int setpgid(pid_t pid, pid_t pgid);
The setpgid() function shall either join an existing process group or create a new process group within the session of the calling process. The process group ID of a session leader shall not change. Upon successful completion, the process group ID of the process with a process ID that matches pid shall be set to pgid. As a special case, if pid is 0, the process ID of the calling process shall be used. Also, if pgid is 0, the process ID of the indicated process shall be used.
Upon successful completion, setpgid() shall return 0; otherwise, -1 shall be returned and errno shall be set to indicate the error.
The setpgid() function shall fail if:
The following sections are informative.
The setpgid() function shall group processes together for the purpose of signaling, placement in foreground or background, and other job control actions.
The setpgid() function is similar to the setpgrp() function of 4.2 BSD, except that 4.2 BSD allowed the specified new process group to assume any value. This presents certain security problems and is more flexible than necessary to support job control.
To provide tighter security, setpgid() only allows the calling process to join a process group already in use inside its session or create a new process group whose process group ID was equal to its process ID.
When a job control shell spawns a new job, the processes in the job must be placed into a new process group via setpgid(). There are two timing constraints involved in this action:
To address these constraints, the following actions are performed. The new processes call setpgid() to alter their own process groups after fork() but before exec. This satisfies the first constraint. Under 4.3 BSD, the second constraint is satisfied by the synchronization property of vfork(); that is, the shell is suspended until the child has completed the exec, thus ensuring that the child has completed the setpgid(). A new version of fork() with this same synchronization property was considered, but it was decided instead to merely allow the parent shell process to adjust the process group of its child processes via setpgid(). Both timing constraints are now satisfied by having both the parent shell and the child attempt to adjust the process group of the child process; it does not matter which succeeds first.
Since it would be confusing to an application to have its process group change after it began executing (that is, after exec), and because the child process would already have adjusted its process group before this, the [EACCES] error was added to disallow this.
One non-obvious use of setpgid() is to allow a job control shell to return itself to its original process group (the one in effect when the job control shell was executed). A job control shell does this before returning control back to its parent when it is terminating or suspending itself as a way of restoring its job control "state" back to what its parent would expect. (Note that the original process group of the job control shell typically matches the process group of its parent, but this is not necessarily always the case.)
exec() , getpgrp() , setsid() , tcsetpgrp() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>