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     getpgrp — get process group


     Standard C Library (libc, -lc)


     #include <unistd.h>


     getpgid(pid_t pid);


     The process group of the current process is returned by getpgrp().  The process group of the
     process identified by pid is returned by getpgid().  If pid is zero, getpgid() returns the
     process group of the current process.

     Process groups are used for distribution of signals, and by terminals to arbitrate requests
     for their input: processes that have the same process group as the terminal are foreground
     and may read, while others will block with a signal if they attempt to read.

     This system call is thus used by programs such as csh(1) to create process groups in
     implementing job control.  The tcgetpgrp() and tcsetpgrp() calls are used to get/set the
     process group of the control terminal.


     The getpgrp() system call always succeeds.  Upon successful completion, the getpgid() system
     call returns the process group of the specified process; otherwise, it returns a value of -1
     and sets errno to indicate the error.


     This version of getpgrp() differs from past Berkeley versions by not taking a pid_t pid
     argument.  This incompatibility is required by ISO/IEC 9945-1:1990 (“POSIX.1”).

     From the ISO/IEC 9945-1:1990 (“POSIX.1”) Rationale:

     4.3BSD provides a getpgrp() system call that returns the process group ID for a specified
     process.  Although this function is used to support job control, all known job-control
     shells always specify the calling process with this function.  Thus, the simpler AT&T
     System V UNIX getpgrp() suffices, and the added complexity of the 4.3BSD getpgrp() has been
     omitted from POSIX.1.  The old functionality is available from the getpgid() system call.


     The getpgid() system call will succeed unless:

     [ESRCH]            there is no process whose process ID equals pid


     getsid(2), setpgid(2), termios(4)


     The getpgrp() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”).


     The getpgrp() system call appeared in 4.0BSD.  The getpgid() system call is derived from its
     usage in AT&T System V Release 4 UNIX.