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 CANCELLATION_P @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 CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS dup ok
90 @c do_cancel dup @ascuplugin @ascuheap @acsmem
91 @c LIBC_CANCEL_RESET ok
92 @c libc_disable_asynccancel ok
93 @c lll_futex_wait dup ok
94 This function executes @var{command} as a shell command. In @theglibc{},
95 it always uses the default shell @code{sh} to run the command.
96 In particular, it searches the directories in @code{PATH} to find
97 programs to execute. The return value is @code{-1} if it wasn't
98 possible to create the shell process, and otherwise is the status of the
99 shell process. @xref{Process Completion}, for details on how this
100 status code can be interpreted.
102 If the @var{command} argument is a null pointer, a return value of zero
103 indicates that no command processor is available.
105 This function is a cancellation point in multi-threaded programs. This
106 is a problem if the thread allocates some resources (like memory, file
107 descriptors, semaphores or whatever) at the time @code{system} is
108 called. If the thread gets canceled these resources stay allocated
109 until the program ends. To avoid this calls to @code{system} should be
110 protected using cancellation handlers.
111 @c ref pthread_cleanup_push / pthread_cleanup_pop
114 The @code{system} function is declared in the header file
118 @strong{Portability Note:} Some C implementations may not have any
119 notion of a command processor that can execute other programs. You can
120 determine whether a command processor exists by executing
121 @w{@code{system (NULL)}}; if the return value is nonzero, a command
122 processor is available.
124 The @code{popen} and @code{pclose} functions (@pxref{Pipe to a
125 Subprocess}) are closely related to the @code{system} function. They
126 allow the parent process to communicate with the standard input and
127 output channels of the command being executed.
129 @node Process Creation Concepts
130 @section Process Creation Concepts
132 This section gives an overview of processes and of the steps involved in
133 creating a process and making it run another program.
135 @cindex creating a process
136 @cindex forking a process
137 @cindex child process
138 @cindex parent process
140 A new processes is created when one of the functions
141 @code{posix_spawn}, @code{fork}, or @code{vfork} is called. (The
142 @code{system} and @code{popen} also create new processes internally.)
143 Due to the name of the @code{fork} function, the act of creating a new
144 process is sometimes called @dfn{forking} a process. Each new process
145 (the @dfn{child process} or @dfn{subprocess}) is allocated a process
146 ID, distinct from the process ID of the parent process. @xref{Process
149 After forking a child process, both the parent and child processes
150 continue to execute normally. If you want your program to wait for a
151 child process to finish executing before continuing, you must do this
152 explicitly after the fork operation, by calling @code{wait} or
153 @code{waitpid} (@pxref{Process Completion}). These functions give you
154 limited information about why the child terminated---for example, its
157 A newly forked child process continues to execute the same program as
158 its parent process, at the point where the @code{fork} call returns.
159 You can use the return value from @code{fork} to tell whether the program
160 is running in the parent process or the child.
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 @node Creating a Process
232 @section Creating a Process
234 The @code{fork} function is the primitive for creating a process.
235 It is declared in the header file @file{unistd.h}.
238 @deftypefun pid_t fork (void)
239 @standards{POSIX.1, unistd.h}
240 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
241 @c The nptl/.../linux implementation safely collects fork_handlers into
242 @c an alloca()ed linked list and increments ref counters; it uses atomic
243 @c ops and retries, avoiding locking altogether. It then takes the
244 @c IO_list lock, resets the thread-local pid, and runs fork. The parent
245 @c restores the thread-local pid, releases the lock, and runs parent
246 @c handlers, decrementing the ref count and signaling futex wait if
247 @c requested by unregister_atfork. The child bumps the fork generation,
248 @c sets the thread-local pid, resets cpu clocks, initializes the robust
249 @c mutex list, the stream locks, the IO_list lock, the dynamic loader
250 @c lock, runs the child handlers, reseting ref counters to 1, and
251 @c initializes the fork lock. These are all safe, unless atfork
252 @c handlers themselves are unsafe.
