1 @node Processes, Inter-Process Communication, Program Basics, Top
2 @c %MENU% How to create processes and run other programs
6 @dfn{Processes} are the primitive units for allocation of system
7 resources. Each process has its own address space and (usually) one
8 thread of control. A process executes a program; you can have multiple
9 processes executing the same program, but each process has its own copy
10 of the program within its own address space and executes it
11 independently of the other copies.
14 @cindex parent process
15 Processes are organized hierarchically. Each process has a @dfn{parent
16 process} which explicitly arranged to create it. The processes created
17 by a given parent are called its @dfn{child processes}. A child
18 inherits many of its attributes from the parent process.
20 This chapter describes how a program can create, terminate, and control
21 child processes. Actually, there are three distinct operations
22 involved: creating a new child process, causing the new process to
23 execute a program, and coordinating the completion of the child process
24 with the original program.
26 The @code{system} function provides a simple, portable mechanism for
27 running another program; it does all three steps automatically. If you
28 need more control over the details of how this is done, you can use the
29 primitive functions to do each step individually instead.
32 * Running a Command:: The easy way to run another program.
33 * Process Creation Concepts:: An overview of the hard way to do it.
34 * Process Identification:: How to get the process ID of a process.
35 * Creating a Process:: How to fork a child process.
36 * Executing a File:: How to make a process execute another program.
37 * Process Completion:: How to tell when a child process has completed.
38 * Process Completion Status:: How to interpret the status value
39 returned from a child process.
40 * BSD Wait Functions:: More functions, for backward compatibility.
41 * Process Creation Example:: A complete example program.
45 @node Running a Command
46 @section Running a Command
47 @cindex running a command
49 The easy way to run another program is to use the @code{system}
50 function. This function does all the work of running a subprogram, but
51 it doesn't give you much control over the details: you have to wait
52 until the subprogram terminates before you can do anything else.
54 @deftypefun int system (const char *@var{command})
55 @standards{ISO, stdlib.h}
57 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{} @ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}}
58 @c system @ascuplugin @ascuheap @asulock @aculock @acsmem
59 @c do_system @ascuplugin @ascuheap @asulock @aculock @acsmem
61 @c libc_lock_lock @asulock @aculock
65 @c libc_lock_unlock @aculock
68 @c CLEANUP_HANDLER @ascuplugin @ascuheap @acsmem
69 @c libc_cleanup_region_start @ascuplugin @ascuheap @acsmem
70 @c pthread_cleanup_push_defer @ascuplugin @ascuheap @acsmem
71 @c __pthread_testcancel @ascuplugin @ascuheap @acsmem
72 @c CANCEL_ENABLED_AND_CANCELED ok
73 @c do_cancel @ascuplugin @ascuheap @acsmem
79 @c libc_lock_unlock ok
84 @c libc_cleanup_region_end ok
85 @c pthread_cleanup_pop_restore ok
87 @c LIBC_CANCEL_ASYNC @ascuplugin @ascuheap @acsmem
88 @c libc_enable_asynccancel @ascuplugin @ascuheap @acsmem
89 @c do_cancel dup @ascuplugin @ascuheap @acsmem
90 @c LIBC_CANCEL_RESET ok
91 @c libc_disable_asynccancel ok
92 @c lll_futex_wait dup ok
93 This function executes @var{command} as a shell command. In @theglibc{},
94 it always uses the default shell @code{sh} to run the command.
95 In particular, it searches the directories in @code{PATH} to find
96 programs to execute. The return value is @code{-1} if it wasn't
97 possible to create the shell process, and otherwise is the status of the
98 shell process. @xref{Process Completion}, for details on how this
99 status code can be interpreted.
101 If the @var{command} argument is a null pointer, a return value of zero
102 indicates that no command processor is available.
104 This function is a cancellation point in multi-threaded programs. This
105 is a problem if the thread allocates some resources (like memory, file
106 descriptors, semaphores or whatever) at the time @code{system} is
107 called. If the thread gets canceled these resources stay allocated
108 until the program ends. To avoid this calls to @code{system} should be
109 protected using cancellation handlers.
110 @c ref pthread_cleanup_push / pthread_cleanup_pop
113 The @code{system} function is declared in the header file
117 @strong{Portability Note:} Some C implementations may not have any
118 notion of a command processor that can execute other programs. You can
119 determine whether a command processor exists by executing
120 @w{@code{system (NULL)}}; if the return value is nonzero, a command
121 processor is available.
