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INTRO(2) UNIX Programmer's Manual INTRO(2) NNAAMMEE iinnttrroo - introduction to system calls and error numbers SSYYNNOOPPSSIISS ##iinncclluuddee <<ssyyss//eerrrrnnoo..hh>> DDEESSCCRRIIPPTTIIOONN This section provides an overview of the system calls, their error re turns, and other common definitions and concepts. DDIIAAGGNNOOSSTTIICCSS Nearly all of the system calls provide an error number in the external variable _e_r_r_n_o, which is defined as: extern int errno When a system call detects an error, it returns an integer value indicat ing failure (usually 1) and sets the variable _e_r_r_n_o accordingly. <This allows interpretation of the failure on receiving a 1 and to take action accordingly.> Successful calls never set _e_r_r_n_o; once set, it remains un til another error occurs. It should only be examined after an error. Note that a number of system calls overload the meanings of these error numbers, and that the meanings must be interpreted according to the type and circumstances of the call. The following is a complete list of the errors and their names as given in <_s_y_s_/_e_r_r_n_o_._h>. 0 _E_r_r_o_r _0. Not used. 1 EPERM _O_p_e_r_a_t_i_o_n _n_o_t _p_e_r_m_i_t_t_e_d _. An attempt was made to perform an oper ation limited to processes with appropriate privileges or to the owner of a file or other resources. 2 ENOENT _N_o _s_u_c_h _f_i_l_e _o_r _d_i_r_e_c_t_o_r_y. A component of a specified pathname did not exist, or the pathname was an empty string. 3 ESRCH _N_o _s_u_c_h _p_r_o_c_e_s_s. No process could be found corresponding to that specified by the given process ID. 4 EINTR _I_n_t_e_r_r_u_p_t_e_d _f_u_n_c_t_i_o_n _c_a_l_l. An asynchronous signal (such as SIGINT or SIGQUIT) was caught by the process during the execution of an interruptible function. If the signal handler performs a normal return, the interupted function call will seem to have returned the error condition. 5 EIO _I_n_p_u_t_/_o_u_t_p_u_t _e_r_r_o_r. Some physical input or output error occurred. This error not be reported until a subsequent operation on the same file descriptor and may be lost (over written) by any subse quent errors. 6 ENXIO _N_o _s_u_c_h _d_e_v_i_c_e _o_r _a_d_d_r_e_s_s. Input or output on a special file re ferred to a device that did not exist, or made a request beyond the limits of the device. This error may also occur when, for example, a tape drive is not online or no disk pack is is loaded on a drive. 7 E2BIG _A_r_g _l_i_s_t _t_o_o _l_o_n_g. The number of bytes used for the argument and environment list of the new process exceeded the current limit of 20480 bytes (NCARGS in <_s_y_s_/_p_a_r_a_m_._h>). 8 ENOEXEC _E_x_e_c _f_o_r_m_a_t _e_r_r_o_r. A request was made to execute a file that, although it has the appropriate permissions, was not in the for mat required for an executable file. 9 EBADF _B_a_d _f_i_l_e _d_e_s_c_r_i_p_t_o_r. A file descriptor argument was out of range, referred to no open file, or a read (write) request was made to a file that was only open for writing (reading). 10 ECHILD _N_o _c_h_i_l_d _p_r_o_c_e_s_s_e_s. A wait or waitpid function was executed by a process that had no existing or unwaitedfor child processes. 11 EDEADLK _R_e_s_o_u_r_c_e _d_e_a_d_l_o_c_k _a_v_o_i_d_e_d. An attempt was made to lock a sys tem resource that would have resulted in a deadlock situation. 12 ENOMEM _C_a_n_n_n_o_t _a_l_l_o_c_a_t_e _m_e_m_o_r_y. The new process image required more memory than was allowed by the hardware or by systemimposed mem ory management constraints. A lack of swap space is normally temporary; however, a lack of core is not. Soft limits may be increased to their corresponding hard limits. 13 EACCES _P_e_r_m_i_s_s_i_o_n _d_e_n_i_e_d. An attempt was made to access a file in a way forbidden by its file access permissions. 14 EFAULT _B_a_d _a_d_d_r_e_s_s. The system detected an invalid address in attempt ing to use an argument of a call. 15 ENOTBLK _N_o_t _a _b_l_o_c_k _d_e_v_i_c_e. A block device operation was attempted on a nonblock device or file. 16 EBUSY _R_e_s_o_u_r_c_e _b_u_s_y. An attempt to use a system resource which was in use at the time in a manner which would have conflicted with the request. 