text: @node Processes, Inter-Process Communication, Program Basics, Top
  @c %MENU% How to create processes and run other programs
  @chapter Processes
  
  @cindex process
  @dfn{Processes} are the primitive units for allocation of system
  resources.  Each process has its own address space and (usually) one
  thread of control.  A process executes a program; you can have multiple
  processes executing the same program, but each process has its own copy
  of the program within its own address space and executes it
  independently of the other copies.
  
  @cindex child process
  @cindex parent process
  Processes are organized hierarchically.  Each process has a @dfn{parent
  process} which explicitly arranged to create it.  The processes created
  by a given parent are called its @dfn{child processes}.  A child
  inherits many of its attributes from the parent process.
  
  This chapter describes how a program can create, terminate, and control
  child processes.  Actually, there are three distinct operations
  involved: creating a new child process, causing the new process to
  execute a program, and coordinating the completion of the child process
  with the original program.
  
  The @code{system} function provides a simple, portable mechanism for
  running another program; it does all three steps automatically.  If you
  need more control over the details of how this is done, you can use the
  primitive functions to do each step individually instead.
  
  @menu
  * Running a Command::           The easy way to run another program.
  * Process Creation Concepts::   An overview of the hard way to do it.
  * Process Identification::      How to get the process ID of a process.
  * Creating a Process::          How to fork a child process.
  * Executing a File::            How to make a process execute another program.
  * Process Completion::          How to tell when a child process has completed.
  * Process Completion Status::   How to interpret the status value
                                   returned from a child process.
  * BSD Wait Functions::  	More functions, for backward compatibility.
  * Process Creation Example::    A complete example program.
  @end menu
  
  
  @node Running a Command
  @section Running a Command
  @cindex running a command
  
  The easy way to run another program is to use the @code{system}
  function.  This function does all the work of running a subprogram, but
  it doesn't give you much control over the details: you have to wait
  until the subprogram terminates before you can do anything else.
  
  @deftypefun int system (const char *@var{command})
  @standards{ISO, stdlib.h}
  @pindex sh
  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{} @ascuheap{} @asulock{}}@acunsafe{@aculock{} @acsmem{}}}
  @c system @ascuplugin @ascuheap @asulock @aculock @acsmem
  @c  do_system @ascuplugin @ascuheap @asulock @aculock @acsmem
  @c   sigemptyset dup ok
  @c   libc_lock_lock @asulock @aculock
  @c   ADD_REF ok
  @c   sigaction dup ok
  @c   SUB_REF ok
  @c   libc_lock_unlock @aculock
  @c   sigaddset dup ok
  @c   sigprocmask dup ok
  @c   CLEANUP_HANDLER @ascuplugin @ascuheap @acsmem
  @c    libc_cleanup_region_start @ascuplugin @ascuheap @acsmem
  @c     pthread_cleanup_push_defer @ascuplugin @ascuheap @acsmem
  @c      CANCELLATION_P @ascuplugin @ascuheap @acsmem
  @c       CANCEL_ENABLED_AND_CANCELED ok
  @c       do_cancel @ascuplugin @ascuheap @acsmem
  @c    cancel_handler ok
  @c     kill syscall ok
  @c     waitpid dup ok
  @c     libc_lock_lock ok
  @c     sigaction dup ok
  @c     libc_lock_unlock ok
  @c   FORK ok
  @c    clone syscall ok
  @c   waitpid dup ok
  @c   CLEANUP_RESET ok
  @c    libc_cleanup_region_end ok
  @c     pthread_cleanup_pop_restore ok
  @c  SINGLE_THREAD_P ok
  @c  LIBC_CANCEL_ASYNC @ascuplugin @ascuheap @acsmem
  @c   libc_enable_asynccancel @ascuplugin @ascuheap @acsmem
  @c    CANCEL_ENABLED_AND_CANCELED_AND_ASYNCHRONOUS dup ok
  @c    do_cancel dup @ascuplugin @ascuheap @acsmem
  @c  LIBC_CANCEL_RESET ok
  @c   libc_disable_asynccancel ok
  @c    lll_futex_wait dup ok
  This function executes @var{command} as a shell command.  In @theglibc{},
  it always uses the default shell @code{sh} to run the command.
  In particular, it searches the directories in @code{PATH} to find
  programs to execute.  The return value is @code{-1} if it wasn't
  possible to create the shell process, and otherwise is the status of the
  shell process.  @xref{Process Completion}, for details on how this
  status code can be interpreted.
  