123 The @code{popen} and @code{pclose} functions (@pxref{Pipe to a
124 Subprocess}) are closely related to the @code{system} function. They
125 allow the parent process to communicate with the standard input and
126 output channels of the command being executed.
128 @node Process Creation Concepts
129 @section Process Creation Concepts
131 This section gives an overview of processes and of the steps involved in
132 creating a process and making it run another program.
134 @cindex creating a process
135 @cindex forking a process
136 @cindex child process
137 @cindex parent process
139 A new processes is created when one of the functions
140 @code{posix_spawn}, @code{fork}, @code{_Fork} or @code{vfork} is called.
141 (The @code{system} and @code{popen} also create new processes internally.)
142 Due to the name of the @code{fork} function, the act of creating a new
143 process is sometimes called @dfn{forking} a process. Each new process
144 (the @dfn{child process} or @dfn{subprocess}) is allocated a process
145 ID, distinct from the process ID of the parent process. @xref{Process
148 After forking a child process, both the parent and child processes
149 continue to execute normally. If you want your program to wait for a
150 child process to finish executing before continuing, you must do this
151 explicitly after the fork operation, by calling @code{wait} or
152 @code{waitpid} (@pxref{Process Completion}). These functions give you
153 limited information about why the child terminated---for example, its
156 A newly forked child process continues to execute the same program as
157 its parent process, at the point where the @code{fork} or @code{_Fork}
158 call returns. You can use the return value from @code{fork} or
159 @code{_Fork} to tell whether the program is running in the parent process
162 @cindex process image
163 Having several processes run the same program is only occasionally
164 useful. But the child can execute another program using one of the
165 @code{exec} functions; see @ref{Executing a File}. The program that the
166 process is executing is called its @dfn{process image}. Starting
167 execution of a new program causes the process to forget all about its
168 previous process image; when the new program exits, the process exits
169 too, instead of returning to the previous process image.
171 @node Process Identification
172 @section Process Identification
175 Each process is named by a @dfn{process ID} number, a value of type
176 @code{pid_t}. A process ID is allocated to each process when it is
177 created. Process IDs are reused over time. The lifetime of a process
178 ends when the parent process of the corresponding process waits on the
179 process ID after the process has terminated. @xref{Process
180 Completion}. (The parent process can arrange for such waiting to
181 happen implicitly.) A process ID uniquely identifies a process only
182 during the lifetime of the process. As a rule of thumb, this means
183 that the process must still be running.
185 Process IDs can also denote process groups and sessions.
191 On Linux, threads created by @code{pthread_create} also receive a
192 @dfn{thread ID}. The thread ID of the initial (main) thread is the
193 same as the process ID of the entire process. Thread IDs for
194 subsequently created threads are distinct. They are allocated from
195 the same numbering space as process IDs. Process IDs and thread IDs
196 are sometimes also referred to collectively as @dfn{task IDs}. In
197 contrast to processes, threads are never waited for explicitly, so a
198 thread ID becomes eligible for reuse as soon as a thread exits or is
199 canceled. This is true even for joinable threads, not just detached
200 threads. Threads are assigned to a @dfn{thread group}. In
201 @theglibc{} implementation running on Linux, the process ID is the
202 thread group ID of all threads in the process.
204 You can get the process ID of a process by calling @code{getpid}. The
205 function @code{getppid} returns the process ID of the parent of the
206 current process (this is also known as the @dfn{parent process ID}).
207 Your program should include the header files @file{unistd.h} and
208 @file{sys/types.h} to use these functions.
212 @deftp {Data Type} pid_t
213 @standards{POSIX.1, sys/types.h}
214 The @code{pid_t} data type is a signed integer type which is capable
215 of representing a process ID. In @theglibc{}, this is an @code{int}.
218 @deftypefun pid_t getpid (void)
219 @standards{POSIX.1, unistd.h}
220 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
221 The @code{getpid} function returns the process ID of the current process.
224 @deftypefun pid_t getppid (void)
225 @standards{POSIX.1, unistd.h}
226 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
227 The @code{getppid} function returns the process ID of the parent of the
231 @deftypefun pid_t gettid (void)
232 @standards{Linux, unistd.h}
233 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
234 The @code{gettid} function returns the thread ID of the current
235 thread. The returned value is obtained from the Linux kernel and is
236 not subject to caching. See the discussion of thread IDs above,
237 especially regarding reuse of the IDs of threads which have exited.