17 EEXIST _F_i_l_e _e_x_i_s_t_s. An existing file was mentioned in an inappropriate context, for instance, as the new link name in a link function. 18 EXDEV _I_m_p_r_o_p_e_r _l_i_n_k. A hard link to a file on another file system was attempted. 19 ENODEV _O_p_e_r_a_t_i_o_n _n_o_t _s_u_p_p_o_r_t_e_d _b_y _d_e_v_i_c_e. An attempt was made to apply an inappropriate function to a device, for example, trying to read a writeonly device such as a printer. 20 ENOTDIR _N_o_t _a _d_i_r_e_c_t_o_r_y. A component of the specified pathname exist ed, but it was not a directory, when a directory was expected. 21 EISDIR _I_s _a _d_i_r_e_c_t_o_r_y. An attempt was made to open a directory with write mode specified. 22 EINVAL _I_n_v_a_l_i_d _a_r_g_u_m_e_n_t. Some invalid argument was supplied. (For ex ample, specifying an undefined signal to a signal or kill func tion). 23 ENFILE _T_o_o _m_a_n_y _o_p_e_n _f_i_l_e_s _i_n _s_y_s_t_e_m. Maximum number of file descrip tors allowable on the system has been reached and a requests for an open cannot be satisfied until at least one has been closed. 24 EMFILE _T_o_o _m_a_n_y _o_p_e_n _f_i_l_e_s. <As released, the limit on the number of open files per process is 64.> Getdtablesize(2) will obtain the current limit. 25 ENOTTY _I_n_a_p_p_r_o_p_r_i_a_t_e _i_o_c_t_l _f_o_r _d_e_v_i_c_e. A control function (see ioctl(2)) was attempted for a file or special device for which the operation was inappropriate. 26 ETXTBSY _T_e_x_t _f_i_l_e _b_u_s_y. The new process was a pure procedure (shared text) file which was open for writing by another process, or the pure procedure file was being executed an open call requested write access. 27 EFBIG _F_i_l_e _t_o_o _l_a_r_g_e. The size of a file exceeded the maximum (about 2.1E9 bytes). 28 ENOSPC _D_e_v_i_c_e _o_u_t _o_f _s_p_a_c_e. A write to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because no more disk blocks are available on the file system, or the allocation of an inode for a newly created file failed because no more inodes are available on the file sys tem. 29 ESPIPE _I_l_l_e_g_a_l _s_e_e_k. An lseek function was issued on a socket, pipe or FIFO. 30 EROFS _R_e_a_d__o_n_l_y _f_i_l_e _s_y_s_t_e_m. An attempt was made to modify a file or directory was made on a file system that was readonly at the time. 31 EMLINK _T_o_o _m_a_n_y _l_i_n_k_s. Maximum allowable hard links to a single file has been exceeded (limit of 32767 hard links per file). 32 EPIPE _B_r_o_k_e_n _p_i_p_e. A write on a pipe, socket or FIFO for which there is no process to read the data. 33 EDOM _N_u_m_e_r_i_c_a_l _a_r_g_u_m_e_n_t _o_u_t _o_f _d_o_m_a_i_n. A numerical input argument was outside the defined domain of the mathematical function. 34 ERANGE _N_u_m_e_r_i_c_a_l _r_e_s_u_l_t _o_u_t _o_f _r_a_n_g_e. A numerical result of the func tion was to large to fit in the available space (perhaps exceeded precision). 35 EAGAIN _R_e_s_o_u_r_c_e _t_e_m_p_o_r_a_r_i_l_y _u_n_a_v_a_i_l_a_b_l_e. This is a temporary condition and later calls to the same routine may complete normally. 36 EINPROGRESS _O_p_e_r_a_t_i_o_n _n_o_w _i_n _p_r_o_g_r_e_s_s. An operation that takes a long time to complete (such as a connect(2)) was attempted on a non blocking object (see fcntl(2)). 37 EALREADY _O_p_e_r_a_t_i_o_n _a_l_r_e_a_d_y _i_n _p_r_o_g_r_e_s_s. An operation was attempted on a nonblocking object that already had an operation in progress. 38 ENOTSOCK _S_o_c_k_e_t _o_p_e_r_a_t_i_o_n _o_n _n_o_n__s_o_c_k_e_t. Selfexplanatory. 39 EDESTADDRREQ _D_e_s_t_i_n_a_t_i_o_n _a_d_d_r_e_s_s _r_e_q_u_i_r_e_d. A required address was omitted from an operation on a socket. 40 EMSGSIZE _M_e_s_s_a_g_e _t_o_o _l_o_n_g. A message sent on a socket was larger than the internal message buffer or some other network limit. 41 EPROTOTYPE _P_r_o_t_o_c_o_l _w_r_o_n_g _t_y_p_e _f_o_r _s_o_c_k_e_t. A protocol was specified that does not support the semantics of the socket type requested. For example, you cannot use the ARPA Internet UDP protocol with type SOCK_STREAM. 