1 Child Processes

Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.

Processes are organized hierarchically. Each process has a parent process which explicitly arranged to create it. The processes created by a given parent are called its child processes. A child inherits many of its attributes from the parent process.

This chapter describes how a program can create, terminate, and control child processes. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program.

The system function provides a simple, portable mechanism for running another program; it does all three steps automatically. If you need more control over the details of how this is done, you can use the primitive functions to do each step individually instead.

1.1 Running a Command		The easy way to run another program.
1.2 Process Creation Concepts		An overview of the hard way to do it.
1.3 Process Identification		How to get the process ID of a process.
1.4 Creating a Process		How to fork a child process.
1.5 Executing a File		How to make a process execute another program.
1.6 Process Completion		How to tell when a child process has completed.
1.7 Process Completion Status		How to interpret the status value returned from a child process.
1.8 BSD Process Wait Functions		More functions, for backward compatibility.
1.9 Process Creation Example		A complete example program.

1.1 Running a Command

The easy way to run another program is to use the system function. This function does all the work of running a subprogram, but it doesn’t give you much control over the details: you have to wait until the subprogram terminates before you can do anything else.

Function: int system (const char *command)

This function executes command as a shell command. In the GNU C library, it always uses the default shell sh to run the command. In particular, it searches the directories in PATH to find programs to execute. The return value is -1 if it wasn’t possible to create the shell process, and otherwise is the status of the shell process. See section Process Completion, for details on how this status code can be interpreted.

The system function is declared in the header file ‘stdlib.h’.

Portability Note: Some C implementations may not have any notion of a command processor that can execute other programs. You can determine whether a command processor exists by executing system (NULL); if the return value is nonzero, a command processor is available.

The popen and pclose functions (@pxref{Pipe to a Subprocess}) are closely related to the system function. They allow the parent process to communicate with the standard input and output channels of the command being executed.

1.2 Process Creation Concepts

This section gives an overview of processes and of the steps involved in creating a process and making it run another program.

Each process is named by a process ID number. A unique process ID is allocated to each process when it is created. The lifetime of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.

Processes are created with the fork system call (so the operation of creating a new process is sometimes called forking a process). The child process created by fork is an exact clone of the original parent process, except that it has its own process ID.

After forking a child process, both the parent and child processes continue to execute normally. If you want your program to wait for a child process to finish executing before continuing, you must do this explicitly after the fork operation, by calling wait or waitpid (see section Process Completion). These functions give you limited information about why the child terminated—for example, its exit status code.

A newly forked child process continues to execute the same program as its parent process, at the point where the fork call returns. You can use the return value from fork to tell whether the program is running in the parent process or the child.

Having several processes run the same program is only occasionally useful. But the child can execute another program using one of the exec functions; see Executing a File. The program that the process is executing is called its process image. Starting execution of a new program causes the process to forget all about its previous process image; when the new program exits, the process exits too, instead of returning to the previous process image.

1.3 Process Identification

The pid_t data type represents process IDs. You can get the process ID of a process by calling getpid. The function getppid returns the process ID of the parent of the current process (this is also known as the parent process ID). Your program should include the header files ‘unistd.h’ and ‘sys/types.h’ to use these functions.

Data Type: pid_t: The pid_t data type is a signed integer type which is capable of representing a process ID. In the GNU library, this is an int.

Function: pid_t getpid (void): The getpid function returns the process ID of the current process.

Function: pid_t getppid (void): The getppid function returns the process ID of the parent of the current process.

1.4 Creating a Process

The fork function is the primitive for creating a process. It is declared in the header file ‘unistd.h’.

Function: pid_t fork (void)

The fork function creates a new process.

If the operation is successful, there are then both parent and child processes and both see fork return, but with different values: it returns a value of 0 in the child process and returns the child’s process ID in the parent process.

If process creation failed, fork returns a value of -1 in the parent process. The following errno error conditions are defined for fork:

EAGAIN: There aren’t enough system resources to create another process, or the user already has too many processes running.
ENOMEM: The process requires more space than the system can supply.

