Waits for a child process to stop or terminate.
#include <wait.h>
pid_t wait4 (pid_t process_id, union wait
*status_location, int options,
struct rusage *resource_usage);
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 wait4 function only returns the status of a child process from this set.
The options argument to the wait4 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:
| 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 suspension of the calling process. If there are child processes that stopped or terminated, one is chosen and the waitpid function returns its process ID, 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 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 wait4 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 status_location value, you can define this information using the macros defined in the <wait.h> header file, which evaluate to integral expressions. In the following macro 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 was returned for a child process that terminated due to the receipt of a signal that was 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 was returned for a child process that has continued. |
If the information stored at the location pointed to by status_ location was stored there by a call to wait4 that specified the WUNTRACED flag, one of the following macros evaluates to a nonzero value:
If the information stored in the location pointed to by status_ location resulted from a call to wait4 without the WUNTRACED flag specified, one of the following macros evaluates to a nonzero value:
The wait4 function is similar to the wait3 function. However, the wait4 function waits for a specific child as indicated by the process_id argument. The resource_usage argument points to a location that contains resource usage information for the child processes as defined in the <resource.h> header file.
See also exit and _exit in this section.
| 0 | Indicates success. There are no stopped or exited child processes, the WNOHANG option is specified. |
| x | The process_id of the child process. Status of a child process is available. |
| -1 | Indicates
an error; errno is set to one of the following values:
|