Don't define exp2 inline.

[glibc/history.git] / manual / process.texi

@node Processes
@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.

@comment stdlib.h
@comment ISO
@deftypefun int system (const char *@var{command})
@pindex sh
This function executes @var{command} as a shell command.  In the GNU C
library, 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 non-zero return value
indicates that a command processor is available and this function can be
used at all.

This function is a cancelation 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 cancelation 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

@comment sys/types.h
@comment POSIX.1
@deftp {Data Type} pid_t
The @code{pid_t} data type is a signed integer type which is capable
of representing a process ID.  In the GNU library, this is an @code{int}.
@end deftp

@comment unistd.h
@comment POSIX.1
@deftypefun pid_t getpid (void)
The @code{getpid} function returns the process ID of the current process.
@end deftypefun

@comment unistd.h
@comment POSIX.1
@deftypefun pid_t getppid (void)
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

@comment unistd.h
@comment POSIX.1
@deftypefun pid_t fork (void)
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


@comment unistd.h
@comment BSD
@deftypefun pid_t vfork (void)
The @code{vfork} function is similar to @code{fork} but on 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 exits 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}.  The GNU C
library 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.

@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}.

@comment unistd.h
@comment POSIX.1
@deftypefun int execv (const char *@var{filename}, char *const @var{argv}@t{[]})
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

@comment unistd.h
@comment POSIX.1
@deftypefun int execl (const char *@var{filename}, const char *@var{arg0}, @dots{})
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

@comment unistd.h
@comment POSIX.1
@deftypefun int execve (const char *@var{filename}, char *const @var{argv}@t{[]}, char *const @var{env}@t{[]})
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

@comment unistd.h
@comment POSIX.1
@deftypefun int execle (const char *@var{filename}, const char *@var{arg0}, char *const @var{env}@t{[]}, @dots{})
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

@comment unistd.h
@comment POSIX.1
@deftypefun int execvp (const char *@var{filename}, char *const @var{argv}@t{[]})
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

@comment unistd.h
@comment POSIX.1
@deftypefun int execlp (const char *@var{filename}, const char *@var{arg0}, @dots{})
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}.  In
the GNU system, 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.  The GNU system has 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}.  In the GNU system, 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 description 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

@comment sys/wait.h
@comment POSIX.1
@deftypefun pid_t waitpid (pid_t @var{pid}, int *@var{status-ptr}, int @var{options})
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 cancelation 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 cancelation 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 the @code{WNOHANG} option was specified and no
child process is waiting to be noticed, the value is zero.  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.
@table @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 table

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.

@table @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 table

@comment sys/wait.h
@comment POSIX.1
@deftypefun pid_t wait (int *@var{status-ptr})
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 cancelation 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 cancelation handlers.
@c ref pthread_cleanup_push / pthread_cleanup_pop
@end deftypefun

@comment sys/wait.h
@comment BSD
@deftypefun pid_t wait4 (pid_t @var{pid}, int *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
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;
  int status;
  while (1)
    @{
      pid = waitpid (WAIT_ANY, &status, WNOHANG);
      if (pid < 0)
        @{
          perror ("waitpid");
          break;
        @}
      if (pid == 0)
        break;
      notice_termination (pid, status);
    @}
@}
@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

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WIFEXITED (int @var{status})
This macro returns a nonzero value if the child process terminated
normally with @code{exit} or @code{_exit}.
@end deftypefn

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WEXITSTATUS (int @var{status})
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

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WIFSIGNALED (int @var{status})
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

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WTERMSIG (int @var{status})
If @code{WIFSIGNALED} is true of @var{status}, this macro returns the
signal number of the signal that terminated the child process.
@end deftypefn

@comment sys/wait.h
@comment BSD
@deftypefn Macro int WCOREDUMP (int @var{status})
This macro returns a nonzero value if the child process terminated
and produced a core dump.
@end deftypefn

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WIFSTOPPED (int @var{status})
This macro returns a nonzero value if the child process is stopped.
@end deftypefn

@comment sys/wait.h
@comment POSIX.1
@deftypefn Macro int WSTOPSIG (int @var{status})
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 Functions

The GNU library also provides these related facilities for compatibility
with BSD Unix.  BSD uses the @code{union wait} data type to represent
status values rather than an @code{int}.  The two representations are
actually interchangeable; they describe the same bit patterns.  The GNU
C Library defines macros such as @code{WEXITSTATUS} so that they will
work on either kind of object, and the @code{wait} function is defined
to accept either type of pointer as its @var{status-ptr} argument.

These functions are declared in @file{sys/wait.h}.
@pindex sys/wait.h

@comment sys/wait.h
@comment BSD
@deftp {Data Type} {union wait}
This data type represents program termination status values.  It has
the following members:

@table @code
@item int w_termsig
The value of this member is the same as the result of the
@code{WTERMSIG} macro.

@item int w_coredump
The value of this member is the same as the result of the
@code{WCOREDUMP} macro.

@item int w_retcode
The value of this member is the same as the result of the
@code{WEXITSTATUS} macro.

@item int w_stopsig
The value of this member is the same as the result of the
@code{WSTOPSIG} macro.
@end table

Instead of accessing these members directly, you should use the
equivalent macros.
@end deftp

The @code{wait3} function is the predecessor to @code{wait4}, which is
more flexible.  @code{wait3} is now obsolete.

@comment sys/wait.h
@comment BSD
@deftypefun pid_t wait3 (union wait *@var{status-ptr}, int @var{options}, struct rusage *@var{usage})
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}.