WAITSection: Linux Programmer's Manual (2 )
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NAMEwait, waitpid - wait for process termination
DESCRIPTIONThe wait function suspends execution of the current process until a child has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.
The waitpid function suspends execution of the current process until a child as specified by the pid argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child as requested by pid has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.
The value of pid can be one of:
The value of options is an OR of zero or more of the following constants:
If status is not NULL, wait or waitpid store status information in the location pointed to by status.
This status can be evaluated with the following macros (these macros take the stat buffer (an int) as an argument - not a pointer to the buffer!):
Some versions of Unix (e.g. Linux, Solaris, but not AIX, SunOS) also define a macro WCOREDUMP(status) to test whether the child process dumped core. Only use this enclosed in #ifdef WCOREDUMP ... #endif.
RETURN VALUEThe process ID of the child which exited, or zero if WNOHANG was used and no child was available, or -1 on error (in which case errno is set to an appropriate value).
NOTESThe Single Unix Specification describes a flag SA_NOCLDWAIT (not present under Linux) such that if either this flag is set, or the action for SIGCHLD is set to SIG_IGN (which, by the way, is not allowed by POSIX), then children that exit do not become zombies and a call to wait() or waitpid() will block until all children have exited, and then fail with errno set to ECHILD.
In the Linux kernel, a kernel-scheduled thread is not a distinct construct from a process. Instead, a thread is simply a process that is created using the Linux-unique clone(2) system call; other routines such as the portable pthread_create(3) call are implemented using clone(2). Thus, if two threads A and B are siblings, then thread A cannot wait on any processes forked by thread B or its descendents, because an uncle cannot wait on his nephews. In some other Unix-like systems, where multiple threads are implemented as belonging to a single process, thread A can wait on any processes forked by sibling thread B; you will have to rewrite any code that makes this assumption for it to work on Linux.
CONFORMING TOSVr4, POSIX.1
SEE ALSOclone(2), signal(2), wait4(2), pthread_create(3), signal(7)
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Time: 02:42:18 GMT, January 20, 2022