wait(2) -- Linux man page
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:
- < -1
- which means to wait for any child process whose process group ID is equal to the absolute value of pid.
- which means to wait for any child process; this is the same behaviour which wait exhibits.
- 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:
- which means to return immediately if no child has exited.
- which means to also return for children which are stopped (but not traced), and whose status has not been reported. Status for traced children which are stopped is provided also without this option.
(For Linux-only options, see below.)
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!):
- returns true if the child terminated normally, that is, by calling exit() or _exit(), or by returning from main().
- 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 _exit() or as the argument for a return statement in the main program. This macro can only be evaluated if WIFEXITED returned true.
- returns true if the child process terminated because of a signal which was not caught.
- returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED returned non-zero.
- returns true if the child process which caused the return is currently stopped; this is only possible if the call was done using WUNTRACED or when the child is being traced (see ptrace(2)).
- 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 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).
- 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 LINUX NOTES section about threads.)
- if the options argument was invalid.
- if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.
NOTESThe Single Unix Specification describes a flag SA_NOCLDWAIT (not supported under Linux) such that if either this flag is set, or the action for SIGCHLD is set to SIG_IGN 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.
The original POSIX standard left the behaviour of setting SIGCHLD to SIG_IGN unspecified. Later standards, including SUSv2 and POSIX 1003.1-2001 specify the behaviour just described as an XSI-compliance option. Linux does not conform to the second of the two points just described: if a wait() or waitpid() call is made while SIGCHLD is being ignored, the call behaves just as though SIGCHLD were not being ignored, that is, the call blocks until the next child terminates and then returns the PID and status of that child.
LINUX NOTESIn 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). Before Linux 2.4, a thread was just a special case of a process, and as a consequence one thread could not wait on the children of another thread, even when the latter belongs to the same thread group. However, POSIX prescribes such functionality, and since Linux 2.4 a thread can, and by default will, wait on children of other threads in the same thread group.
The following Linux-specific options are for use with children created using clone(2).
- Wait for "clone" children only. If omitted then wait for "non-clone" children only. (A "clone" child is one which delivers no signal, or a signal other than SIGCHLD to its parent upon termination.) This option is ignored if __WALL is also specified.
- (Since Linux 2.4) Wait for all children, regardless of type ("clone" or "non-clone").
- (Since Linux 2.4) Do not wait for children of other threads in the same thread group. This was the default before Linux 2.4.
CONFORMING TOSVr4, POSIX.1
SEE ALSOclone(2), ptrace(2), signal(2), wait4(2), pthread_create(3), signal(7)