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等待子进程结束

2010-05-06 13:25 197 查看

Name

wait3, wait4 — wait for process to change state, BSD style

Synopsis

#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
#include <sys/wait.h>

pid_t wait3(	int *status,
	int options,
	struct rusage *rusage ) ;

pid_t wait4(	pid_t pid,
	int *status,
	int options,
	struct rusage *rusage ) ;

DESCRIPTION

The

wait3

() and

wait4

() system calls are similar to waitpid(2), but additionally return resource usage information about the child in the structure pointed to by

rusage

.

Other than the use of the

rusage

argument, the following

wait3

() call:

wait3(status, options, rusage);

is equivalent to:

waitpid(−1, status, options);

Similarly, the following

wait4

() call:

wait4(pid, status, options, rusage);

is equivalent to:

waitpid(pid, status, options);

In other words,

wait3

() waits of any child, while

wait4

() can be used to select a specific child, or children, on which to wait. See wait(2) for further details.

If

rusage

is not NULL, the struct rusage to which it points will be filled with accounting information about the child. See getrusage(2) for details.

The wait3 function suspends execution of the current process until a child has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.

The wait4 function suspends execution of the current process until a child as specified by the pid argument has exited, or until a signal is delivered whose action is to terminate the current process or to call a signal handling function. If a child as requested by pid has already exited by the time of the call (a so-called "zombie" process), the function returns immediately. Any system resources used by the child are freed.

The value of pid can be one of:

< -1 which means to wait for any child process whose process group ID is equal to the absolute value of pid. -1 which means to wait for any child process; this is equivalent to calling wait3. 0 which means to wait for any child process whose process group ID is equal to that of the calling process. > 0 which means to wait for the child whose process ID is equal to the value of pid.
The value of options is a bitwise OR of zero or more of the following constants:

WNOHANG which means to return immediately if no child is there to be waited for. WUNTRACED which means to also return for children which are stopped, and whose status has not been reported.
If status is not NULL, wait3 or wait4 store status information in the location pointed to by status.

This status can be evaluated with the following macros (these macros take the stat buffer (an int) as an argument --- not a pointer to the buffer!):

WIFEXITED(status) is non-zero if the child exited normally. WEXITSTATUS(status) evaluates to the least significant eight bits of the return code of the child which terminated, which may have been set as the argument to a call to exit() or as the argument for a return statement in the main program. This macro can only be evaluated if WIFEXITED returned non-zero. WIFSIGNALED(status) returns true if the child process exited because of a signal which was not caught. WTERMSIG(status) returns the number of the signal that caused the child process to terminate. This macro can only be evaluated if WIFSIGNALED returned non-zero. WIFSTOPPED(status) returns true if the child process which caused the return is currently stopped; this is only possible if the call was done using WUNTRACED. WSTOPSIG(status) returns the number of the signal which caused the child to stop. This macro can only be evaluated if WIFSTOPPED returned non-zero.
If rusage is not NULL, the struct rusage as defined in <sys/resource.h> it points to will be filled with accounting information. See getrusage(2) for details.

RETURN VALUE

The process ID of the child which exited, -1 on error (in particular, when no unwaited-for child processes of the specified kind exist) or zero if WNOHANG was used and no child was available yet. In the latter two cases errno will be set appropriately.

ERRORS

ECHILD No unwaited-for child process as specified does exist. EINTR if WNOHANG was not set and an unblocked signal or a SIGCHLD was caught.