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waitpid(3c)
waitpid(3C) Standard C Library Functions waitpid(3C)
NAME
waitpid - wait for child process to change state
SYNOPSIS
#include <sys/types.h>
#include <sys/wait.h>
pid_t waitpid(pid_t pid, int *stat_loc, int options);
DESCRIPTION
The waitpid() function will suspend execution of the calling thread un‐
til status information for one of its terminated child processes is
available, or until delivery of a signal whose action is either to exe‐
cute a signal-catching function or to terminate the process, or an er‐
ror occurs. If more than one thread is suspended in waitpid(),
wait(3C), or waitid(2) awaiting termination of the same process, ex‐
actly one thread will return the process status at the time of the tar‐
get process termination. If status information is available prior to
the call to waitpid(), return will be immediate.
The pid argument specifies a set of child processes for which status is
requested, as follows:
o If pid is less than (pid_t)−1, status is requested for any
child process whose process group ID is equal to the ab‐
solute value of pid.
o If pid is equal to (pid_t)−1, status is requested for any
child process.
o If pid is equal to (pid_t)0 status is requested for any
child process whose process group ID is equal to that of the
calling process.
o If pid is greater than (pid_t)0, it specifies the process ID
of the child process for which status is requested.
One instance of a SIGCHLD signal is queued for each child process whose
status has changed. If waitpid() returns because the status of a child
process is available, and WNOWAIT was not specified in options, any
pending SIGCHLD signal associated with the process ID of that child
process is discarded. Any other pending SIGCHLD signals remain pending.
If the calling process has SA_NOCLDWAIT set or has SIGCHLD set to
SIG_IGN and the process has no unwaited children that were transformed
into zombie processes, it will block until all of its children termi‐
nate, and waitpid() will fail and set errno to ECHILD.
If waitpid() returns because the status of a child process is avail‐
able, then that status may be evaluated with the macros defined by
wait.h(3HEAD). If the calling process had specified a non-zero value of
stat_loc, the status of the child process will be stored in the loca‐
tion pointed to by stat_loc.
The options argument is constructed from the bitwise-inclusive OR of
zero or more of the following flags, defined in the header
<sys/wait.h>:
WCONTINUED The status of any continued child process specified by
pid, whose status has not been reported since it contin‐
ued, is also reported to the calling process.
WNOHANG The waitpid() function will not suspend execution of the
calling process if status is not immediately available
for one of the child processes specified by pid.
WNOWAIT Keep the process whose status is returned in stat_loc in
a waitable state. The process may be waited for again
with identical results.
WUNTRACED The status of any child processes specified by pid that
are stopped, and whose status has not yet been reported
since they stopped, is also reported to the calling
process. WSTOPPED is a synonym for WUNTRACED.
RETURN VALUES
If waitpid() returns because the status of a child process is avail‐
able, it returns a value equal to the process ID of the child process
for which status is reported. If waitpid() returns due to the delivery
of a signal to the calling process, −1 is returned and errno is set to
EINTR. If waitpid() was invoked with WNOHANG set in options, it has at
least one child process specified by pid for which status is not avail‐
able, and status is not available for any process specified by pid,
then 0 is returned. Otherwise, −1 is returned and errno is set to indi‐
cate the error.
ERRORS
The waitpid() function will fail if:
ECHILD The process or process group specified by pid does not exist
or is not a child of the calling process or can never be in
the states specified by options.
EINTR The waitpid() function was interrupted due to the receipt of
a signal sent by the calling process.
EINVAL An invalid value was specified for options.
USAGE
With options equal to 0 and pid equal to (pid_t)−1, waitpid() is iden‐
tical to wait(3C). The waitpid() function is implemented as a call to
the more general waitid(2) function.
ATTRIBUTES
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelAsync-Signal-
Safe _ StandardSee standards(7).
SEE ALSO
exec(2), exit(2), fork(2), Intro(2), pause(2), sigaction(2),
ptrace(3C), signal(3C), wait(3C), siginfo.h(3HEAD), wait.h(3HEAD), at‐
tributes(7), standards(7)
HISTORY
The waitpid() function has been included in all Sun and Oracle releases
of Solaris.
Oracle Solaris 11.4 27 Aug 2025 waitpid(3C)