239 This function is specific to Linux.
242 @node Creating a Process
243 @section Creating a Process
245 The @code{fork} function is the primitive for creating a process.
246 It is declared in the header file @file{unistd.h}.
249 @deftypefun pid_t fork (void)
250 @standards{POSIX.1, unistd.h}
251 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
252 @c The posix/fork.c implementation iterates over the fork_handlers
253 @c using a lock. It then takes the IO_list lock, resets the thread-local
254 @c pid, and runs fork. The parent releases the lock, and runs parent
255 @c handlers, and unlocks the internal lock. The child bumps the fork
256 @c generation, sets the thread-local pid, resets cpu clocks, initializes
257 @c the robust mutex list, the stream locks, the IO_list lock, the dynamic
258 @c loader lock, runs the child handlers, reseting ref counters to 1, and
259 @c initializes the fork lock. These are all safe, unless atfork
260 @c handlers themselves are unsafe.
261 The @code{fork} function creates a new process.
263 If the operation is successful, there are then both parent and child
264 processes and both see @code{fork} return, but with different values: it
265 returns a value of @code{0} in the child process and returns the child's
266 process ID in the parent process.
268 If process creation failed, @code{fork} returns a value of @code{-1} in
269 the parent process. The following @code{errno} error conditions are
270 defined for @code{fork}:
274 There aren't enough system resources to create another process, or the
275 user already has too many processes running. This means exceeding the
276 @code{RLIMIT_NPROC} resource limit, which can usually be increased;
277 @pxref{Limits on Resources}.
280 The process requires more space than the system can supply.
284 The specific attributes of the child process that differ from the
289 The child process has its own unique process ID.
292 The parent process ID of the child process is the process ID of its
296 The child process gets its own copies of the parent process's open file
297 descriptors. Subsequently changing attributes of the file descriptors
298 in the parent process won't affect the file descriptors in the child,
299 and vice versa. @xref{Control Operations}. However, the file position
300 associated with each descriptor is shared by both processes;
301 @pxref{File Position}.
304 The elapsed processor times for the child process are set to zero;
305 see @ref{Processor Time}.
308 The child doesn't inherit file locks set by the parent process.
309 @c !!! flock locks shared
310 @xref{Control Operations}.
313 The child doesn't inherit alarms set by the parent process.
314 @xref{Setting an Alarm}.
317 The set of pending signals (@pxref{Delivery of Signal}) for the child
318 process is cleared. (The child process inherits its mask of blocked
319 signals and signal actions from the parent process.)
322 @deftypefun pid_t _Fork (void)
323 @standards{GNU, unistd.h}
324 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
325 The @code{_Fork} function is similar to @code{fork}, but it does not invoke
326 any callbacks registered with @code{pthread_atfork}, nor does it reset
327 any internal state or locks (such as the @code{malloc} locks). In the
328 new subprocess, only async-signal-safe functions may be called, such as
329 @code{dup2} or @code{execve}.
331 The @code{_Fork} function is an async-signal-safe replacement of @code{fork}.
332 It is a GNU extension.
336 @deftypefun pid_t vfork (void)
337 @standards{BSD, unistd.h}
338 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
339 @c The vfork implementation proper is a safe syscall, but it may fall
340 @c back to fork if the vfork syscall is not available.
341 The @code{vfork} function is similar to @code{fork} but on some systems
342 it is more efficient; however, there are restrictions you must follow to
345 While @code{fork} makes a complete copy of the calling process's address
346 space and allows both the parent and child to execute independently,
347 @code{vfork} does not make this copy. Instead, the child process
348 created with @code{vfork} shares its parent's address space until it
349 calls @code{_exit} or one of the @code{exec} functions. In the
350 meantime, the parent process suspends execution.
352 You must be very careful not to allow the child process created with
353 @code{vfork} to modify any global data or even local variables shared
354 with the parent. Furthermore, the child process cannot return from (or
355 do a long jump out of) the function that called @code{vfork}! This
356 would leave the parent process's control information very confused. If
357 in doubt, use @code{fork} instead.
