waitpid(3)waitpid(3)NAMEwaitpid - allows the calling process to obtain status information per‐
taining to one of its child processes.
SYNOPSIS
#include <sys/wait.h>
pid_t waitpid(
pid_t process_id,
int ∗status_location,
int options);
LIBRARY
SVR4 Library (libsvr4)
PARAMETERS
Points to a location that contains the termination status of the child
process as defined in the <sys/wait.h> header file. Specifies the
child process or set of child processes. Modifies the behavior of the
function. The flags for the options parameter are defined in the
DESCRIPTION section.
DESCRIPTION
The waitpid() function allows the calling process to obtain status
information pertaining to one of its child processes. Various options
permit status information to be obtained for child processes that have
terminated or stopped. If status information is available for two or
more child processes, the order in which their status is reported is
unspecified.
The waitpid() function behaves identically to wait(), if the process_id
parameter has a value of (pid_t)-1 and the options parameter specifies
a value of zero (0). Should these parameters contain other values, the
waitpid() function is changed as specified by those values.
In a multithreaded environment, the waitpid() function blocks only the
calling thread.
The process_id parameter allows the calling process to gather status
from a specific set of child processes. The waitpid() function returns
the status only of child processes from this set. The following rules
define the set: If the process_id parameter is equal to (pid_t)-1, sta‐
tus is requested for any child process. In this respect, the waitpid()
function is equivalent to the wait() function. If the process_id
parameter is greater than zero (0), it specifies the process ID of a
single child process for which status is requested. If the process_id
parameter is equal to zero (0), status is requested for any child
process whose process group ID is equal to that of the calling process.
If the process_id parameter is less than (pid_t)-1, status is requested
for any child process whose process group ID is equal to the absolute
value of the process_id parameter.
The options parameter to the waitpid() function modifies the behavior
of the function. The flags for the options parameter can be combined by
specifying them in a bitwise-inclusive OR operation. The flags are as
follows: Prevents the calling process from being suspended. If there
are child processes that have been stopped or terminated, one is chosen
and waitpid() returns its pid, just as when the WNOHANG flag is not
specified. If there are no such processes (that is, if conditions are
such that waitpid() without the WNOHANG flag would have suspended the
calling process), zero (0) is returned. Because you can never wait for
process 0, there is no confusion arising from this return. Specifies
that the call return additional information when the child process is
stopped because it received a SIGTTIN, SIGTTOU, SIGSTOP, or SIGTSTOP
signal.
If the waitpid() function returns the process ID of the child process,
in this instance, information is stored in the location pointed to by
the status_location parameter if the value of the status_location is
not null. The value stored in the location pointed to by the sta‐
tus_location parameter is zero (0) if, and only if, the status is
returned from a terminated child process that did one of the following:
Returned zero (0) from the main() routine. Passed zero (0) as the sta‐
tus parameter to the _exit or exit function.
Regardless of its value, this information can be defined using the
macros defined in the <sys/wait.h> header file that evaluate to inte‐
gral expressions.
RETURN VALUES
If the waitpid() function returns because the status of a child process
is available, the process ID of the child is returned to the calling
process. If they return because a signal was caught by the calling
process, -1 is returned and errno is set to [EINTR].
If the waitpid() function is invoked with the WNOHANG option specified,
it has at least one child process specified by pid for which status is
not available, and status is not available for any process specified by
pid, a value of 0 (zero) is returned. Otherwise, (pid_t-1) is
returned, and errno is set to indicate the error.
ERRORS
The waitpid() function fails if one or both of the following are true:
The process or process group ID specified by the process_id parameter
does not exist or is not a child process of the calling process. The
value of the options parameter is not valid.
RELATED INFORMATION
Functions: exec(2), exit(2), fork(2), ptrace(2), getrusage(2), sigac‐
tion(2), pause(3)
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waitpid(3)