The specific attributes of the child process that differ from the parent process are:

The child process has its own unique process ID.
The parent process ID of the child process is the process ID of its parent process.
The child process gets its own copies of the parent process’s open file descriptors. Subsequently changing attributes of the file descriptors in the parent process won’t affect the file descriptors in the child, and vice versa. @xref{Control Operations}.
The elapsed processor times for the child process are set to zero; see @ref{Processor Time}.
The child doesn’t inherit file locks set by the parent process. @xref{Control Operations}.
The child doesn’t inherit alarms set by the parent process. @xref{Setting an Alarm}.
The set of pending signals (@pxref{Delivery of Signal}) for the child process is cleared. (The child process inherits its mask of blocked signals and signal actions from the parent process.)

Function: pid_t vfork (void)

The vfork function is similar to fork but more efficient; however, there are restrictions you must follow to use it safely.

While fork makes a complete copy of the calling process’s address space and allows both the parent and child to execute independently, vfork does not make this copy. Instead, the child process created with vfork shares its parent’s address space until it calls one of the exec functions. In the meantime, the parent process suspends execution.

You must be very careful not to allow the child process created with vfork to modify any global data or even local variables shared with the parent. Furthermore, the child process cannot return from (or do a long jump out of) the function that called vfork! This would leave the parent process’s control information very confused. If in doubt, use fork instead.

Some operating systems don’t really implement vfork. The GNU C library permits you to use vfork on all systems, but actually executes fork if vfork isn’t available. If you follow the proper precautions for using vfork, your program will still work even if the system uses fork instead.

1.5 Executing a File

This section describes the exec family of functions, for executing a file as a process image. You can use these functions to make a child process execute a new program after it has been forked.

The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file ‘unistd.h’.

Function: int execv (const char *filename, char *const argv[])

The execv function executes the file named by filename as a new process image.

The argv argument is an array of null-terminated strings that is used to provide a value for the argv argument to the main function of the program to be executed. The last element of this array must be a null pointer. By convention, the first element of this array is the file name of the program sans directory names. @xref{Program Arguments}, for full details on how programs can access these arguments.

The environment for the new process image is taken from the environ variable of the current process image; see @ref{Environment Variables}, for information about environments.

Function: int execl (const char *filename, const char *arg0, …): This is similar to execv, but the argv strings are specified individually instead of as an array. A null pointer must be passed as the last such argument.

Function: int execve (const char *filename, char *const argv[], char *const env[]): This is similar to execv, but permits you to specify the environment for the new program explicitly as the env argument. This should be an array of strings in the same format as for the environ variable; see @ref{Environment Access}.

Function: int execle (const char *filename, const char *arg0, char *const env[], …): This is similar to execl, but permits you to specify the environment for the new program explicitly. The environment argument is passed following the null pointer that marks the last argv argument, and should be an array of strings in the same format as for the environ variable.

Function: int execvp (const char *filename, char *const argv[])

The execvp function is similar to execv, except that it searches the directories listed in the PATH environment variable (@pxref{Standard Environment}) to find the full file name of a file from filename if filename does not contain a slash.

This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type.

Function: int execlp (const char *filename, const char *arg0, …): This function is like execl, except that it performs the same file name searching as the execvp function.

The size of the argument list and environment list taken together must not be greater than ARG_MAX bytes. @xref{General Limits}. In the GNU system, the size (which compares against ARG_MAX) includes, for each string, the number of characters in the string, plus the size of a char *, plus one, rounded up to a multiple of the size of a char *. Other systems may have somewhat different rules for counting.

These functions normally don’t return, since execution of a new program causes the currently executing program to go away completely. A value of -1 is returned in the event of a failure. In addition to the usual file name syntax errors (@pxref{File Name Errors}), the following errno error conditions are defined for these functions:

E2BIG: The combined size of the new program’s argument list and environment list is larger than ARG_MAX bytes. The GNU system has no specific limit on the argument list size, so this error code cannot result, but you may get ENOMEM instead if the arguments are too big for available memory.
ENOEXEC: The specified file can’t be executed because it isn’t in the right format.
ENOMEM: Executing the specified file requires more storage than is available.

If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. @xref{File Times}, for more details about access times of files.

