Processes
*********

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.

Processes are organized hierarchically.  Child processes are created by a parent process, and inherit many of their attributes from the parent process.

This chapter describes how a program can create, terminate, and control child processes.

Program Arguments
=================

When your C program starts, it begins by executing the function called main.  You can define main either to take no arguments, or to take two arguments that represent the command line arguments to the program, like this:

     int main (int ARGC, char *ARGV[])

The command line arguments are the whitespace-separated tokens typed by the user to the shell in invoking the program.  The value of the ARGC argument is the number of command line arguments.  The ARGV argument is a vector of pointers to char; sometimes it is also declared as char **ARGV.  The elements of ARGV are the individual command line argument strings.  By convention, ARGV[0] is the file name of the program being run, and ARGV[ARGC] is a null pointer.

If the syntax for the command line arguments to your program is simple enough, you can simply pick the arguments off from ARGV by hand. But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using getopt to do the parsing.

Program Argument Syntax Conventions
-----------------------------------

The getopt function decodes options following the usual conventions for POSIX utilities:

  * Arguments are options if they begin with a hyphen delimiter (-).

  * Multiple options may follow a hyphen delimiter in a single token if the options do not take arguments.  Thus, -abc is equivalent to -a -b -c.

  * Option names are single alphanumeric (as for isalnum; see Classification of Characters).

  * Certain options require an argument.  For example, the -o command of the ld command requires an argument---an output file name.

  * An option and its argument may or may appear as separate tokens.  (In other words, the whitespace separating them is optional.)  Thus, -o foo and -ofoo are equivalent.

  * Options typically precede other non-option arguments.

     The implementation of getopt in the GNU C library normally makes it appear as if all the option arguments were specified before all the non-option arguments for the purposes of parsing, even if the user of your program intermixed option and non-option arguments.  It does this by reordering the elements of the ARGV array.  This behavior is nonstandard; if you want to suppress it, define the _POSIX_OPTION_ORDER environment variable.  See Standard Environment Variables.

  * The argument -- terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen.

  * A token consisting of a single hyphen character is interpreted as an ordinary non-option argument.  By convention, it is used to specify input from or output to the standard input and output streams.

  * Options may be supplied in any order, or appear multiple times.  The interpretation is left up to the particular application program.

Parsing Program Arguments
-------------------------

Here are the details about how to call the getopt function.  To use this facility, your program must include the header file unistd.h.

 -- Variable: int opterr
     If the value of this variable is nonzero, then getopt prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument.  This is the default behavior.  If you set this variable to zero, getopt does not print any messages, but it still returns ? to indicate an error.

 -- Variable: int optopt
     When getopt encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable.  You can use this for providing your own diagnostic messages.

 -- Variable: int optind
     This variable is set by getopt to the index of the next element of the ARGV array to be processed.  Once getopt has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin.  The initial value of this variable is 1.

 -- Variable: char * optarg
     This variable is set by getopt to point at the value of the option argument, for those options that accept arguments.

 -- Function: int getopt (int ARGC, char **ARGV, const char *OPTIONS)
     The getopt function gets the next option argument from the argument list specified by the ARGV and ARGC arguments. Normally these arguments' values come directly from the arguments of main.

     The OPTIONS argument is a string that specifies the option characters that are valid for this program.  An option character in this string can be followed by a colon (:) to indicate that it takes a required argument.

     If the OPTIONS argument string begins with a hyphen (-), this is treated specially.  It permits arguments without an option to be returned as if they were associated with option character \0.

     The getopt function returns the option character for the next command line option.  When no more option arguments are available, it returns -1.  There may still be more non-option arguments; you must compare the external variable optind against the ARGV parameter to check this.

     If the options has an argument, getopt returns the argument by storing it in the varables OPTARG.  You don't ordinarily need to copy the optarg string, since it is a pointer into the original ARGV array, not into a static area that might be overwritten.

     If getopt finds an option character in ARGV that was not included in OPTIONS, or a missing option argument, it returns ? and sets the external variable optopt to the actual option character.  In addition, if the external variable opterr is nonzero, getopt prints an error message.