  If the @var{command} argument is a null pointer, a return value of zero
  indicates that no command processor is available.
  
  This function is a cancellation point in multi-threaded programs.  This
  is a problem if the thread allocates some resources (like memory, file
  descriptors, semaphores or whatever) at the time @code{system} is
  called.  If the thread gets canceled these resources stay allocated
  until the program ends.  To avoid this calls to @code{system} should be
  protected using cancellation handlers.
  @c ref pthread_cleanup_push / pthread_cleanup_pop
  
  @pindex stdlib.h
  The @code{system} function is declared in the header file
  @file{stdlib.h}.
  @end deftypefun
  
  @strong{Portability Note:} Some C implementations may not have any
  notion of a command processor that can execute other programs.  You can
  determine whether a command processor exists by executing
  @w{@code{system (NULL)}}; if the return value is nonzero, a command
  processor is available.
  
  The @code{popen} and @code{pclose} functions (@pxref{Pipe to a
  Subprocess}) are closely related to the @code{system} function.  They
  allow the parent process to communicate with the standard input and
  output channels of the command being executed.
  
  @node Process Creation Concepts
  @section Process Creation Concepts
  
  This section gives an overview of processes and of the steps involved in
  creating a process and making it run another program.
  
  @cindex process ID
  @cindex process lifetime
  Each process is named by a @dfn{process ID} number.  A unique process ID
  is allocated to each process when it is created.  The @dfn{lifetime} of
  a process ends when its termination is reported to its parent process;
  at that time, all of the process resources, including its process ID,
  are freed.
  
  @cindex creating a process
  @cindex forking a process
  @cindex child process
  @cindex parent process
  Processes are created with the @code{fork} system call (so the operation
  of creating a new process is sometimes called @dfn{forking} a process).
  The @dfn{child process} created by @code{fork} is a copy of the original
  @dfn{parent process}, except that it has its own process ID.
  
  After forking a child process, both the parent and child processes
  continue to execute normally.  If you want your program to wait for a
  child process to finish executing before continuing, you must do this
  explicitly after the fork operation, by calling @code{wait} or
  @code{waitpid} (@pxref{Process Completion}).  These functions give you
  limited information about why the child terminated---for example, its
  exit status code.
  
  A newly forked child process continues to execute the same program as
  its parent process, at the point where the @code{fork} call returns.
  You can use the return value from @code{fork} to tell whether the program
  is running in the parent process or the child.
  
  @cindex process image
  Having several processes run the same program is only occasionally
  useful.  But the child can execute another program using one of the
  @code{exec} functions; see @ref{Executing a File}.  The program that the
  process is executing is called its @dfn{process image}.  Starting
  execution of a new program causes the process to forget all about its
  previous process image; when the new program exits, the process exits
  too, instead of returning to the previous process image.
  
  @node Process Identification
  @section Process Identification
  
  The @code{pid_t} data type represents process IDs.  You can get the
  process ID of a process by calling @code{getpid}.  The function
  @code{getppid} returns the process ID of the parent of the current
  process (this is also known as the @dfn{parent process ID}).  Your
  program should include the header files @file{unistd.h} and
  @file{sys/types.h} to use these functions.
  @pindex sys/types.h
  @pindex unistd.h
  
  @deftp {Data Type} pid_t
  @standards{POSIX.1, sys/types.h}
  The @code{pid_t} data type is a signed integer type which is capable
  of representing a process ID.  In @theglibc{}, this is an @code{int}.
  @end deftp
  
  @deftypefun pid_t getpid (void)
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  The @code{getpid} function returns the process ID of the current process.
  @end deftypefun
  
  @deftypefun pid_t getppid (void)
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  The @code{getppid} function returns the process ID of the parent of the
  current process.
  @end deftypefun
  
  @node Creating a Process
  @section Creating a Process
  
  The @code{fork} function is the primitive for creating a process.
  It is declared in the header file @file{unistd.h}.
  @pindex unistd.h
  