The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.

Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged:

The process ID and the parent process ID. See section Process Creation Concepts.
Session and process group membership. @xref{Concepts of Job Control}.
Real user ID and group ID, and supplementary group IDs. @xref{Process Persona}.
Pending alarms. @xref{Setting an Alarm}.
Current working directory and root directory. @xref{Working Directory}.
File mode creation mask. @xref{Setting Permissions}.
Process signal mask; see @ref{Process Signal Mask}.
Pending signals; see @ref{Blocking Signals}.
Elapsed processor time associated with the process; see @ref{Processor Time}.

If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in @ref{Process Persona}.

Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see @ref{Signal Handling}.

File descriptors open in the existing process image remain open in the new process image, unless they have the FD_CLOEXEC (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in @ref{Low-Level I/O}.

Streams, by contrast, cannot survive through exec functions, because they are located in the memory of the process itself. The new process image has no streams except those it creates afresh. Each of the streams in the pre-exec process image has a descriptor inside it, and these descriptors do survive through exec (provided that they do not have FD_CLOEXEC set. The new process image can reconnect these to new streams using fdopen (@pxref{Descriptors and Streams}).

1.6 Process Completion

The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file ‘sys/wait.h’.

Function: pid_t waitpid (pid_t pid, int *status_ptr, int options)

The waitpid function is used to request status information from a child process whose process ID is pid. Normally, the calling process is suspended until the child process makes status information available by terminating.

Other values for the pid argument have special interpretations. A value of -1 or WAIT_ANY requests status information for any child process; a value of 0 or WAIT_MYPGRP requests information for any child process in the same process group as the calling process; and any other negative value - pgid requests information for any child process whose process group ID is pgid.

If status information for a child process is available immediately, this function returns immediately without waiting. If more than one eligible child process has status information available, one of them is chosen randomly, and its status is returned immediately. To get the status from the other eligible child processes, you need to call waitpid again.

The options argument is a bit mask. Its value should be the bitwise OR (that is, the ‘|’ operator) of zero or more of the WNOHANG and WUNTRACED flags. You can use the WNOHANG flag to indicate that the parent process shouldn’t wait; and the WUNTRACED flag to request status information from stopped processes as well as processes that have terminated.

The status information from the child process is stored in the object that status_ptr points to, unless status_ptr is a null pointer.

The return value is normally the process ID of the child process whose status is reported. If the WNOHANG option was specified and no child process is waiting to be noticed, the value is zero. A value of -1 is returned in case of error. The following errno error conditions are defined for this function:

EINTR: The function was interrupted by delivery of a signal to the calling process. @xref{Interrupted Primitives}.
ECHILD: There are no child processes to wait for, or the specified pid is not a child of the calling process.
EINVAL: An invalid value was provided for the options argument.

These symbolic constants are defined as values for the pid argument to the waitpid function.

WAIT_ANY: This constant macro (whose value is -1) specifies that waitpid should return status information about any child process.
WAIT_MYPGRP: This constant (with value 0) specifies that waitpid should return status information about any child process in the same process group as the calling process.

These symbolic constants are defined as flags for the options argument to the waitpid function. You can bitwise-OR the flags together to obtain a value to use as the argument.

WNOHANG: This flag specifies that waitpid should return immediately instead of waiting, if there is no child process ready to be noticed.
WUNTRACED: This flag specifies that waitpid should report the status of any child processes that have been stopped as well as those that have terminated.

Function: pid_t wait (int *status_ptr)

This is a simplified version of waitpid, and is used to wait until any one child process terminates. The call:

wait (&status)

is exactly equivalent to:

waitpid (-1, &status, 0)

Here’s an example of how to use waitpid to get the status from all child processes that have terminated, without ever waiting. This function is designed to be a handler for SIGCHLD, the signal that indicates that at least one child process has terminated.

void
sigchld_handler (int signum)
{
  int pid;
  int status;
  while (1)
    {
      pid = waitpid (WAIT_ANY, &status, WNOHANG);
      if (pid < 0)
        {
          perror ("waitpid");
          break;
        }
      if (pid == 0)
        break;
      notice_termination (pid, status);
    }
}

1.7 Process Completion Status

If the exit status value (@pxref{Program Termination}) of the child process is zero, then the status value reported by waitpid or wait is also zero. You can test for other kinds of information encoded in the returned status value using the following macros. These macros are defined in the header file ‘sys/wait.h’.