Example of Parsing Program Arguments
------------------------------------

Here is an example showing how getopt is typically used.  The key points to notice are:

  * Normally, getopt is called in a loop.  When getopt returns -1, indicating no more options are present, the loop terminates.

  * A switch statement is used to dispatch on the return value from getopt.  In typical use, each case just sets a variable that is used later in the program.

  * A second loop is used to process the remaining non-option arguments.

          [file testopt.c.texi not included]

Here are some examples showing what this program prints with different combinations of arguments:

     % testopt
     aflag = 0, bflag = 0, cvalue = (null)

     % testopt -a -b
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -ab
     aflag = 1, bflag = 1, cvalue = (null)

     % testopt -c foo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt -cfoo
     aflag = 0, bflag = 0, cvalue = foo

     % testopt arg1
     aflag = 0, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -a arg1
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument arg1

     % testopt -c foo arg1
     aflag = 0, bflag = 0, cvalue = foo
     Non-option argument arg1

     % testopt -a -- -b
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -b

     % testopt -a -
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -

Environment Variables
=====================

When a program is executed, it receives information about the context in which it was invoked in two ways.  The first mechanism uses the ARGV and ARGC arguments to its main function, and is discussed in Program Arguments.  The second mechanism is uses environment variables and is discussed in this section.

The ARGV mechanism is typically used to pass command-line arguments specific to the particular program being invoked.  The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently accessed.

The environment variables discussed in this section are the same environment variables that you set using the assignments and the export command in the shell.  Programs executed from the shell inherit all of the environment variables from the shell.

Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes.  The set of all environment variables that have values is collectively known as the environment.

Names of environment variables are case-sensitive and must not contain the character =.  System-defined environment variables are invariably uppercase.

The values of environment variables can be anything that can be represented as a string.  A value must not contain an embedded null character, since this is assumed to terminate the string.

Environment Access
------------------

The value of an environment variable can be accessed with the getenv function.  This is declared in the header file stdlib.h.

 -- Function: char * getenv (const char *NAME)
     This function returns a string that is the value of the environment variable NAME.  You must not modify this string.  In some systems not using the GNU library, it might be overwritten by subsequent calls to getenv (but not by any other library function).  If the environment variable NAME is not defined, the value is a null pointer.

 -- Function: int putenv (const char *STRING)
     The putenv function adds or removes definitions from the environment. If the STRING is of the form NAME=VALUE, the definition is added to the environment.  Otherwise, the STRING is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

     The GNU library provides this function for compatibility with SVID; it may not be available in other systems.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see Executing a File).

 -- Variable: char ** environ
     The environment is represented as an array of strings.  Each string is of the format NAME=VALUE.  The order in which strings appear in the environment is not significant, but the same NAME must not appear more than once.  The last element of the array is a null pointer.

     This variable is not declared in any header file, but if you declare it in your own program as extern, the right thing will happen.

     If you just want to get the value of an environment variable, use getenv.

Standard Environment Variables
------------------------------

These environment variables have standard meanings. This doesn't mean that they are always present in the environment, though; it just means that if these variables are present, they have these meanings, and that you shouldn't try to use these environment variable names for some other purpose.

HOME
     This is a string representing the user's home directory, or initial default working directory.  See User Database, for a more secure way of determining this information.

LOGNAME
     This is the name that the user used to log in.  Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a process; a function like getlogin (see User Identification Functions) is better for that purpose.

PATH
     A path is a sequence of directory names which is used for searching for a file.  The variable PATH holds a path The execlp and execvp functions (see Executing a File) uses this environment variable, as do many shells and other utilities which are implemented in terms of those functions.

     The syntax of a path is a sequence of directory names separated by colons.  An empty string instead of a directory name stands for the current directory.  (see Working Directory.)

     A typical value for this environment variable might be a string like:

          .:/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local:/usr/local/bin

     This means that if the user tries to execute a program named foo, the system will look for files named ./foo, /bin/foo, /etc/foo, and so on.  The first of these files that exists is the one that is executed.

TERM
     This specifies the kind of terminal that is receiving program output. Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals.  Many programs which use the termcap library (see Find (The Termcap Library Manual)) use the TERM environment variable, for example.

TZ
     This specifies the time zone.  See Time Zone, for information about the format of this string and how it is used.

