WAIT(2) BSD System Calls Manual WAIT(2)NAME
wait, waitpid, wait4, wait3 — wait for process termination
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/wait.h>
pid_t
wait(int *status);
pid_t
waitpid(pid_t wpid, int *status, int options);
#include <sys/resource.h>
pid_t
wait3(int *status, int options, struct rusage *rusage);
pid_t
wait4(pid_t wpid, int *status, int options, struct rusage *rusage);
DESCRIPTION
The wait() function suspends execution of its calling process until
status information is available for a terminated child process, or a sig‐
nal is received. On return from a successful wait() call, the status
area contains termination information about the process that exited as
defined below.
The wait4() call provides a more general interface for programs that need
to wait for certain child processes, that need resource utilization sta‐
tistics accumulated by child processes, or that require options. The
other wait functions are implemented using wait4().
The wpid parameter specifies the set of child processes for which to
wait. If wpid is -1, the call waits for any child process. If wpid is
0, the call waits for any child process in the process group of the call‐
er. If wpid is greater than zero, the call waits for the process with
process id wpid. If wpid is less than -1, the call waits for any process
whose process group id equals the absolute value of wpid.
The status parameter is defined below.
The options parameter contains the bitwise OR of any of the following
options:
WNOHANG This option is used to indicate that the call should not block
if there are no processes that wish to report status.
WUNTRACED If this option is set, children of the current process that
are stopped due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP
signal also have their status reported.
WALTSIG If this option is specified, the call will wait only for pro‐
cesses that are configured to post a signal other than SIGCHLD
when they exit. If WALTSIG is not specified, the call will
wait only for processes that are configured to post SIGCHLD.
__WCLONE This is an alias for WALTSIG. It is provided for compatibil‐
ity with the Linux clone(2) API.
WALLSIG If this option is specified, the call will wait for all chil‐
dren regardless of what exit signal they post.
__WALL This is an alias for WALLSIG. It is provided for compatibil‐
ity with the Linux clone(2) API .
If rusage is non-zero, a summary of the resources used by the terminated
process and all its children is returned (this information is currently
not available for stopped processes).
When the WNOHANG option is specified and no processes wish to report sta‐
tus, wait4() returns a process id of 0.
The waitpid() call is identical to wait4() with an rusage value of zero.
The older wait3() call is the same as wait4() with a wpid value of -1.
The following macros may be used to test the manner of exit of the
process. Note that these macros expect the status value itself, not a
pointer to the status value. One of the first three macros will evaluate
to a non-zero (true) value:
WIFEXITED(status)
True if the process terminated normally by a call to _exit(2) or
exit(3).
WIFSIGNALED(status)
True if the process terminated due to receipt of a signal.
WIFSTOPPED(status)
True if the process has not terminated, but has stopped and can
be restarted. This macro can be true only if the wait call spec‐
ified the WUNTRACED option or if the child process is being
traced (see ptrace(2)).
Depending on the values of those macros, the following macros produce the
remaining status information about the child process:
WEXITSTATUS(status)
If WIFEXITED(status) is true, evaluates to the low-order 8 bits
of the argument passed to _exit(2) or exit(3) by the child.
WTERMSIG(status)
If WIFSIGNALED(status) is true, evaluates to the number of the
signal that caused the termination of the process.
WCOREDUMP(status)
If WIFSIGNALED(status) is true, evaluates as true if the termina‐
tion of the process was accompanied by the creation of a core
file containing an image of the process when the signal was
received.
WSTOPSIG(status)
If WIFSTOPPED(status) is true, evaluates to the number of the
signal that caused the process to stop.
NOTES
See sigaction(2) for a list of termination signals. A status of 0 indi‐
cates normal termination.
If a parent process terminates without waiting for all of its child pro‐
cesses to terminate, the remaining child processes are assigned the par‐
ent process 1 ID (the init process ID).
If a signal is caught while any of the wait() calls is pending, the call
may be interrupted or restarted when the signal-catching routine returns,
depending on the options in effect for the signal; see intro(2), System
call restart.
RETURN VALUES
If wait() returns due to a stopped or terminated child process, the
process ID of the child is returned to the calling process. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
If wait4(), wait3() or waitpid() returns due to a stopped or terminated
child process, the process ID of the child is returned to the calling
process. If there are no children not previously awaited, -1 is returned
with errno set to [ECHILD]. Otherwise, if WNOHANG is specified and there
are no stopped or exited children, 0 is returned. If an error is
detected or a caught signal aborts the call, a value of -1 is returned
and errno is set to indicate the error.
ERRORSwait() will fail and return immediately if:
[ECHILD] The calling process has no existing unwaited-for child
processes.
[EFAULT] The status or rusage arguments point to an illegal
address. (May not be detected before exit of a child
process.)
[EINTR] The call was interrupted by a caught signal, or the
signal did not have the SA_RESTART flag set.
In addition, wait3(), wait4(), and waitpid() will fail and return immedi‐
ately if:
[EINVAL] An invalid value was specified for options.
SEE ALSO_exit(2), sigaction(2)STANDARDS
The wait() and waitpid() functions conform to ISO/IEC 9945-1:1990
(“POSIX.1”); the wait3() function conforms to X/Open Portability Guide
Issue 4 (“XPG4”); wait4() is an extension. The WCOREDUMP() macro and the
ability to restart a pending wait() call are extensions to the POSIX
interface.
HISTORY
A wait() function call appeared in Version 2 AT&T UNIX.
BSD April 3, 2010 BSD