253 The @code{fork} function creates a new process.
255 If the operation is successful, there are then both parent and child
256 processes and both see @code{fork} return, but with different values: it
257 returns a value of @code{0} in the child process and returns the child's
258 process ID in the parent process.
260 If process creation failed, @code{fork} returns a value of @code{-1} in
261 the parent process. The following @code{errno} error conditions are
262 defined for @code{fork}:
266 There aren't enough system resources to create another process, or the
267 user already has too many processes running. This means exceeding the
268 @code{RLIMIT_NPROC} resource limit, which can usually be increased;
269 @pxref{Limits on Resources}.
272 The process requires more space than the system can supply.
276 The specific attributes of the child process that differ from the
281 The child process has its own unique process ID.
284 The parent process ID of the child process is the process ID of its
288 The child process gets its own copies of the parent process's open file
289 descriptors. Subsequently changing attributes of the file descriptors
290 in the parent process won't affect the file descriptors in the child,
291 and vice versa. @xref{Control Operations}. However, the file position
292 associated with each descriptor is shared by both processes;
293 @pxref{File Position}.
296 The elapsed processor times for the child process are set to zero;
297 see @ref{Processor Time}.
300 The child doesn't inherit file locks set by the parent process.
301 @c !!! flock locks shared
302 @xref{Control Operations}.
305 The child doesn't inherit alarms set by the parent process.
306 @xref{Setting an Alarm}.
309 The set of pending signals (@pxref{Delivery of Signal}) for the child
310 process is cleared. (The child process inherits its mask of blocked
311 signals and signal actions from the parent process.)
315 @deftypefun pid_t vfork (void)
316 @standards{BSD, unistd.h}
317 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
318 @c The vfork implementation proper is a safe syscall, but it may fall
319 @c back to fork if the vfork syscall is not available.
320 The @code{vfork} function is similar to @code{fork} but on some systems
321 it is more efficient; however, there are restrictions you must follow to
324 While @code{fork} makes a complete copy of the calling process's address
325 space and allows both the parent and child to execute independently,
326 @code{vfork} does not make this copy. Instead, the child process
327 created with @code{vfork} shares its parent's address space until it
328 calls @code{_exit} or one of the @code{exec} functions. In the
329 meantime, the parent process suspends execution.
331 You must be very careful not to allow the child process created with
332 @code{vfork} to modify any global data or even local variables shared
333 with the parent. Furthermore, the child process cannot return from (or
334 do a long jump out of) the function that called @code{vfork}! This
335 would leave the parent process's control information very confused. If
336 in doubt, use @code{fork} instead.
338 Some operating systems don't really implement @code{vfork}. @Theglibc{}
339 permits you to use @code{vfork} on all systems, but actually
340 executes @code{fork} if @code{vfork} isn't available. If you follow
341 the proper precautions for using @code{vfork}, your program will still
342 work even if the system uses @code{fork} instead.
345 @node Executing a File
346 @section Executing a File
347 @cindex executing a file
348 @cindex @code{exec} functions
350 This section describes the @code{exec} family of functions, for executing
351 a file as a process image. You can use these functions to make a child
352 process execute a new program after it has been forked.
354 To see the effects of @code{exec} from the point of view of the called
355 program, see @ref{Program Basics}.
358 The functions in this family differ in how you specify the arguments,
359 but otherwise they all do the same thing. They are declared in the
360 header file @file{unistd.h}.
362 @deftypefun int execv (const char *@var{filename}, char *const @var{argv}@t{[]})
363 @standards{POSIX.1, unistd.h}
364 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
365 The @code{execv} function executes the file named by @var{filename} as a
368 The @var{argv} argument is an array of null-terminated strings that is
369 used to provide a value for the @code{argv} argument to the @code{main}
370 function of the program to be executed. The last element of this array
371 must be a null pointer. By convention, the first element of this array
372 is the file name of the program sans directory names. @xref{Program
373 Arguments}, for full details on how programs can access these arguments.
375 The environment for the new process image is taken from the
376 @code{environ} variable of the current process image; see
377 @ref{Environment Variables}, for information about environments.