42 ENOPROTOOPT _P_r_o_t_o_c_o_l _n_o_t _a_v_a_i_l_a_b_l_e. A bad option or level was speci fied in a getsockopt(2) or setsockopt(2) call. 43 EPROTONOSUPPORT _P_r_o_t_o_c_o_l _n_o_t _s_u_p_p_o_r_t_e_d. The protocol has not been con figured into the system or no implementation for it exists. 44 ESOCKTNOSUPPORT _S_o_c_k_e_t _t_y_p_e _n_o_t _s_u_p_p_o_r_t_e_d. The support for the socket type has not been configured into the system or no implementation for it exists. 45 EOPNOTSUPP _O_p_e_r_a_t_i_o_n _n_o_t _s_u_p_p_o_r_t_e_d. The attempted operation is not supported for the type of object referenced. Usually this occurs when a file descriptor refers to a file or socket that cannot support this operation, for example, trying to _a_c_c_e_p_t a connec tion on a datagram socket. 46 EPFNOSUPPORT _P_r_o_t_o_c_o_l _f_a_m_i_l_y _n_o_t _s_u_p_p_o_r_t_e_d. The protocol family has not been configured into the system or no implementation for it exists. 47 EAFNOSUPPORT _A_d_d_r_e_s_s _f_a_m_i_l_y _n_o_t _s_u_p_p_o_r_t_e_d _b_y _p_r_o_t_o_c_o_l _f_a_m_i_l_y. An ad dress incompatible with the requested protocol was used. For ex ample, you shouldn't necessarily expect to be able to use NS ad dresses with ARPA Internet protocols. 48 EADDRINUSE _A_d_d_r_e_s_s _a_l_r_e_a_d_y _i_n _u_s_e. Only one usage of each address is normally permitted. 49 EADDRNOTAVAIL _C_a_n_n_o_t _a_s_s_i_g_n _r_e_q_u_e_s_t_e_d _a_d_d_r_e_s_s. Normally results from an attempt to create a socket with an address not on this ma chine. 50 ENETDOWN _N_e_t_w_o_r_k _i_s _d_o_w_n. A socket operation encountered a dead net work. 51 ENETUNREACH _N_e_t_w_o_r_k _i_s _u_n_r_e_a_c_h_a_b_l_e. A socket operation was attempted to an unreachable network. 52 ENETRESET _N_e_t_w_o_r_k _d_r_o_p_p_e_d _c_o_n_n_e_c_t_i_o_n _o_n _r_e_s_e_t. The host you were con nected to crashed and rebooted. 53 ECONNABORTED _S_o_f_t_w_a_r_e _c_a_u_s_e_d _c_o_n_n_e_c_t_i_o_n _a_b_o_r_t. A connection abort was caused internal to your host machine. 54 ECONNRESET _C_o_n_n_e_c_t_i_o_n _r_e_s_e_t _b_y _p_e_e_r. A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or a reboot. 55 ENOBUFS _N_o _b_u_f_f_e_r _s_p_a_c_e _a_v_a_i_l_a_b_l_e. An operation on a socket or pipe was not performed because the system lacked sufficient buffer space or because a queue was full. 56 EISCONN _S_o_c_k_e_t _i_s _a_l_r_e_a_d_y _c_o_n_n_e_c_t_e_d. A connect request was made on an already connected socket; or, a sendto or sendmsg request on a connected socket specified a destination when already connected. 57 ENOTCONN _S_o_c_k_e_t _i_s _n_o_t _c_o_n_n_e_c_t_e_d. An request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket) no address was supplied. 58 ESHUTDOWN _C_a_n_n_o_t _s_e_n_d _a_f_t_e_r _s_o_c_k_e_t _s_h_u_t_d_o_w_n. A request to send data was disallowed because the socket had already been shut down with a previous shutdown(2) call. 60 ETIMEDOUT _C_o_n_n_e_c_t_i_o_n _t_i_m_e_d _o_u_t. A connect or send request failed be cause the connected party did not properly respond after a period of time. (The timeout period is dependent on the communication protocol.) 61 ECONNREFUSED _C_o_n_n_e_c_t_i_o_n _r_e_f_u_s_e_d. No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the for eign host. 62 ELOOP _T_o_o _m_a_n_y _l_e_v_e_l_s _o_f _s_y_m_b_o_l_i_c _l_i_n_k_s. A path name lookup involved more than 8 symbolic links. 63 ENAMETOOLONG _F_i_l_e _n_a_m_e _t_o_o _l_o_n_g. A component of a path name exceeded 255 (MAXNAMELEN) characters, or an entire path name exceeded 1023 (MAXPATHLEN1) characters. 64 EHOSTDOWN _H_o_s_t _i_s _d_o_w_n. A socket operation failed because the destina tion host was down. 65 EHOSTUNREACH _N_o _r_o_u_t_e _t_o _h_o_s_t. A socket operation was attempted to an unreachable host. 66 ENOTEMPTY _D_i_r_e_c_t_o_r_y _n_o_t _e_m_p_t_y. A directory with entries other than `.' and `..' was supplied to a remove directory or rename call. 67 EPROCLIM _T_o_o _m_a_n_y _p_r_o_c_e_s_s_e_s. 68 EUSERS _T_o_o _m_a_n_y _u_s_e_r_s. The quota system ran out of table entries. 69 EDQUOT _D_i_s_c _q_u_o_t_a _e_x_c_e_e_d_e_d. A write to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because the user's quota of disk blocks was exhaust ed, or the allocation of an inode for a newly created file failed because the user's quota of inodes was exhausted. 