359 Some operating systems don't really implement @code{vfork}. @Theglibc{}
360 permits you to use @code{vfork} on all systems, but actually
361 executes @code{fork} if @code{vfork} isn't available. If you follow
362 the proper precautions for using @code{vfork}, your program will still
363 work even if the system uses @code{fork} instead.
366 @node Executing a File
367 @section Executing a File
368 @cindex executing a file
369 @cindex @code{exec} functions
371 This section describes the @code{exec} family of functions, for executing
372 a file as a process image. You can use these functions to make a child
373 process execute a new program after it has been forked.
375 To see the effects of @code{exec} from the point of view of the called
376 program, see @ref{Program Basics}.
379 The functions in this family differ in how you specify the arguments,
380 but otherwise they all do the same thing. They are declared in the
381 header file @file{unistd.h}.
383 @deftypefun int execv (const char *@var{filename}, char *const @var{argv}@t{[]})
384 @standards{POSIX.1, unistd.h}
385 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
386 The @code{execv} function executes the file named by @var{filename} as a
389 The @var{argv} argument is an array of null-terminated strings that is
390 used to provide a value for the @code{argv} argument to the @code{main}
391 function of the program to be executed. The last element of this array
392 must be a null pointer. By convention, the first element of this array
393 is the file name of the program sans directory names. @xref{Program
394 Arguments}, for full details on how programs can access these arguments.
396 The environment for the new process image is taken from the
397 @code{environ} variable of the current process image; see
398 @ref{Environment Variables}, for information about environments.
401 @deftypefun int execl (const char *@var{filename}, const char *@var{arg0}, @dots{})
402 @standards{POSIX.1, unistd.h}
403 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
404 This is similar to @code{execv}, but the @var{argv} strings are
405 specified individually instead of as an array. A null pointer must be
406 passed as the last such argument.
409 @deftypefun int execve (const char *@var{filename}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]})
410 @standards{POSIX.1, unistd.h}
411 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
412 This is similar to @code{execv}, but permits you to specify the environment
413 for the new program explicitly as the @var{env} argument. This should
414 be an array of strings in the same format as for the @code{environ}
415 variable; see @ref{Environment Access}.
418 @deftypefun int fexecve (int @var{fd}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]})
419 @standards{POSIX.1, unistd.h}
420 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
421 This is similar to @code{execve}, but instead of identifying the program
422 executable by its pathname, the file descriptor @var{fd} is used. The
423 descriptor must have been opened with the @code{O_RDONLY} flag or (on
424 Linux) the @code{O_PATH} flag.
426 On Linux, @code{fexecve} can fail with an error of @code{ENOSYS} if
427 @file{/proc} has not been mounted and the kernel lacks support for the
428 underlying @code{execveat} system call.
431 @deftypefun int execle (const char *@var{filename}, const char *@var{arg0}, @dots{}, char *const @var{env}@t{[]})
432 @standards{POSIX.1, unistd.h}
433 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
434 This is similar to @code{execl}, but permits you to specify the
435 environment for the new program explicitly. The environment argument is
436 passed following the null pointer that marks the last @var{argv}
437 argument, and should be an array of strings in the same format as for
438 the @code{environ} variable.
441 @deftypefun int execvp (const char *@var{filename}, char *const @var{argv}@t{[]})
442 @standards{POSIX.1, unistd.h}
443 @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
444 The @code{execvp} function is similar to @code{execv}, except that it
445 searches the directories listed in the @code{PATH} environment variable
446 (@pxref{Standard Environment}) to find the full file name of a
447 file from @var{filename} if @var{filename} does not contain a slash.
449 This function is useful for executing system utility programs, because
450 it looks for them in the places that the user has chosen. Shells use it
451 to run the commands that users type.
454 @deftypefun int execlp (const char *@var{filename}, const char *@var{arg0}, @dots{})
455 @standards{POSIX.1, unistd.h}
456 @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
457 This function is like @code{execl}, except that it performs the same
458 file name searching as the @code{execvp} function.