  @deftypefun pid_t fork (void)
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
  @c The nptl/.../linux implementation safely collects fork_handlers into
  @c an alloca()ed linked list and increments ref counters; it uses atomic
  @c ops and retries, avoiding locking altogether.  It then takes the
  @c IO_list lock, resets the thread-local pid, and runs fork.  The parent
  @c restores the thread-local pid, releases the lock, and runs parent
  @c handlers, decrementing the ref count and signaling futex wait if
  @c requested by unregister_atfork.  The child bumps the fork generation,
  @c sets the thread-local pid, resets cpu clocks, initializes the robust
  @c mutex list, the stream locks, the IO_list lock, the dynamic loader
  @c lock, runs the child handlers, reseting ref counters to 1, and
  @c initializes the fork lock.  These are all safe, unless atfork
  @c handlers themselves are unsafe.
  The @code{fork} function creates a new process.
  
  If the operation is successful, there are then both parent and child
  processes and both see @code{fork} return, but with different values: it
  returns a value of @code{0} in the child process and returns the child's
  process ID in the parent process.
  
  If process creation failed, @code{fork} returns a value of @code{-1} in
  the parent process.  The following @code{errno} error conditions are
  defined for @code{fork}:
  
  @table @code
  @item EAGAIN
  There aren't enough system resources to create another process, or the
  user already has too many processes running.  This means exceeding the
  @code{RLIMIT_NPROC} resource limit, which can usually be increased;
  @pxref{Limits on Resources}.
  
  @item ENOMEM
  The process requires more space than the system can supply.
  @end table
  @end deftypefun
  
  The specific attributes of the child process that differ from the
  parent process are:
  
  @itemize @bullet
  @item
  The child process has its own unique process ID.
  
  @item
  The parent process ID of the child process is the process ID of its
  parent process.
  
  @item
  The child process gets its own copies of the parent process's open file
  descriptors.  Subsequently changing attributes of the file descriptors
  in the parent process won't affect the file descriptors in the child,
  and vice versa.  @xref{Control Operations}.  However, the file position
  associated with each descriptor is shared by both processes;
  @pxref{File Position}.
  
  @item
  The elapsed processor times for the child process are set to zero;
  see @ref{Processor Time}.
  
  @item
  The child doesn't inherit file locks set by the parent process.
  @c !!! flock locks shared
  @xref{Control Operations}.
  
  @item
  The child doesn't inherit alarms set by the parent process.
  @xref{Setting an Alarm}.
  
  @item
  The set of pending signals (@pxref{Delivery of Signal}) for the child
  process is cleared.  (The child process inherits its mask of blocked
  signals and signal actions from the parent process.)
  @end itemize
  
  
  @deftypefun pid_t vfork (void)
  @standards{BSD, unistd.h}
  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuplugin{}}@acunsafe{@aculock{}}}
  @c The vfork implementation proper is a safe syscall, but it may fall
  @c back to fork if the vfork syscall is not available.
  The @code{vfork} function is similar to @code{fork} but on some systems
  it is more efficient; however, there are restrictions you must follow to
  use it safely.
  
  While @code{fork} makes a complete copy of the calling process's address
  space and allows both the parent and child to execute independently,
  @code{vfork} does not make this copy.  Instead, the child process
  created with @code{vfork} shares its parent's address space until it
  calls @code{_exit} or one of the @code{exec} functions.  In the
  meantime, the parent process suspends execution.
  
  You must be very careful not to allow the child process created with
  @code{vfork} to modify any global data or even local variables shared
  with the parent.  Furthermore, the child process cannot return from (or
  do a long jump out of) the function that called @code{vfork}!  This
  would leave the parent process's control information very confused.  If
  in doubt, use @code{fork} instead.
  
  Some operating systems don't really implement @code{vfork}.  @Theglibc{}
  permits you to use @code{vfork} on all systems, but actually
  executes @code{fork} if @code{vfork} isn't available.  If you follow
  the proper precautions for using @code{vfork}, your program will still
  work even if the system uses @code{fork} instead.
  @end deftypefun
  
  @node Executing a File
  @section Executing a File
  @cindex executing a file
  @cindex @code{exec} functions
  
  This section describes the @code{exec} family of functions, for executing
  a file as a process image.  You can use these functions to make a child
  process execute a new program after it has been forked.
  
  To see the effects of @code{exec} from the point of view of the called
  program, see @ref{Program Basics}.
  
