WAIT(2)

NAME
wait, waitpid - wait for process termination

SYNOPSIS
#include <sys/types.h>
#include <sys/wait.h>

pid_t wait(int *status)
pid_t waitpid(pid_t pid, int *status, int options);

DESCRIPTION
The 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:

< -1
    which means to wait for any child process whose process group ID is equal to the absolute value of pid.

-1  which means to wait for any child process; this is the same behaviour which wait exhibits.

0   which means to wait for any child process whose process group ID is equal to that of the calling process.

> 0 which means to wait for the child whose process ID is equal to the value of pid.

The value of options is an OR of zero or more of the following constants:

WNOHANG
    which means to return immediately if no child has exited.

WUNTRACED
    which means to also return for children which are stopped, and whose status has not been reported.

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!):

WIFEXITED(status)
    is non-zero if the child exited normally.

WEXITSTATUS(status)
    evaluates to the least significant eight bits of the return code of the child which terminated, which may have been set as the argument to a call to exit() or as the argument for a return statement in the main program. This macro can only be evaluated if WIFEXITED returned non-zero.

WIFSIGNALED(status)
    returns true if the child process exited because of a signal which was not caught.

WTERMSIG(status)
    returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED returned non-zero.

WIFSTOPPED(status)
    returns true if the child process which caused the return is currently stopped; this is only possible if the call was done using WUNTRACED.

WSTOPSIG(status)
    returns the number of the signal which caused the child to stop. This macro can only be evaluated if WIFSTOPPED returned non-zero.

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 VALUE
The 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).

ERRORS

ECHILD
    if the process specified in pid does not exist or is not a child of the calling process. (This can happen for one's own child if the action for SIGCHLD is set to SIG_IGN. See also the NOTES section about threads.)

EINVAL
    if the options argument was invalid.

EINTR
    if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.

NOTES
The 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 TO
SVr4, POSIX.1

SEE ALSO
clone(2), signal(2), wait4(2), pthread_create(3), signal(7)