461 The size of the argument list and environment list taken together must
462 not be greater than @code{ARG_MAX} bytes. @xref{General Limits}. On
463 @gnuhurdsystems{}, the size (which compares against @code{ARG_MAX})
464 includes, for each string, the number of characters in the string, plus
465 the size of a @code{char *}, plus one, rounded up to a multiple of the
466 size of a @code{char *}. Other systems may have somewhat different
469 These functions normally don't return, since execution of a new program
470 causes the currently executing program to go away completely. A value
471 of @code{-1} is returned in the event of a failure. In addition to the
472 usual file name errors (@pxref{File Name Errors}), the following
473 @code{errno} error conditions are defined for these functions:
477 The combined size of the new program's argument list and environment
478 list is larger than @code{ARG_MAX} bytes. @gnuhurdsystems{} have no
479 specific limit on the argument list size, so this error code cannot
480 result, but you may get @code{ENOMEM} instead if the arguments are too
481 big for available memory.
484 The specified file can't be executed because it isn't in the right format.
487 Executing the specified file requires more storage than is available.
490 If execution of the new file succeeds, it updates the access time field
491 of the file as if the file had been read. @xref{File Times}, for more
492 details about access times of files.
494 The point at which the file is closed again is not specified, but
495 is at some point before the process exits or before another process
498 Executing a new process image completely changes the contents of memory,
499 copying only the argument and environment strings to new locations. But
500 many other attributes of the process are unchanged:
504 The process ID and the parent process ID. @xref{Process Creation Concepts}.
507 Session and process group membership. @xref{Concepts of Job Control}.
510 Real user ID and group ID, and supplementary group IDs. @xref{Process
514 Pending alarms. @xref{Setting an Alarm}.
517 Current working directory and root directory. @xref{Working
518 Directory}. On @gnuhurdsystems{}, the root directory is not copied when
519 executing a setuid program; instead the system default root directory
520 is used for the new program.
523 File mode creation mask. @xref{Setting Permissions}.
526 Process signal mask; see @ref{Process Signal Mask}.
529 Pending signals; see @ref{Blocking Signals}.
532 Elapsed processor time associated with the process; see @ref{Processor Time}.
535 If the set-user-ID and set-group-ID mode bits of the process image file
536 are set, this affects the effective user ID and effective group ID
537 (respectively) of the process. These concepts are discussed in detail
538 in @ref{Process Persona}.
540 Signals that are set to be ignored in the existing process image are
541 also set to be ignored in the new process image. All other signals are
542 set to the default action in the new process image. For more
543 information about signals, see @ref{Signal Handling}.
545 File descriptors open in the existing process image remain open in the
546 new process image, unless they have the @code{FD_CLOEXEC}
547 (close-on-exec) flag set. The files that remain open inherit all
548 attributes of the open file descriptors from the existing process image,
549 including file locks. File descriptors are discussed in @ref{Low-Level I/O}.
551 Streams, by contrast, cannot survive through @code{exec} functions,
552 because they are located in the memory of the process itself. The new
553 process image has no streams except those it creates afresh. Each of
554 the streams in the pre-@code{exec} process image has a descriptor inside
555 it, and these descriptors do survive through @code{exec} (provided that
556 they do not have @code{FD_CLOEXEC} set). The new process image can
557 reconnect these to new streams using @code{fdopen} (@pxref{Descriptors
560 @node Process Completion
561 @section Process Completion
562 @cindex process completion
563 @cindex waiting for completion of child process
564 @cindex testing exit status of child process
566 The functions described in this section are used to wait for a child
567 process to terminate or stop, and determine its status. These functions
568 are declared in the header file @file{sys/wait.h}.
571 @deftypefun pid_t waitpid (pid_t @var{pid}, int *@var{status-ptr}, int @var{options})
572 @standards{POSIX.1, sys/wait.h}
573 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
574 The @code{waitpid} function is used to request status information from a
575 child process whose process ID is @var{pid}. Normally, the calling
576 process is suspended until the child process makes status information
577 available by terminating.
579 Other values for the @var{pid} argument have special interpretations. A
580 value of @code{-1} or @code{WAIT_ANY} requests status information for
581 any child process; a value of @code{0} or @code{WAIT_MYPGRP} requests
582 information for any child process in the same process group as the
583 calling process; and any other negative value @minus{} @var{pgid}
584 requests information for any child process whose process group ID is
587 If status information for a child process is available immediately, this
588 function returns immediately without waiting. If more than one eligible
589 child process has status information available, one of them is chosen
590 randomly, and its status is returned immediately. To get the status
591 from the other eligible child processes, you need to call @code{waitpid}
594 The @var{options} argument is a bit mask. Its value should be the
595 bitwise OR (that is, the @samp{|} operator) of zero or more of the
596 @code{WNOHANG} and @code{WUNTRACED} flags. You can use the
597 @code{WNOHANG} flag to indicate that the parent process shouldn't wait;
598 and the @code{WUNTRACED} flag to request status information from stopped
599 processes as well as processes that have terminated.