  @pindex unistd.h
  The functions in this family differ in how you specify the arguments,
  but otherwise they all do the same thing.  They are declared in the
  header file @file{unistd.h}.
  
  @deftypefun int execv (const char *@var{filename}, char *const @var{argv}@t{[]})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  The @code{execv} function executes the file named by @var{filename} as a
  new process image.
  
  The @var{argv} argument is an array of null-terminated strings that is
  used to provide a value for the @code{argv} argument to the @code{main}
  function of the program to be executed.  The last element of this array
  must be a null pointer.  By convention, the first element of this array
  is the file name of the program sans directory names.  @xref{Program
  Arguments}, for full details on how programs can access these arguments.
  
  The environment for the new process image is taken from the
  @code{environ} variable of the current process image; see
  @ref{Environment Variables}, for information about environments.
  @end deftypefun
  
  @deftypefun int execl (const char *@var{filename}, const char *@var{arg0}, @dots{})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
  This is similar to @code{execv}, but the @var{argv} strings are
  specified individually instead of as an array.  A null pointer must be
  passed as the last such argument.
  @end deftypefun
  
  @deftypefun int execve (const char *@var{filename}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This is similar to @code{execv}, but permits you to specify the environment
  for the new program explicitly as the @var{env} argument.  This should
  be an array of strings in the same format as for the @code{environ}
  variable; see @ref{Environment Access}.
  @end deftypefun
  
  @deftypefun int execle (const char *@var{filename}, const char *@var{arg0}, @dots{}, char *const @var{env}@t{[]})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
  This is similar to @code{execl}, but permits you to specify the
  environment for the new program explicitly.  The environment argument is
  passed following the null pointer that marks the last @var{argv}
  argument, and should be an array of strings in the same format as for
  the @code{environ} variable.
  @end deftypefun
  
  @deftypefun int execvp (const char *@var{filename}, char *const @var{argv}@t{[]})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
  The @code{execvp} function is similar to @code{execv}, except that it
  searches the directories listed in the @code{PATH} environment variable
  (@pxref{Standard Environment}) to find the full file name of a
  file from @var{filename} if @var{filename} does not contain a slash.
  
  This function is useful for executing system utility programs, because
  it looks for them in the places that the user has chosen.  Shells use it
  to run the commands that users type.
  @end deftypefun
  
  @deftypefun int execlp (const char *@var{filename}, const char *@var{arg0}, @dots{})
  @standards{POSIX.1, unistd.h}
  @safety{@prelim{}@mtsafe{@mtsenv{}}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{}}}
  This function is like @code{execl}, except that it performs the same
  file name searching as the @code{execvp} function.
  @end deftypefun
  
  The size of the argument list and environment list taken together must
  not be greater than @code{ARG_MAX} bytes.  @xref{General Limits}.  On
  @gnuhurdsystems{}, the size (which compares against @code{ARG_MAX})
  includes, for each string, the number of characters in the string, plus
  the size of a @code{char *}, plus one, rounded up to a multiple of the
  size of a @code{char *}.  Other systems may have somewhat different
  rules for counting.
  
  These functions normally don't return, since execution of a new program
  causes the currently executing program to go away completely.  A value
  of @code{-1} is returned in the event of a failure.  In addition to the
  usual file name errors (@pxref{File Name Errors}), the following
  @code{errno} error conditions are defined for these functions:
  
  @table @code
  @item E2BIG
  The combined size of the new program's argument list and environment
  list is larger than @code{ARG_MAX} bytes.  @gnuhurdsystems{} have no
  specific limit on the argument list size, so this error code cannot
  result, but you may get @code{ENOMEM} instead if the arguments are too
  big for available memory.
  
  @item ENOEXEC
  The specified file can't be executed because it isn't in the right format.
  
  @item ENOMEM
  Executing the specified file requires more storage than is available.
  @end table
  
  If execution of the new file succeeds, it updates the access time field
  of the file as if the file had been read.  @xref{File Times}, for more
  details about access times of files.
  
  The point at which the file is closed again is not specified, but
  is at some point before the process exits or before another process
  image is executed.
  
  Executing a new process image completely changes the contents of memory,
  copying only the argument and environment strings to new locations.  But
  many other attributes of the process are unchanged:
  
  @itemize @bullet
  @item
  The process ID and the parent process ID.  @xref{Process Creation Concepts}.
  