LANG
     This specifies the default locale to use for attribute categories where neither LC_ALL nor the specific environment variable for that category is set.  See Locales, for more information about locales.

LC_ALL
     This is similar to the LANG environment variable.  However, its value takes precedence over any values provided for the individual attribute category environment variables, or for the LANG environment variable.

LC_COLLATE
     This specifies what locale to use for string sorting.

LC_CTYPE
     This specifies what locale to use for character sets and character classification.

LC_MONETARY
     This specifies what locale to use for formatting monetary values.

LC_NUMERIC
     This specifies what locale to use for formatting numbers.

LC_TIME
     This specifies what locale to use for formatting date/time values.

_POSIX_OPTION_ORDER
     If this environment variable is defined, it suppresses the usual reordering of command line arguments by getopt.  See Program Argument Syntax Conventions.

Program Termination
===================

The usual way for a program to terminate is simply for its main function to return.  The exit status value returned from the main function is used to report information back to the process's parent process or shell.

A program can also terminate normally calling the exit function

In addition, programs can be terminated by signals; this is discussed in more detail in Signal Handling.  The abort function causes a terminal that kills the program.

Normal Program Termination
--------------------------

 -- Function: void exit (int STATUS)
     The exit function causes normal program termination with status STATUS.  This function does not return.

When a program terminates normally by returning from its main function or by calling exit, the following actions occur in sequence:

 1. Functions that were registered with the atexit or on_exit functions are called in the reverse order of their registration.  This mechanism allows your application to specify its own ``cleanup'' actions to be performed at program termination.  Typically, this is used to do things like saving program state information in a file, or unlock locks in shared data bases.

 2. All open streams are closed; writing out any buffered output data.  See Opening and Closing Streams.  In addition, temporary files opened with the tmpfile function are removed; see Temporary Files.

 3. _exit is called.  See Termination Internals

Exit Status
-----------

When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the exit status.  This is a value between 0 and 255 that the exiting process passes as an argument to exit.

Normally you should use the exit status to report very broad information about success or failure.  You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.

There are conventions for what sorts of status values certain programs should return.  The most common convention is simply 0 for success and 1 for failure.  Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare.  Your program should follow an existing convention if an existing convention makes sense for it.

A general convention reserves status values 128 and up for special purposes.  In particular, the value 128 is used to indicate failure to execute another program in a subprocess.  This convention is not universally obeyed, but it is a good idea to follow it in your programs.

Warning: Don't try to use the number of errors as the exit status.  This is actually not very useful; a parent process would generally not care how many errors occurred.  Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors---that is, success.

For the same reason, it does not work to use the value of errno as the exit status---these can exceed 255.

Portability note: Some non-POSIX systems use different conventions for exit status values.  For greater portability, you can use the macros EXIT_SUCCESS and EXIT_FAILURE for the conventional status value for success and failure, respectively.  They are declared in the file stdlib.h.

 -- Macro: int EXIT_SUCCESS
     This macro can be used with the exit function to indicate successful program completion.

     On POSIX systems, the value of this macro is 0.  On other systems, the value might be some other (possibly non-constant) integer expression.

 -- Macro: int EXIT_FAILURE
     This macro can be used with the exit function to indicate unsuccessful program completion in a general sense.

     On POSIX systems, the value of this macro is 1.  On other systems, the value might be some other (possibly non-constant) integer expression.  Other nonzero status values also indicate future.  Certain programs use different nonzero status values to indicate particular kinds of "non-success".  For example, diff uses status value 1 to mean that the files are different, and 2 or more to mean that there was difficulty in opening the files.

Cleanups on Exit
----------------

 -- Function: int atexit (void (*FUNCTION))
     The atexit function registers the function FUNCTION to be called at normal program termination.  The FUNCTION is called with no arguments.

     The return value from atexit is zero on success and nonzero if the function cannot be registered.

 -- Function: int on_exit (void (*FUNCTION)(int STATUS, void *ARG), void *ARG)
     This function is a somewhat more powerful variant of atexit.  It accepts two arguments, a function FUNCTION and an arbitrary pointer ARG.  At normal program termination, the FUNCTION is called with two arguments:  the STATUS value passed to exit, and the ARG.

