use the function from a setuid or setgid program (@pxref{How Change Persona}), it gives information relative to the user who actually ran the program.

The return value is 0 if the access is permitted, and -1 otherwise. (In other words, treated as a predicate function, access returns true if the requested access is denied.)

In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCES

The access specified by how is denied.

ENOENT

The file doesn’t exist.

EROFS

Write permission was requested for a file on a read-only file system.

These macros are defined in the header file ‘unistd.h’ for use as the how argument to the access function. The values are integer constants.

Macro: int R_OK

Argument that means, test for read permission.

Macro: int W_OK

Argument that means, test for write permission.

Macro: int X_OK

Argument that means, test for execute/search permission.

Macro: int F_OK

Argument that means, test for existence of the file.

1.8.9 File Times

Each file has three timestamps associated with it: its access time, its modification time, and its attribute modification time. These correspond to the st_atime, st_mtime, and st_ctime members of the stat structure; see File Attributes.

All of these times are represented in calendar time format, as time_t objects. This data type is defined in ‘time.h’. For more information about representation and manipulation of time values, see @ref{Calendar Time}.

When an existing file is opened, its attribute change time and modification time fields are updated. Reading from a file updates its access time attribute, and writing updates its modification time.

When a file is created, all three timestamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.

Adding a new name for a file with the link function updates the attribute change time field of the file being linked, and both the attribute change time and modification time fields of the directory containing the new name. These same fields are affected if a file name is deleted with unlink, remove, or rmdir. Renaming a file with rename affects only the attribute change time and modification time fields of the two parent directories involved, and not the times for the file being renamed.

Changing attributes of a file (for example, with chmod) updates its attribute change time field.

You can also change some of the timestamps of a file explicitly using the utime function—all except the attribute change time. You need to include the header file ‘utime.h’ to use this facility.

Data Type: struct utimbuf

The utimbuf structure is used with the utime function to specify new access and modification times for a file. It contains the following members:

time_t actime

This is the access time for the file.

time_t modtime

This is the modification time for the file.

Function: int utime (const char *filename, const struct utimbuf *times)

This function is used to modify the file times associated with the file named filename.

If times is a null pointer, then the access and modification times of the file are set to the current time. Otherwise, they are set to the values from the actime and modtime members (respectively) of the utimbuf structure pointed at by times.

The attribute modification time for the file is set to the current time in either case (since changing the timestamps is itself a modification of the file attributes).

The utime function returns 0 if successful and -1 on failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EACCES

There is a permission problem in the case where a null pointer was passed as the times argument. In order to update the timestamp on the file, you must either be the owner of the file, have write permission on the file, or be a privileged user.

ENOENT

The file doesn’t exist.

EPERM

If the times argument is not a null pointer, you must either be the owner of the file or be a privileged user. This error is used to report the problem.

EROFS

The file lives on a read-only file system.

Each of the three time stamps has a corresponding microsecond part, which extends its resolution. These fields are called st_atime_usec, st_mtime_usec, and st_ctime_usec; each has a value between 0 and 999,999, which indicates the time in microseconds. They correspond to the tv_usec field of a timeval structure; see @ref{High-Resolution Calendar}.

The utimes function is like utime, but also lets you specify the fractional part of the file times. The prototype for this function is in the header file ‘sys/time.h’.

Function: int utimes (const char *filename, struct timeval tvp[2])

This function sets the file access and modification times for the file named by filename. The new file access time is specified by tvp[0], and the new modification time by tvp[1]. This function comes from BSD.

The return values and error conditions are the same as for the utime function.

1.9 Making Special Files

The mknod function is the primitive for making special files, such as files that correspond to devices. The GNU library includes this function for compatibility with BSD.

The prototype for mknod is declared in ‘sys/stat.h’.

Function: int mknod (const char *filename, int mode, int dev)

The mknod function makes a special file with name filename. The mode specifies the mode of the file, and may include the various special file bits, such as S_IFCHR (for a character special file) or S_IFBLK (for a block special file). See section Testing the Type of a File.