  @item
  Session and process group membership.  @xref{Concepts of Job Control}.
  
  @item
  Real user ID and group ID, and supplementary group IDs.  @xref{Process
  Persona}.
  
  @item
  Pending alarms.  @xref{Setting an Alarm}.
  
  @item
  Current working directory and root directory.  @xref{Working
  Directory}.  On @gnuhurdsystems{}, the root directory is not copied when
  executing a setuid program; instead the system default root directory
  is used for the new program.
  
  @item
  File mode creation mask.  @xref{Setting Permissions}.
  
  @item
  Process signal mask; see @ref{Process Signal Mask}.
  
  @item
  Pending signals; see @ref{Blocking Signals}.
  
  @item
  Elapsed processor time associated with the process; see @ref{Processor Time}.
  @end itemize
  
  If the set-user-ID and set-group-ID mode bits of the process image file
  are set, this affects the effective user ID and effective group ID
  (respectively) of the process.  These concepts are discussed in detail
  in @ref{Process Persona}.
  
  Signals that are set to be ignored in the existing process image are
  also set to be ignored in the new process image.  All other signals are
  set to the default action in the new process image.  For more
  information about signals, see @ref{Signal Handling}.
  
  File descriptors open in the existing process image remain open in the
  new process image, unless they have the @code{FD_CLOEXEC}
  (close-on-exec) flag set.  The files that remain open inherit all
  attributes of the open file descriptors from the existing process image,
  including file locks.  File descriptors are discussed in @ref{Low-Level I/O}.
  
  Streams, by contrast, cannot survive through @code{exec} functions,
  because they are located in the memory of the process itself.  The new
  process image has no streams except those it creates afresh.  Each of
  the streams in the pre-@code{exec} process image has a descriptor inside
  it, and these descriptors do survive through @code{exec} (provided that
  they do not have @code{FD_CLOEXEC} set).  The new process image can
  reconnect these to new streams using @code{fdopen} (@pxref{Descriptors
  and Streams}).
  
  @node Process Completion
  @section Process Completion
  @cindex process completion
  @cindex waiting for completion of child process
  @cindex testing exit status of child process
  
  The functions described in this section are used to wait for a child
  process to terminate or stop, and determine its status.  These functions
  are declared in the header file @file{sys/wait.h}.
  @pindex sys/wait.h
  
  @deftypefun pid_t waitpid (pid_t @var{pid}, int *@var{status-ptr}, int @var{options})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  The @code{waitpid} function is used to request status information from a
  child process whose process ID is @var{pid}.  Normally, the calling
  process is suspended until the child process makes status information
  available by terminating.
  
  Other values for the @var{pid} argument have special interpretations.  A
  value of @code{-1} or @code{WAIT_ANY} requests status information for
  any child process; a value of @code{0} or @code{WAIT_MYPGRP} requests
  information for any child process in the same process group as the
  calling process; and any other negative value @minus{} @var{pgid}
  requests information for any child process whose process group ID is
  @var{pgid}.
  
  If status information for a child process is available immediately, this
  function returns immediately without waiting.  If more than one eligible
  child process has status information available, one of them is chosen
  randomly, and its status is returned immediately.  To get the status
  from the other eligible child processes, you need to call @code{waitpid}
  again.
  
  The @var{options} argument is a bit mask.  Its value should be the
  bitwise OR (that is, the @samp{|} operator) of zero or more of the
  @code{WNOHANG} and @code{WUNTRACED} flags.  You can use the
  @code{WNOHANG} flag to indicate that the parent process shouldn't wait;
  and the @code{WUNTRACED} flag to request status information from stopped
  processes as well as processes that have terminated.
  
  The status information from the child process is stored in the object
  that @var{status-ptr} points to, unless @var{status-ptr} is a null pointer.
  
  This function is a cancellation point in multi-threaded programs.  This
  is a problem if the thread allocates some resources (like memory, file
  descriptors, semaphores or whatever) at the time @code{waitpid} is
  called.  If the thread gets canceled these resources stay allocated
  until the program ends.  To avoid this calls to @code{waitpid} should be
  protected using cancellation handlers.
  @c ref pthread_cleanup_push / pthread_cleanup_pop
  
  The return value is normally the process ID of the child process whose
  status is reported.  If there are child processes but none of them is
  waiting to be noticed, @code{waitpid} will block until one is.  However,
  if the @code{WNOHANG} option was specified, @code{waitpid} will return
  zero instead of blocking.
  