380 @deftypefun int execl (const char *@var{filename}, const char *@var{arg0}, @dots{})
381 @standards{POSIX.1, unistd.h}
382 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
383 This is similar to @code{execv}, but the @var{argv} strings are
384 specified individually instead of as an array. A null pointer must be
385 passed as the last such argument.
388 @deftypefun int execve (const char *@var{filename}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]})
389 @standards{POSIX.1, unistd.h}
390 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
391 This is similar to @code{execv}, but permits you to specify the environment
392 for the new program explicitly as the @var{env} argument. This should
393 be an array of strings in the same format as for the @code{environ}
394 variable; see @ref{Environment Access}.
397 @deftypefun int execle (const char *@var{filename}, const char *@var{arg0}, @dots{}, char *const @var{env}@t{[]})
398 @standards{POSIX.1, unistd.h}
399 @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
400 This is similar to @code{execl}, but permits you to specify the
401 environment for the new program explicitly. The environment argument is
402 passed following the null pointer that marks the last @var{argv}
403 argument, and should be an array of strings in the same format as for
404 the @code{environ} variable.
407 @deftypefun int execvp (const char *@var{filename}, char *const @var{argv}@t{[]})
408 @standards{POSIX.1, unistd.h}
409 @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
410 The @code{execvp} function is similar to @code{execv}, except that it
411 searches the directories listed in the @code{PATH} environment variable
412 (@pxref{Standard Environment}) to find the full file name of a
413 file from @var{filename} if @var{filename} does not contain a slash.
415 This function is useful for executing system utility programs, because
416 it looks for them in the places that the user has chosen. Shells use it
417 to run the commands that users type.
420 @deftypefun int execlp (const char *@var{filename}, const char *@var{arg0}, @dots{})
421 @standards{POSIX.1, unistd.h}
422 @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
423 This function is like @code{execl}, except that it performs the same
424 file name searching as the @code{execvp} function.
427 The size of the argument list and environment list taken together must
428 not be greater than @code{ARG_MAX} bytes. @xref{General Limits}. On
429 @gnuhurdsystems{}, the size (which compares against @code{ARG_MAX})
430 includes, for each string, the number of characters in the string, plus
431 the size of a @code{char *}, plus one, rounded up to a multiple of the
432 size of a @code{char *}. Other systems may have somewhat different
435 These functions normally don't return, since execution of a new program
436 causes the currently executing program to go away completely. A value
437 of @code{-1} is returned in the event of a failure. In addition to the
438 usual file name errors (@pxref{File Name Errors}), the following
439 @code{errno} error conditions are defined for these functions:
443 The combined size of the new program's argument list and environment
444 list is larger than @code{ARG_MAX} bytes. @gnuhurdsystems{} have no
445 specific limit on the argument list size, so this error code cannot
446 result, but you may get @code{ENOMEM} instead if the arguments are too
447 big for available memory.
450 The specified file can't be executed because it isn't in the right format.
453 Executing the specified file requires more storage than is available.
456 If execution of the new file succeeds, it updates the access time field
457 of the file as if the file had been read. @xref{File Times}, for more
458 details about access times of files.
460 The point at which the file is closed again is not specified, but
461 is at some point before the process exits or before another process
464 Executing a new process image completely changes the contents of memory,
465 copying only the argument and environment strings to new locations. But
466 many other attributes of the process are unchanged:
470 The process ID and the parent process ID. @xref{Process Creation Concepts}.
473 Session and process group membership. @xref{Concepts of Job Control}.
476 Real user ID and group ID, and supplementary group IDs. @xref{Process
480 Pending alarms. @xref{Setting an Alarm}.
483 Current working directory and root directory. @xref{Working
484 Directory}. On @gnuhurdsystems{}, the root directory is not copied when
485 executing a setuid program; instead the system default root directory
486 is used for the new program.
