suspend the calling process
pid_t waitpid( pid_t pid,
int options );
The waitpid() function suspends execution of the calling
process until status information from one of its terminated child
processes is available, or until the delivery of a signal whose action
is either to terminate the process or execute a signal handler. If
status information is available prior to the call to waitpid(),
the return is immediate.
The waitpid() function behaves the same as the wait()
function when passed a pid argument of -1 and the
options argument has a value of zero.
The pid argument modifies the behavior of waitpid() in
that it allows the calling process to specify a set of child processes
for which status information is requested:
- If pid is equal to -1, status is requested for any child
- If pid is greater than zero, it specifies the process ID of a
single child process for which status is requested.
- If pid is equal to zero, status is requested from any child
process whose process group ID is equal to that of the calling process.
- If pid is less than -1, status is requested for any child
process whose process group ID is equal to the absolute value of
If stat_loc is non-NULL, the terminating
status of the child process is placed in the integer pointed to by
stat_loc. The macros listed below extract
information from stat_loc. The stat_val argument to
these macros is the integer value pointed to by stat_loc. At
least the following macros are defined in <sys/wait.h>:
- WIFEXITED( stat_val )
- Evaluates to a nonzero value if the status returned was from a
normally terminated child process.
- WEXITSTATUS( stat_val )
- If the value of WIFEXITED(stat_val) is
nonzero, this macro evaluates to the low-order 8 bits of the
termination status of the child process.
- WIFSIGNALED( stat_val )
- Evaluates to nonzero value if the child process terminated from
receiving a signal that wasn't caught.
- WTERMSIG( stat_val )
- If the value of WIFSIGNALED(stat_val) is
nonzero, this macro evaluates to the number of the signal that
terminated the child process.
- WIFSTOPPED( stat_val )
- Evaluates to a nonzero value if the status returned is for a child
process that is stopped.
- WSTOPSIG( stat_val )
- If the value of WIFSTOPPED(stat_val) is
nonzero, this macro evaluates to the number of the signal that caused
the child process to stop.
The options argument is constructed from the bitwise
inclusive OR of zero or more of the following flags:
- The waitpid() function will not suspend execution of the
calling process if status isn't immediately available for one of the
child processes specified by pid.
Only one of the WIFEXITED(stat_val) and
WIFSIGNALED(stat_val) macros can evaluate to a
If successful, waitpid() returns the process ID of the
terminating child process.
If waitpid() was invoked with
WNOHANG set in options and there are no
children waiting, it returns 0.
On delivery of a signal,
waitpid() returns -1, and errno is set to
- The calling process has no existing unwaited-for child processes that
meet the criteria set by pid.
- The function was interrupted by a signal. The value of the location
pointed to by stat_loc is undefined.
- The value of the options argument isn't valid.