601 The status information from the child process is stored in the object
602 that @var{status-ptr} points to, unless @var{status-ptr} is a null pointer.
604 This function is a cancellation point in multi-threaded programs. This
605 is a problem if the thread allocates some resources (like memory, file
606 descriptors, semaphores or whatever) at the time @code{waitpid} is
607 called. If the thread gets canceled these resources stay allocated
608 until the program ends. To avoid this calls to @code{waitpid} should be
609 protected using cancellation handlers.
610 @c ref pthread_cleanup_push / pthread_cleanup_pop
612 The return value is normally the process ID of the child process whose
613 status is reported. If there are child processes but none of them is
614 waiting to be noticed, @code{waitpid} will block until one is. However,
615 if the @code{WNOHANG} option was specified, @code{waitpid} will return
616 zero instead of blocking.
618 If a specific PID to wait for was given to @code{waitpid}, it will
619 ignore all other children (if any). Therefore if there are children
620 waiting to be noticed but the child whose PID was specified is not one
621 of them, @code{waitpid} will block or return zero as described above.
623 A value of @code{-1} is returned in case of error. The following
624 @code{errno} error conditions are defined for this function:
628 The function was interrupted by delivery of a signal to the calling
629 process. @xref{Interrupted Primitives}.
632 There are no child processes to wait for, or the specified @var{pid}
633 is not a child of the calling process.
636 An invalid value was provided for the @var{options} argument.
640 These symbolic constants are defined as values for the @var{pid} argument
641 to the @code{waitpid} function.
643 @comment Extra blank lines make it look better.
647 This constant macro (whose value is @code{-1}) specifies that
648 @code{waitpid} should return status information about any child process.
652 This constant (with value @code{0}) specifies that @code{waitpid} should
653 return status information about any child process in the same process
654 group as the calling process.
657 These symbolic constants are defined as flags for the @var{options}
658 argument to the @code{waitpid} function. You can bitwise-OR the flags
659 together to obtain a value to use as the argument.
664 This flag specifies that @code{waitpid} should return immediately
665 instead of waiting, if there is no child process ready to be noticed.
669 This flag specifies that @code{waitpid} should report the status of any
670 child processes that have been stopped as well as those that have
674 @deftypefun pid_t wait (int *@var{status-ptr})
675 @standards{POSIX.1, sys/wait.h}
676 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
677 This is a simplified version of @code{waitpid}, and is used to wait
678 until any one child process terminates. The call:
685 is exactly equivalent to:
688 waitpid (-1, &status, 0)
691 This function is a cancellation point in multi-threaded programs. This
692 is a problem if the thread allocates some resources (like memory, file
693 descriptors, semaphores or whatever) at the time @code{wait} is
694 called. If the thread gets canceled these resources stay allocated
695 until the program ends. To avoid this calls to @code{wait} should be
696 protected using cancellation handlers.
697 @c ref pthread_cleanup_push / pthread_cleanup_pop
700 @deftypefun pid_t wait4 (pid_t @var{pid}, int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
701 @standards{BSD, sys/wait.h}
702 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
703 If @var{usage} is a null pointer, @code{wait4} is equivalent to
704 @code{waitpid (@var{pid}, @var{status-ptr}, @var{options})}.
706 If @var{usage} is not null, @code{wait4} stores usage figures for the
707 child process in @code{*@var{rusage}} (but only if the child has
708 terminated, not if it has stopped). @xref{Resource Usage}.
710 This function is a BSD extension.
713 Here's an example of how to use @code{waitpid} to get the status from
714 all child processes that have terminated, without ever waiting. This
715 function is designed to be a handler for @code{SIGCHLD}, the signal that
716 indicates that at least one child process has terminated.
721 sigchld_handler (int signum)
723 int pid, status, serrno;
727 pid = waitpid (WAIT_ANY, &status, WNOHANG);
735 notice_termination (pid, status);
742 @node Process Completion Status
743 @section Process Completion Status
745 If the exit status value (@pxref{Program Termination}) of the child
746 process is zero, then the status value reported by @code{waitpid} or
747 @code{wait} is also zero. You can test for other kinds of information
748 encoded in the returned status value using the following macros.