489 File mode creation mask. @xref{Setting Permissions}.
492 Process signal mask; see @ref{Process Signal Mask}.
495 Pending signals; see @ref{Blocking Signals}.
498 Elapsed processor time associated with the process; see @ref{Processor Time}.
501 If the set-user-ID and set-group-ID mode bits of the process image file
502 are set, this affects the effective user ID and effective group ID
503 (respectively) of the process. These concepts are discussed in detail
504 in @ref{Process Persona}.
506 Signals that are set to be ignored in the existing process image are
507 also set to be ignored in the new process image. All other signals are
508 set to the default action in the new process image. For more
509 information about signals, see @ref{Signal Handling}.
511 File descriptors open in the existing process image remain open in the
512 new process image, unless they have the @code{FD_CLOEXEC}
513 (close-on-exec) flag set. The files that remain open inherit all
514 attributes of the open file descriptors from the existing process image,
515 including file locks. File descriptors are discussed in @ref{Low-Level I/O}.
517 Streams, by contrast, cannot survive through @code{exec} functions,
518 because they are located in the memory of the process itself. The new
519 process image has no streams except those it creates afresh. Each of
520 the streams in the pre-@code{exec} process image has a descriptor inside
521 it, and these descriptors do survive through @code{exec} (provided that
522 they do not have @code{FD_CLOEXEC} set). The new process image can
523 reconnect these to new streams using @code{fdopen} (@pxref{Descriptors
526 @node Process Completion
527 @section Process Completion
528 @cindex process completion
529 @cindex waiting for completion of child process
530 @cindex testing exit status of child process
532 The functions described in this section are used to wait for a child
533 process to terminate or stop, and determine its status. These functions
534 are declared in the header file @file{sys/wait.h}.
537 @deftypefun pid_t waitpid (pid_t @var{pid}, int *@var{status-ptr}, int @var{options})
538 @standards{POSIX.1, sys/wait.h}
539 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
540 The @code{waitpid} function is used to request status information from a
541 child process whose process ID is @var{pid}. Normally, the calling
542 process is suspended until the child process makes status information
543 available by terminating.
545 Other values for the @var{pid} argument have special interpretations. A
546 value of @code{-1} or @code{WAIT_ANY} requests status information for
547 any child process; a value of @code{0} or @code{WAIT_MYPGRP} requests
548 information for any child process in the same process group as the
549 calling process; and any other negative value @minus{} @var{pgid}
550 requests information for any child process whose process group ID is
553 If status information for a child process is available immediately, this
554 function returns immediately without waiting. If more than one eligible
555 child process has status information available, one of them is chosen
556 randomly, and its status is returned immediately. To get the status
557 from the other eligible child processes, you need to call @code{waitpid}
560 The @var{options} argument is a bit mask. Its value should be the
561 bitwise OR (that is, the @samp{|} operator) of zero or more of the
562 @code{WNOHANG} and @code{WUNTRACED} flags. You can use the
563 @code{WNOHANG} flag to indicate that the parent process shouldn't wait;
564 and the @code{WUNTRACED} flag to request status information from stopped
565 processes as well as processes that have terminated.
567 The status information from the child process is stored in the object
568 that @var{status-ptr} points to, unless @var{status-ptr} is a null pointer.
570 This function is a cancellation point in multi-threaded programs. This
571 is a problem if the thread allocates some resources (like memory, file
572 descriptors, semaphores or whatever) at the time @code{waitpid} is
573 called. If the thread gets canceled these resources stay allocated
574 until the program ends. To avoid this calls to @code{waitpid} should be
575 protected using cancellation handlers.