  If a specific PID to wait for was given to @code{waitpid}, it will
  ignore all other children (if any).  Therefore if there are children
  waiting to be noticed but the child whose PID was specified is not one
  of them, @code{waitpid} will block or return zero as described above.
  
  A value of @code{-1} is returned in case of error.  The following
  @code{errno} error conditions are defined for this function:
  
  @table @code
  @item EINTR
  The function was interrupted by delivery of a signal to the calling
  process.  @xref{Interrupted Primitives}.
  
  @item ECHILD
  There are no child processes to wait for, or the specified @var{pid}
  is not a child of the calling process.
  
  @item EINVAL
  An invalid value was provided for the @var{options} argument.
  @end table
  @end deftypefun
  
  These symbolic constants are defined as values for the @var{pid} argument
  to the @code{waitpid} function.
  
  @comment Extra blank lines make it look better.
  @vtable @code
  @item WAIT_ANY
  
  This constant macro (whose value is @code{-1}) specifies that
  @code{waitpid} should return status information about any child process.
  
  
  @item WAIT_MYPGRP
  This constant (with value @code{0}) specifies that @code{waitpid} should
  return status information about any child process in the same process
  group as the calling process.
  @end vtable
  
  These symbolic constants are defined as flags for the @var{options}
  argument to the @code{waitpid} function.  You can bitwise-OR the flags
  together to obtain a value to use as the argument.
  
  @vtable @code
  @item WNOHANG
  
  This flag specifies that @code{waitpid} should return immediately
  instead of waiting, if there is no child process ready to be noticed.
  
  @item WUNTRACED
  
  This flag specifies that @code{waitpid} should report the status of any
  child processes that have been stopped as well as those that have
  terminated.
  @end vtable
  
  @deftypefun pid_t wait (int *@var{status-ptr})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This is a simplified version of @code{waitpid}, and is used to wait
  until any one child process terminates.  The call:
  
  @smallexample
  wait (&status)
  @end smallexample
  
  @noindent
  is exactly equivalent to:
  
  @smallexample
  waitpid (-1, &status, 0)
  @end smallexample
  
  This function is a cancellation point in multi-threaded programs.  This
  is a problem if the thread allocates some resources (like memory, file
  descriptors, semaphores or whatever) at the time @code{wait} is
  called.  If the thread gets canceled these resources stay allocated
  until the program ends.  To avoid this calls to @code{wait} should be
  protected using cancellation handlers.
  @c ref pthread_cleanup_push / pthread_cleanup_pop
  @end deftypefun
  
  @deftypefun pid_t wait4 (pid_t @var{pid}, int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
  @standards{BSD, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  If @var{usage} is a null pointer, @code{wait4} is equivalent to
  @code{waitpid (@var{pid}, @var{status-ptr}, @var{options})}.
  
  If @var{usage} is not null, @code{wait4} stores usage figures for the
  child process in @code{*@var{rusage}} (but only if the child has
  terminated, not if it has stopped).  @xref{Resource Usage}.
  
  This function is a BSD extension.
  @end deftypefun
  
  Here's an example of how to use @code{waitpid} to get the status from
  all child processes that have terminated, without ever waiting.  This
  function is designed to be a handler for @code{SIGCHLD}, the signal that
  indicates that at least one child process has terminated.
  
  @smallexample
  @group
  void
  sigchld_handler (int signum)
  @{
    int pid, status, serrno;
    serrno = errno;
    while (1)
      @{
        pid = waitpid (WAIT_ANY, &status, WNOHANG);
        if (pid < 0)
          @{
            perror ("waitpid");
            break;
          @}
        if (pid == 0)
          break;
        notice_termination (pid, status);
      @}
    errno = serrno;
  @}
  @end group
  @end smallexample
  
  @node Process Completion Status
  @section Process Completion Status
  
  If the exit status value (@pxref{Program Termination}) of the child
  process is zero, then the status value reported by @code{waitpid} or
  @code{wait} is also zero.  You can test for other kinds of information
  encoded in the returned status value using the following macros.
  These macros are defined in the header file @file{sys/wait.h}.
  @pindex sys/wait.h
  