70 ESTALE _S_t_a_l_e _N_F_S _f_i_l_e _h_a_n_d_l_e. An attempt was made to access an open file (on an NFS filesystem) which is now unavailable as refer enced by the file descriptor. This may indicate the file was deleted on the NFS server or some other catastrophic event oc cured. 72 EBADRPC _R_P_C _s_t_r_u_c_t _i_s _b_a_d. Exchange of RPC information was unsuccess ful. 73 ERPCMISMATCH _R_P_C _v_e_r_s_i_o_n _w_r_o_n_g. The version of RPC on the remote peer is not compatible with the local version. 74 EPROGUNAVAIL _R_P_C _p_r_o_g_. _n_o_t _a_v_a_i_l. The requested program is not regis tered on the remote host. 75 EPROGMISMATCH _P_r_o_g_r_a_m _v_e_r_s_i_o_n _w_r_o_n_g. The requested version of the pro gram is not available on the remote host (RPC). 76 EPROCUNAVAIL _B_a_d _p_r_o_c_e_d_u_r_e _f_o_r _p_r_o_g_r_a_m. An RPC call was attempted for a procedure which doesn't exist in the remote program. 77 ENOLCK _N_o _l_o_c_k_s _a_v_a_i_l_a_b_l_e. A systemimposed limit on the number of si multaneous file locks was reached. 78 ENOSYS _F_u_n_c_t_i_o_n _n_o_t _i_m_p_l_e_m_e_n_t_e_d. Attempted a system call that is not available on this system. DDEEFFIINNIITTIIOONNSS Process ID. Each active process in the system is uniquely identified by a nonnegative integer called a process ID. The range of this ID is from 0 to 30000. Parent process ID A new process is created by a currently active process; (see fork(2)). The parent process ID of a process is the process ID of its creator. Process Group ID Each active process is a member of a process group that is iden tified by a nonnegative integer called the process group ID. This is the process ID of the group leader. This grouping per mits the signaling of related processes (see killpg(2)) and the job control mechanisms of csh(1). Tty Group ID Each active process can be a member of a terminal group that is identified by a nonnegative integer called the tty group ID. This grouping is used to arbitrate between multiple jobs contend ing for the same terminal; (see csh(1) and tty(4)). Real User ID and Real Group ID Each user on the system is identified by a positive integer termed the real user ID. Each user is also a member of one or more groups. One of these groups is distinguished from others and used in implementing ac counting facilities. The positive integer corresponding to this distinguished group is termed the real group ID. All processes have a real user ID and real group ID. These are initialized from the equivalent attributes of the process that created it. Effective User Id, Effective Group Id, and Access Groups Access to system resources is governed by three values: the ef fective user ID, the effective group ID, and the group access list. The effective user ID and effective group ID are initially the process's real user ID and real group ID respectively. Either may be modified through execution of a setuserID or setgroup ID file (possibly by one its ancestors) (see execve(2)). The group access list is an additional set of group ID's used on ly in determining resource accessibility. Access checks are per formed as described below in ``File Access Permissions''. Superuser A process is recognized as a _s_u_p_e_r__u_s_e_r process and is granted special privileges if its effective user ID is 0. Special Processes The processes with a process ID's of 0, 1, and 2 are special. Process 0 is the scheduler. Process 1 is the initialization pro cess init, and is the ancestor of every other process in the system. It is used to control the process structure. Process 2 is the paging daemon. Descriptor An integer assigned by the system when a file is referenced by open(2) or dup(2), or when a socket is created by pipe(2), socket(2) or socketpair(2), which uniquely identifies an access path to that file or socket from a given process or any of its children. File Name Names consisting of up to 255 (MAXNAMELEN) characters may be used to name an ordinary file, special file, or directory. These characters may be selected from