     This function is a GNU extension, and may not be supported by other implementations.

Here's a trivial program that illustrates the use of exit and atexit:

     #include <stdio.h>
     #include <stdlib.h>

     void bye (void)
     {
       printf ("Goodbye, cruel world....\n");
     }

     void main (void)
     {
       atexit (bye);
       exit (EXIT_SUCCESS);
     }

When this program is executed, it just prints the message and exits.

Aborting a Program
------------------

You can abort your program using the abort function.  The prototype for this function is in stdlib.h.

 -- Function: void abort ()
     The abort function causes abnormal program termination, without executing functions registered with atexit or on_exit.

     This function actually terminates the process by raising a SIGABRT signal, and your program can include a handler to intercept this signal; see Signal Handling.

     Incomplete:  Why would you want to define such a handler?

Termination Internals
---------------------

The _exit function is the primitive used for process termination by exit.  It is declared in the header file unistd.h.

 -- Function: void _exit (int STATUS)
     The _exit function is the primitive for causing a process to terminate with status STATUS.  Calling this function does not execute cleanup functions registered with atexit or on_exit.

When a process terminates for any reason---either by an explicit termination call, or termination as a result of a signal---the following things happen:

  * All open file descriptors in the process are closed.  See Low-Level Input/Output.

  * The low-order 8 bits of the return status code are saved to be reported back to the parent process via wait or waitpid; see Process Completion.

  * Any child processes of the process being terminated are assigned a new parent process.  (This is the init process, with process ID 1.)

  * A SIGCHLD signal is sent to the parent process.

  * If the process is a session leader that has a controlling terminal, then a SIGHUP signal is sent to each process in the foreground job, and the controlling terminal is disassociated from that session. See Job Control.

  * If termination of a process causes a process group to become orphaned, and any member of that process group is stopped, then a SIGHUP signal and a SIGCONT signal are sent to each process in the group.  See Job Control.

Creating New Processes
======================

This section describes how your program can cause other programs to be executed.  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 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.

Running a Command
-----------------

The easy way to run another program is to use the 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.

 -- Function: int system (const char *COMMAND)
     This function executes COMMAND as a shell command.  In the GNU C library, it always uses the default shell sh to run the command. In particular, it searching the directories in PATH to find programs to execute.  The return value is -1 if it wasn't possible to create the shell process, and otherwise is the status of the shell process.  See Process Completion, for details on how this status code can be interpreted.

The system function is declared in the header file stdlib.h.

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 system (o); in this case the return value is nonzero if and only if such a processor is available.

The popen and pclose functions (see Pipe to a Subprocess) are closely related to the system function.  They allow the parent process to communicate with the standard input and output channels of the command being executed.

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.

Each process is named by a process ID number.  A unique process ID is allocated to each process when it is created.  The 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.

Processes are created with the fork system call (so the operation of creating a new process is sometimes called forking a process). The child process created by fork is an exact clone of the original 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.  This is done with the wait or waitpid functions (see Process Completion).  These functions give the parent 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 fork call returns. You can use the return value from fork to tell whether the program is running in the parent process or the child.

Having all processes run the same program is usually not very useful. But the child can execute another program using one of the exec functions; see Executing a File.  The program that the process is executing is called its process image.  Starting execution of a new program causes the process to forget all about its current process image; when the new program exits, the process exits too, instead of returning to the previous process image.

Process Identification
----------------------

The pid_t data type represents process IDs.  You can get the process ID of a process by calling getpid.  The function getppid returns the process ID of the parent of the parent of the current process (this is also known as the parent process ID). Your program should include the header files unistd.h and sys/types.h to use these functions.

 -- Data Type: pid_t
     The pid_t data type is a signed integer type which is capable of representing a process ID.  In the GNU library, this is an int.

 -- Function: pid_t getpid ()
     The getpid function returns the process ID of the current process.

 -- Function: pid_t getppid ()
     The getppid function returns the process ID of the parent of the current process.

Creating a Process
------------------

The fork function is the primitive for creating a process. It is declared in the header file unistd.h.

 -- Function: pid_t fork ()
     The fork function creates a new process.