749 These macros are defined in the header file @file{sys/wait.h}.
752 @deftypefn Macro int WIFEXITED (int @var{status})
753 @standards{POSIX.1, sys/wait.h}
754 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
755 This macro returns a nonzero value if the child process terminated
756 normally with @code{exit} or @code{_exit}.
759 @deftypefn Macro int WEXITSTATUS (int @var{status})
760 @standards{POSIX.1, sys/wait.h}
761 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
762 If @code{WIFEXITED} is true of @var{status}, this macro returns the
763 low-order 8 bits of the exit status value from the child process.
767 @deftypefn Macro int WIFSIGNALED (int @var{status})
768 @standards{POSIX.1, sys/wait.h}
769 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
770 This macro returns a nonzero value if the child process terminated
771 because it received a signal that was not handled.
772 @xref{Signal Handling}.
775 @deftypefn Macro int WTERMSIG (int @var{status})
776 @standards{POSIX.1, sys/wait.h}
777 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
778 If @code{WIFSIGNALED} is true of @var{status}, this macro returns the
779 signal number of the signal that terminated the child process.
782 @deftypefn Macro int WCOREDUMP (int @var{status})
783 @standards{BSD, sys/wait.h}
784 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
785 This macro returns a nonzero value if the child process terminated
786 and produced a core dump.
789 @deftypefn Macro int WIFSTOPPED (int @var{status})
790 @standards{POSIX.1, sys/wait.h}
791 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
792 This macro returns a nonzero value if the child process is stopped.
795 @deftypefn Macro int WSTOPSIG (int @var{status})
796 @standards{POSIX.1, sys/wait.h}
797 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
798 If @code{WIFSTOPPED} is true of @var{status}, this macro returns the
799 signal number of the signal that caused the child process to stop.
803 @node BSD Wait Functions
804 @section BSD Process Wait Function
806 @Theglibc{} also provides the @code{wait3} function for compatibility
807 with BSD. This function is declared in @file{sys/wait.h}. It is the
808 predecessor to @code{wait4}, which is more flexible. @code{wait3} is
812 @deftypefun pid_t wait3 (int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
813 @standards{BSD, sys/wait.h}
814 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
815 If @var{usage} is a null pointer, @code{wait3} is equivalent to
816 @code{waitpid (-1, @var{status-ptr}, @var{options})}.
818 If @var{usage} is not null, @code{wait3} stores usage figures for the
819 child process in @code{*@var{rusage}} (but only if the child has
820 terminated, not if it has stopped). @xref{Resource Usage}.
823 @node Process Creation Example
824 @section Process Creation Example
826 Here is an example program showing how you might write a function
827 similar to the built-in @code{system}. It executes its @var{command}
828 argument using the equivalent of @samp{sh -c @var{command}}.
834 #include <sys/types.h>
835 #include <sys/wait.h>
837 /* @r{Execute the command using this shell program.} */
838 #define SHELL "/bin/sh"
842 my_system (const char *command)
851 /* @r{This is the child process. Execute the shell command.} */
852 execl (SHELL, SHELL, "-c", command, NULL);
853 _exit (EXIT_FAILURE);
856 /* @r{The fork failed. Report failure.} */
859 /* @r{This is the parent process. Wait for the child to complete.} */
860 if (waitpid (pid, &status, 0) != pid)
866 @comment Yes, this example has been tested.
868 There are a couple of things you should pay attention to in this
871 Remember that the first @code{argv} argument supplied to the program
872 represents the name of the program being executed. That is why, in the
873 call to @code{execl}, @code{SHELL} is supplied once to name the program
874 to execute and a second time to supply a value for @code{argv[0]}.
876 The @code{execl} call in the child process doesn't return if it is
877 successful. If it fails, you must do something to make the child
878 process terminate. Just returning a bad status code with @code{return}
879 would leave two processes running the original program. Instead, the
880 right behavior is for the child process to report failure to its parent
883 Call @code{_exit} to accomplish this. The reason for using @code{_exit}
884 instead of @code{exit} is to avoid flushing fully buffered streams such
885 as @code{stdout}. The buffers of these streams probably contain data
886 that was copied from the parent process by the @code{fork}, data that
887 will be output eventually by the parent process. Calling @code{exit} in
888 the child would output the data twice. @xref{Termination Internals}.