576 @c ref pthread_cleanup_push / pthread_cleanup_pop
578 The return value is normally the process ID of the child process whose
579 status is reported. If there are child processes but none of them is
580 waiting to be noticed, @code{waitpid} will block until one is. However,
581 if the @code{WNOHANG} option was specified, @code{waitpid} will return
582 zero instead of blocking.
584 If a specific PID to wait for was given to @code{waitpid}, it will
585 ignore all other children (if any). Therefore if there are children
586 waiting to be noticed but the child whose PID was specified is not one
587 of them, @code{waitpid} will block or return zero as described above.
589 A value of @code{-1} is returned in case of error. The following
590 @code{errno} error conditions are defined for this function:
594 The function was interrupted by delivery of a signal to the calling
595 process. @xref{Interrupted Primitives}.
598 There are no child processes to wait for, or the specified @var{pid}
599 is not a child of the calling process.
602 An invalid value was provided for the @var{options} argument.
606 These symbolic constants are defined as values for the @var{pid} argument
607 to the @code{waitpid} function.
609 @comment Extra blank lines make it look better.
613 This constant macro (whose value is @code{-1}) specifies that
614 @code{waitpid} should return status information about any child process.
618 This constant (with value @code{0}) specifies that @code{waitpid} should
619 return status information about any child process in the same process
620 group as the calling process.
623 These symbolic constants are defined as flags for the @var{options}
624 argument to the @code{waitpid} function. You can bitwise-OR the flags
625 together to obtain a value to use as the argument.
630 This flag specifies that @code{waitpid} should return immediately
631 instead of waiting, if there is no child process ready to be noticed.
635 This flag specifies that @code{waitpid} should report the status of any
636 child processes that have been stopped as well as those that have
640 @deftypefun pid_t wait (int *@var{status-ptr})
641 @standards{POSIX.1, sys/wait.h}
642 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
643 This is a simplified version of @code{waitpid}, and is used to wait
644 until any one child process terminates. The call:
651 is exactly equivalent to:
654 waitpid (-1, &status, 0)
657 This function is a cancellation point in multi-threaded programs. This
658 is a problem if the thread allocates some resources (like memory, file
659 descriptors, semaphores or whatever) at the time @code{wait} is
660 called. If the thread gets canceled these resources stay allocated
661 until the program ends. To avoid this calls to @code{wait} should be
662 protected using cancellation handlers.
663 @c ref pthread_cleanup_push / pthread_cleanup_pop
666 @deftypefun pid_t wait4 (pid_t @var{pid}, int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
667 @standards{BSD, sys/wait.h}
668 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
669 If @var{usage} is a null pointer, @code{wait4} is equivalent to
670 @code{waitpid (@var{pid}, @var{status-ptr}, @var{options})}.
672 If @var{usage} is not null, @code{wait4} stores usage figures for the
673 child process in @code{*@var{rusage}} (but only if the child has
674 terminated, not if it has stopped). @xref{Resource Usage}.
676 This function is a BSD extension.
679 Here's an example of how to use @code{waitpid} to get the status from
680 all child processes that have terminated, without ever waiting. This
681 function is designed to be a handler for @code{SIGCHLD}, the signal that
682 indicates that at least one child process has terminated.
687 sigchld_handler (int signum)
689 int pid, status, serrno;
693 pid = waitpid (WAIT_ANY, &status, WNOHANG);
701 notice_termination (pid, status);
708 @node Process Completion Status
709 @section Process Completion Status
711 If the exit status value (@pxref{Program Termination}) of the child
712 process is zero, then the status value reported by @code{waitpid} or
713 @code{wait} is also zero. You can test for other kinds of information
714 encoded in the returned status value using the following macros.
715 These macros are defined in the header file @file{sys/wait.h}.
718 @deftypefn Macro int WIFEXITED (int @var{status})
719 @standards{POSIX.1, sys/wait.h}
720 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
721 This macro returns a nonzero value if the child process terminated
722 normally with @code{exit} or @code{_exit}.