the set of all ASCII char acter excluding 0 (NUL) and the ASCII code for `/' (slash). (The parity bit, bit 7, must be 0.) Note that it is generally unwise to use `*', `?', `[' or `]' as part of file names because of the special meaning attached to these characters by the shell. Path Name A path name is a NULterminated character string starting with an optional slash `/', followed by zero or more directory names sep arated by slashes, optionally followed by a file name. The total length of a path name must be less than 1024 (MAXPATHLEN) charac ters. If a path name begins with a slash, the path search begins at the _r_o_o_t directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. An empty pathname refers to the current directory. Directory A directory is a special type of file that contains entries that are references to other files. Directory entries are called links. By convention, a directory contains at least two links, `.' and `..', referred to as _d_o_t and _d_o_t__d_o_t respectively. Dot refers to the directory itself and dotdot refers to its parent directory. Root Directory and Current Working Directory Each process has associated with it a concept of a root directory and a current working directory for the purpose of resolving path name searches. A process's root directory need not be the root directory of the root file system. File Access Permissions Every file in the file system has a set of access permissions. These permissions are used in determining whether a process may perform a requested operation on the file (such as opening a file for writing). Access permissions are established at the time a file is created. They may be changed at some later time through the chmod(2) call. File access is broken down according to whether a file may be: read, written, or executed. Directory files use the execute per mission to control if the directory may be searched. File access permissions are interpreted by the system as they ap ply to three different classes of users: the owner of the file, those users in the file's group, anyone else. Every file has an independent set of access permissions for each of these classes. When an access check is made, the system decides if permission should be granted by checking the access information applicable to the caller. Read, write, and execute/search permissions on a file are granted to a process if: The process's effective user ID is that of the superuser. (Note: even the superuser cannot execute a nonexecutable file.) The process's effective user ID matches the user ID of the owner of the file and the owner permissions allow the access. The process's effective user ID does not match the user ID of the owner of the file, and either the process's effective group ID matches the group ID of the file, or the group ID of the file is in the process's group access list, and the group permissions al low the access. Neither the effective user ID nor effective group ID and group access list of the process match the corresponding user ID and group ID of the file, but the permissions for ``other users'' al low access. Otherwise, permission is denied. Sockets and Address Families A socket is an endpoint for communication between processes. Each socket has queues for sending and receiving data. Sockets are typed according to their communications properties. These properties include whether messages sent and received at a socket require the name of the partner, whether communication is reliable, the format used in naming message recipients, etc. Each instance of the system supports some collection of socket types; consult socket(2) for more information about the types available and their properties. Each instance of the system supports some number of sets of com munications protocols. Each protocol set supports addresses of a certain format. An Address Family is the set of addresses for a specific group of protocols. Each socket has an address chosen from the address family in which the socket was created. SSEEEE AALLSSOO intro(3), perror(3) HHIISSTTOORRYY An iinnttrroo appeared in Version 6 AT&T UNIX. 4th Berkeley Distribution July 23, 1991 8