  @deftypefn Macro int WIFEXITED (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This macro returns a nonzero value if the child process terminated
  normally with @code{exit} or @code{_exit}.
  @end deftypefn
  
  @deftypefn Macro int WEXITSTATUS (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  If @code{WIFEXITED} is true of @var{status}, this macro returns the
  low-order 8 bits of the exit status value from the child process.
  @xref{Exit Status}.
  @end deftypefn
  
  @deftypefn Macro int WIFSIGNALED (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This macro returns a nonzero value if the child process terminated
  because it received a signal that was not handled.
  @xref{Signal Handling}.
  @end deftypefn
  
  @deftypefn Macro int WTERMSIG (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  If @code{WIFSIGNALED} is true of @var{status}, this macro returns the
  signal number of the signal that terminated the child process.
  @end deftypefn
  
  @deftypefn Macro int WCOREDUMP (int @var{status})
  @standards{BSD, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This macro returns a nonzero value if the child process terminated
  and produced a core dump.
  @end deftypefn
  
  @deftypefn Macro int WIFSTOPPED (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  This macro returns a nonzero value if the child process is stopped.
  @end deftypefn
  
  @deftypefn Macro int WSTOPSIG (int @var{status})
  @standards{POSIX.1, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  If @code{WIFSTOPPED} is true of @var{status}, this macro returns the
  signal number of the signal that caused the child process to stop.
  @end deftypefn
  
  
  @node BSD Wait Functions
  @section BSD Process Wait Function
  
  @Theglibc{} also provides the @code{wait3} function for compatibility
  with BSD.  This function is declared in @file{sys/wait.h}.  It is the
  predecessor to @code{wait4}, which is more flexible.  @code{wait3} is
  now obsolete.
  @pindex sys/wait.h
  
  @deftypefun pid_t wait3 (int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
  @standards{BSD, sys/wait.h}
  @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
  If @var{usage} is a null pointer, @code{wait3} is equivalent to
  @code{waitpid (-1, @var{status-ptr}, @var{options})}.
  
  If @var{usage} is not null, @code{wait3} stores usage figures for the
  child process in @code{*@var{rusage}} (but only if the child has
  terminated, not if it has stopped).  @xref{Resource Usage}.
  @end deftypefun
  
  @node Process Creation Example
  @section Process Creation Example
  
  Here is an example program showing how you might write a function
  similar to the built-in @code{system}.  It executes its @var{command}
  argument using the equivalent of @samp{sh -c @var{command}}.
  
  @smallexample
  #include <stddef.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/types.h>
  #include <sys/wait.h>
  
  /* @r{Execute the command using this shell program.}  */
  #define SHELL "/bin/sh"
  
  @group
  int
  my_system (const char *command)
  @{
    int status;
    pid_t pid;
  @end group
  
    pid = fork ();
    if (pid == 0)
      @{
        /* @r{This is the child process.  Execute the shell command.} */
        execl (SHELL, SHELL, "-c", command, NULL);
        _exit (EXIT_FAILURE);
      @}
    else if (pid < 0)
      /* @r{The fork failed.  Report failure.}  */
      status = -1;
    else
      /* @r{This is the parent process.  Wait for the child to complete.}  */
      if (waitpid (pid, &status, 0) != pid)
        status = -1;
    return status;
  @}
  @end smallexample
  
  @comment Yes, this example has been tested.
  
  There are a couple of things you should pay attention to in this
  example.
  
  Remember that the first @code{argv} argument supplied to the program
  represents the name of the program being executed.  That is why, in the
  call to @code{execl}, @code{SHELL} is supplied once to name the program
  to execute and a second time to supply a value for @code{argv[0]}.
  
  The @code{execl} call in the child process doesn't return if it is
  successful.  If it fails, you must do something to make the child
  process terminate.  Just returning a bad status code with @code{return}
  would leave two processes running the original program.  Instead, the
  right behavior is for the child process to report failure to its parent
  process.
  
  Call @code{_exit} to accomplish this.  The reason for using @code{_exit}
  instead of @code{exit} is to avoid flushing fully buffered streams such
  as @code{stdout}.  The buffers of these streams probably contain data
  that was copied from the parent process by the @code{fork}, data that
  will be output eventually by the parent process.  Calling @code{exit} in
  the child would output the data twice.  @xref{Termination Internals}.
  