Process Group Functions - The GNU C Library

Here are descriptions of the functions for manipulating process groups. Your program should include the header files sys/types.h and unistd.h to use these functions.

— Function: pid_t setsid (void)

The setsid function creates a new session. The calling process becomes the session leader, and is put in a new process group whose process group ID is the same as the process ID of that process. There are initially no other processes in the new process group, and no other process groups in the new session.
This function also makes the calling process have no controlling terminal.
The setsid function returns the new process group ID of the calling process if successful. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EPERM
The calling process is already a process group leader, or there is already another process group around that has the same process group ID.

— Function: pid_t getsid (pid_t pid)

The getsid function returns the process group ID of the session leader of the specified process. If a pid is 0, the process group ID of the session leader of the current process is returned.
In case of error -1 is returned and errno is set. The following errno error conditions are defined for this function:

ESRCH
There is no process with the given process ID pid.
EPERM
The calling process and the process specified by pid are in different sessions, and the implementation doesn't allow to access the process group ID of the session leader of the process with ID pid from the calling process.

The getpgrp function has two definitions: one derived from BSD Unix, and one from the POSIX.1 standard. The feature test macros you have selected (see Feature Test Macros) determine which definition you get. Specifically, you get the BSD version if you define _BSD_SOURCE; otherwise, you get the POSIX version if you define _POSIX_SOURCE or _GNU_SOURCE. Programs written for old BSD systems will not include unistd.h, which defines getpgrp specially under _BSD_SOURCE. You must link such programs with the -lbsd-compat option to get the BSD definition.

— POSIX.1 Function: pid_t getpgrp (void)

The POSIX.1 definition of getpgrp returns the process group ID of the calling process.

— BSD Function: pid_t getpgrp (pid_t pid)

The BSD definition of getpgrp returns the process group ID of the process pid. You can supply a value of 0 for the pid argument to get information about the calling process.

— System V Function: int getpgid (pid_t pid)

getpgid is the same as the BSD function getpgrp. It returns the process group ID of the process pid. You can supply a value of 0 for the pid argument to get information about the calling process.
In case of error -1 is returned and errno is set. The following errno error conditions are defined for this function:

ESRCH
There is no process with the given process ID pid. The calling process and the process specified by pid are in different sessions, and the implementation doesn't allow to access the process group ID of the process with ID pid from the calling process.

— Function: int setpgid (pid_t pid, pid_t pgid)

The setpgid function puts the process pid into the process group pgid. As a special case, either pid or pgid can be zero to indicate the process ID of the calling process.
This function fails on a system that does not support job control. See Job Control is Optional, for more information.
If the operation is successful, setpgid returns zero. Otherwise it returns -1. The following errno error conditions are defined for this function:

EACCES
The child process named by pid has executed an exec function since it was forked.
EINVAL
The value of the pgid is not valid.
ENOSYS
The system doesn't support job control.
EPERM
The process indicated by the pid argument is a session leader, or is not in the same session as the calling process, or the value of the pgid argument doesn't match a process group ID in the same session as the calling process.
ESRCH
The process indicated by the pid argument is not the calling process or a child of the calling process.

27.7.2 Process Group Functions