The dev argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.

The return value is 0 on success and -1 on error. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for this function:

EPERM

The calling process is not privileged. Only the superuser can create special files.

ENOSPC

The directory or file system that would contain the new file is “full” and cannot be extended.

EROFS

The directory containing the new file can’t be modified because it’s on a read-only file system.

EEXIST

There is already a file named filename. If you want to replace this file, you must remove the old file explicitly first.

1.10 Temporary Files

If you need to use a temporary file in your program, you can use the tmpfile function to open it. Or you can use the tmpnam function make a name for a temporary file and then open it in the usual way with fopen.

The tempnam function is like tmpnam but lets you choose what directory temporary files will go in, and something about what their file names will look like.

These facilities are declared in the header file ‘stdio.h’.

Function: FILE * tmpfile (void)

This function creates a temporary binary file for update mode, as if by calling fopen with mode "wb+". The file is deleted automatically when it is closed or when the program terminates. (On some other ANSI C systems the file may fail to be deleted if the program terminates abnormally).

Function: char * tmpnam (char *result)

This function constructs and returns a file name that is a valid file name and that does not name any existing file. If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls. Otherwise, the result argument should be a pointer to an array of at least L_tmpnam characters, and the result is written into that array.

It is possible for tmpnam to fail if you call it too many times. This is because the fixed length of a temporary file name gives room for only a finite number of different names. If tmpnam fails, it returns a null pointer.

Macro: int L_tmpnam

The value of this macro is an integer constant expression that represents the minimum allocation size of a string large enough to hold the file name generated by the tmpnam function.

Macro: int TMP_MAX

The macro TMP_MAX is a lower bound for how many temporary names you can create with tmpnam. You can rely on being able to call tmpnam at least this many times before it might fail saying you have made too many temporary file names.

With the GNU library, you can create a very large number of temporary file names—if you actually create the files, you will probably run out of disk space before you run out of names. Some other systems have a fixed, small limit on the number of temporary files. The limit is never less than 25.

Function: char * tempnam (const char *dir, const char *prefix)

This function generates a unique temporary filename. If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name.

The directory prefix for the temporary file name is determined by testing each of the following, in sequence. The directory must exist and be writable.

• The environment variable TMPDIR, if it is defined.
• The dir argument, if it is not a null pointer.
• The value of the P_tmpdir macro.
• The directory ‘/tmp’.

This function is defined for SVID compatibility.

SVID Macro: char * P_tmpdir

This macro is the name of the default directory for temporary files.

Older Unix systems did not have the functions just described. Instead they used mktemp and mkstemp. Both of these functions work by modifying a file name template string you pass. The last six characters of this string must be ‘XXXXXX’. These six ‘X’s are replaced with six characters which make the whole string a unique file name. Usually the template string is something like ‘/tmp/prefixXXXXXX’, and each program uses a unique prefix.

Note: Because mktemp and mkstemp modify the template string, you must not pass string constants to them. String constants are normally in read-only storage, so your program would crash when mktemp or mkstemp tried to modify the string.

Function: char * mktemp (char *template)

The mktemp function generates a unique file name by modifying template as described above. If successful, it returns template as modified. If mktemp cannot find a unique file name, it makes template an empty string and returns that. If template does not end with ‘XXXXXX’, mktemp returns a null pointer.

Function: int mkstemp (char *template)

The mkstemp function generates a unique file name just as mktemp does, but it also opens the file for you with open (@pxref{Opening and Closing Files}). If successful, it modifies template in place and returns a file descriptor open on that file for reading and writing. If mkstemp cannot create a uniquely-named file, it makes template an empty string and returns -1. If template does not end with ‘XXXXXX’, mkstemp returns -1 and does not modify template.

Unlike mktemp, mkstemp is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file. This is because it works by calling open with the O_EXCL flag bit, which says you want to always create a new file, and get an error if the file already exists.

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