Macro: int WIFEXITED (int status): This macro returns a nonzero value if the child process terminated normally with exit or _exit.

Macro: int WEXITSTATUS (int status): If WIFEXITED is true of status, this macro returns the low-order 8 bits of the exit status value from the child process. @xref{Exit Status}.

Macro: int WIFSIGNALED (int status): This macro returns a nonzero value if the child process terminated because it received a signal that was not handled. @xref{Signal Handling}.

Macro: int WTERMSIG (int status): If WIFSIGNALED is true of status, this macro returns the signal number of the signal that terminated the child process.

Macro: int WCOREDUMP (int status): This macro returns a nonzero value if the child process terminated and produced a core dump.

Macro: int WIFSTOPPED (int status): This macro returns a nonzero value if the child process is stopped.

Macro: int WSTOPSIG (int status): If WIFSTOPPED is true of status, this macro returns the signal number of the signal that caused the child process to stop.

1.8 BSD Process Wait Functions

The GNU library also provides these related facilities for compatibility with BSD Unix. BSD uses the union wait data type to represent status values rather than an int. The two representations are actually interchangeable; they describe the same bit patterns. The GNU C Library defines macros such as WEXITSTATUS so that they will work on either kind of object, and the wait function is defined to accept either type of pointer as its status_ptr argument.

These functions are declared in ‘sys/wait.h’.

Data Type: union wait

This data type represents program termination status values. It has the following members:

int w_termsig: The value of this member is the same as the result of the WTERMSIG macro.
int w_coredump: The value of this member is the same as the result of the WCOREDUMP macro.
int w_retcode: The value of this member is the same as the result of the WEXITSTATUS macro.
int w_stopsig: The value of this member is the same as the result of the WSTOPSIG macro.

Instead of accessing these members directly, you should use the equivalent macros.

Function: pid_t wait3 (union wait *status_ptr, int options, struct rusage *usage)

If usage is a null pointer, wait3 is equivalent to waitpid (-1, status_ptr, options).

If usage is not null, wait3 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped). @xref{Resource Usage}.

Function: pid_t wait4 (pid_t pid, union wait *status_ptr, int options, struct rusage *usage)

If usage is a null pointer, wait4 is equivalent to waitpid (pid, status_ptr, options).

If usage is not null, wait4 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped). @xref{Resource Usage}.

1.9 Process Creation Example

Here is an example program showing how you might write a function similar to the built-in system. It executes its command argument using the equivalent of ‘sh -c command’.

#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

/* Execute the command using this shell program.  */
#define SHELL "/bin/sh"

int 
my_system (const char *command)
{
  int status;
  pid_t pid;

  pid = fork ();
  if (pid == 0)
    {
      /* This is the child process.  Execute the shell command. */
      execl (SHELL, SHELL, "-c", command, NULL);
      _exit (EXIT_FAILURE);
    }
  else if (pid < 0)
    /* The fork failed.  Report failure.  */
    status = -1;
  else
    /* This is the parent process.  Wait for the child to complete.  */
    if (waitpid (pid, &status, 0) != pid)
      status = -1;
  return status;
}

There are a couple of things you should pay attention to in this example.

Remember that the first argv argument supplied to the program represents the name of the program being executed. That is why, in the call to execl, SHELL is supplied once to name the program to execute and a second time to supply a value for argv[0].

The execl call in the child process doesn’t return if it is successful. If it fails, you must do something to make the child process terminate. Just returning a bad status code with return would leave two processes running the original program. Instead, the right behavior is for the child process to report failure to its parent process.

Call _exit to accomplish this. The reason for using _exit instead of exit is to avoid flushing fully buffered streams such as stdout. The buffers of these streams probably contain data that was copied from the parent process by the fork, data that will be output eventually by the parent process. Calling exit in the child would output the data twice. @xref{Termination Internals}.

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