Waits for a child process to stop or terminate.
#include <wait.h>
pid_t waitpid (pid_t process_id, int
*status_location, int options);
If the process_id argument is -1 and the options argument is 0, the waitpid function behaves the same as the wait function. If these arguments have other values, the waitpid function is changed as specified by those values.
The process_id argument allows the calling process to gather status from a specific set of child processes, according to the following rules:
| If the process_id is | Then status is requested |
|---|---|
| Equal to -1 | For any child process. In this respect, the waitpid function is equivalent to the wait function. |
| Greater than 0 | For a single child process and specifies the process ID. |
The waitpid function only returns the status of a child process from this set.
The options argument to the waitpid function modifies the behavior of the function. You can combine the flags for the options argument by specifying their bitwise-inclusive OR. The flags are:
| WCONTINUED | Specifies that the following is reported to the calling process: the status of any continued child process specified by the process_id argument whose status is unreported since it continued. |
| WNOWAIT | Specifies that the process whose status is returned in status_location is kept in a waitable state. You can wait for the process again with the same results. |
| WNOHANG | Prevents the calling process from being suspended. If there are child processes that stopped or terminated, one is chosen and waitpid returns its pid, as when you do not specify the WNOHANG flag. If there are no terminated processes (that is, if waitpid suspends the calling process without the WNOHANG flag), 0 (zero) is returned. Because you can never wait for process 0, there is no confusion arising from this return. |
| WUNTRACED | Specifies that the call return additional information when the child processes of the current process stop because the child process received a SIGTTIN, SIGTTOU, SIGSTOP, or SIGTSTOP signal. |
If the waitpid function returns because the status of a child process is available, the process ID of the child process is returned. Information is stored in the location pointed to by status_location, if this pointer is not null. The value stored in the location pointed to by status_location is 0 only if the status is returned from a terminated child process that did one of the following:
Regardless of the value of status_location, you can define this information using the macros defined in the <wait.h> header file, which evaluate to integral expressions. In the following function descriptions, status_value is equal to the integer value pointed to by status_location:
| WIFEXITED(status_value) | Evaluates to a nonzero value if status was returned for a child process that terminated normally. |
| WEXITSTATUS(status_value) | If the value of WIFEXITED(status_value) is nonzero, this macro evaluates to the low-order 8 bits of the status argument that the child process passed to the _exit or exit function, or to the value the child process returned from the main function. |
| WIFSIGNALED(status_value) | Evaluates to nonzero value if status returned for a child process that terminated due to the receipt of a signal not caught. |
| WTERMSIG(status_value) | If the value of WIFSIGNALED(status_value) is nonzero, this macro evaluates to the number of the signal that caused the termination of the child process. |
| WIFSTOPPED(status_value) | Evaluates to a nonzero value if status was returned for a child process that is currently stopped. |
| WSTOPSIG(status_ value) | If the value of WIFSTOPPED(status_ value) is nonzero, this macro evaluates to the number of the signal that caused the child process to stop. |
| WIFCONTINUED(status_value) | Evaluates to a nonzero value if status returned for a child process that continued. |
If the information stored at the location pointed to by status_ location is stored there by a call to waitpid that specified the WUNTRACED flag, one of the following macros evaluates to a nonzero value:
If the information stored in the buffer pointed to by status_ location resulted from a call to waitpid without the WUNTRACED flag specified, one of the following macros evaluates to a nonzero value:
If a parent process terminates without waiting for all of its child processes to terminate, the remaining child processes is assigned a parent process ID equal to the process ID of the init process.
See also exit, _exit, and wait in this section.
| 0 | Indicates success. If the WNOHANG option was specified, and there are no stopped or exited child processes, the waitpid function also returns a value of 0. |
| -1 | Indicates an error;
errno is set to one of the following values:
|