#### 27.7.2 Process Group Functions

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.

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

> This is the BSD Unix name for `setpgid`. Both functions do exactly the same thing.