725 @deftypefn Macro int WEXITSTATUS (int @var{status})
726 @standards{POSIX.1, sys/wait.h}
727 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
728 If @code{WIFEXITED} is true of @var{status}, this macro returns the
729 low-order 8 bits of the exit status value from the child process.
733 @deftypefn Macro int WIFSIGNALED (int @var{status})
734 @standards{POSIX.1, sys/wait.h}
735 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
736 This macro returns a nonzero value if the child process terminated
737 because it received a signal that was not handled.
738 @xref{Signal Handling}.
741 @deftypefn Macro int WTERMSIG (int @var{status})
742 @standards{POSIX.1, sys/wait.h}
743 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
744 If @code{WIFSIGNALED} is true of @var{status}, this macro returns the
745 signal number of the signal that terminated the child process.
748 @deftypefn Macro int WCOREDUMP (int @var{status})
749 @standards{BSD, sys/wait.h}
750 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
751 This macro returns a nonzero value if the child process terminated
752 and produced a core dump.
755 @deftypefn Macro int WIFSTOPPED (int @var{status})
756 @standards{POSIX.1, sys/wait.h}
757 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
758 This macro returns a nonzero value if the child process is stopped.
761 @deftypefn Macro int WSTOPSIG (int @var{status})
762 @standards{POSIX.1, sys/wait.h}
763 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
764 If @code{WIFSTOPPED} is true of @var{status}, this macro returns the
765 signal number of the signal that caused the child process to stop.
769 @node BSD Wait Functions
770 @section BSD Process Wait Function
772 @Theglibc{} also provides the @code{wait3} function for compatibility
773 with BSD. This function is declared in @file{sys/wait.h}. It is the
774 predecessor to @code{wait4}, which is more flexible. @code{wait3} is
778 @deftypefun pid_t wait3 (int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
779 @standards{BSD, sys/wait.h}
780 @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
781 If @var{usage} is a null pointer, @code{wait3} is equivalent to
782 @code{waitpid (-1, @var{status-ptr}, @var{options})}.
784 If @var{usage} is not null, @code{wait3} stores usage figures for the
785 child process in @code{*@var{rusage}} (but only if the child has
786 terminated, not if it has stopped). @xref{Resource Usage}.
789 @node Process Creation Example
790 @section Process Creation Example
792 Here is an example program showing how you might write a function
793 similar to the built-in @code{system}. It executes its @var{command}
794 argument using the equivalent of @samp{sh -c @var{command}}.
800 #include <sys/types.h>
801 #include <sys/wait.h>
803 /* @r{Execute the command using this shell program.} */
804 #define SHELL "/bin/sh"
808 my_system (const char *command)
817 /* @r{This is the child process. Execute the shell command.} */
818 execl (SHELL, SHELL, "-c", command, NULL);
819 _exit (EXIT_FAILURE);
822 /* @r{The fork failed. Report failure.} */
825 /* @r{This is the parent process. Wait for the child to complete.} */
826 if (waitpid (pid, &status, 0) != pid)
832 @comment Yes, this example has been tested.
834 There are a couple of things you should pay attention to in this
837 Remember that the first @code{argv} argument supplied to the program
838 represents the name of the program being executed. That is why, in the
839 call to @code{execl}, @code{SHELL} is supplied once to name the program
840 to execute and a second time to supply a value for @code{argv[0]}.
842 The @code{execl} call in the child process doesn't return if it is
843 successful. If it fails, you must do something to make the child
844 process terminate. Just returning a bad status code with @code{return}
845 would leave two processes running the original program. Instead, the
846 right behavior is for the child process to report failure to its parent
849 Call @code{_exit} to accomplish this. The reason for using @code{_exit}
850 instead of @code{exit} is to avoid flushing fully buffered streams such
851 as @code{stdout}. The buffers of these streams probably contain data
852 that was copied from the parent process by the @code{fork}, data that
853 will be output eventually by the parent process. Calling @code{exit} in
854 the child would output the data twice. @xref{Termination Internals}.