     If the operation is successful, there are then both parent and child processes and both see fork return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process.  If the child process could not be created, a value of -1 is returned in the parent process.  The following errno error conditions are defined for this function:

     EAGAIN
          There aren't enough system resources to create another process, or the user already has too many processes running.

     ENOMEM
          The process requires more space than the system can supply.

The specific attributes of the child process that differ from the parent process are:

  * The child process has its own unique process ID.

  * The parent process ID of the child process is the process ID of its parent process.

  * 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.  See Control Operations.

  * The elapsed processor times for the child process are set to zero; see Processor Time.

  * The child doesn't inherit file locks set by the parent process. See Control Operations.

  * The child doesn't inherit alarms set by the parent process. See Setting an Alarm.

  * The set of pending signals (see 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.)

 -- Function: pid_t vfork (void)
     The vfork function is similar to fork but more efficient; however, there are restrictions you must follow to use it safely.

     While fork makes a complete copy of the calling process's address space and allows both the parent and child to execute independently, vfork does not make this copy.  Instead, the child process created with vfork shares its parent's address space until it calls one of the exec functions.  In the meantime, the parent process suspends execution.

     You must be very careful not to allow the child process created with 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 vfork!  This would leave the parent process's control information very confused.  If in doubt, use fork instead.

     Some operating systems don't really implement vfork.  The GNU C library permits you to use vfork on all systems, but actually executes fork if vfork isn't available.

Executing a File
----------------

This section describes the 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.

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 unistd.h.

 -- Function: int execv (const char *FILENAME, char *const ARGV[])
     The execv function executes the file named by FILENAME as a new process image.

     The ARGV argument is an array of null-terminated strings that is used to provide a value for the argv argument to the main function of the program to be executed.  The last element of this array must be a null pointer.  See Program Arguments, for information on how programs can access these arguments.

     The environment for the new process image is taken from the environ variable of the current process image; see Environment Variables, for information about environments.

 -- Function: int execl (const char *FILENAME, const char *ARG0, ...)
     This is similar to execv, but the ARGV strings are specified individually instead of as an array.  A null pointer must be passed as the last such argument.

 -- Function: int execve (const char *FILENAME, char *const ARGV[], char *const ENV[])
     This is similar to execv, but permits you to specify the environment for the new program explicitly as the ENV argument.  This should be an array of strings in the same format as for the environ variable; see Environment Access.

 -- Function: int execle (const char *FILENAME, const char *ARG0, char *const ENV[], ...)
     This is similar to 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 ARGV argument, and should be an array of strings in the same format as for the environ variable.

 -- Function: int execvp (const char *FILENAME, char *const ARGV[])
     The execvp function is similar to execv, except that it searches the directories listed in the PATH environment variable (see Standard Environment Variables) to find the full file name of a file from FILENAME if FILENAME does not contain a slash.

     This function is useful for executing installed system utility programs, so that the user can control where to look for them.  It is also useful in shells, for executing commands typed by the user.

 -- Function: int execlp (const char *FILENAME, const char *ARG0, ...)
     This function is like execl, except that it performs the same file name searching as the execvp function.

The size of the argument list and environment list taken together must not be greater than ARG_MAX bytes.  See System Parameters.

Incomplete:  The POSIX.1 standard requires some statement here about how null terminators, null pointers, and alignment requirements affect the total size of the argument and environment lists.

These functions normally don't return, since execution of a new program causes the currently executing program to go away completely.  A value of -1 is returned in the event of a failure.  In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for these functions:

E2BIG
     The combined size of the new program's argument list and environment list is larger than ARG_MAX bytes.

ENOEXEC
     The specified file can't be executed because it isn't in the right format.

ENOMEM
     Executing the specified file requires more storage than is available.

If execution of the new file is successful, the access time field of the file is updated as if the file had been opened.  See 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, except for the arguments and the environment, but many other attributes of the process are unchanged:

  * The process ID and the parent process ID.  See Process Creation Concepts.

  * Session and process group membership.  See Job Control Concepts.

  * Real user ID and group ID, and supplementary group IDs.  See User/Group IDs of a Process.

  * Pending alarms.  See Setting an Alarm.

  * Current working directory and root directory.  See Working Directory.

  * File mode creation mask.  See Setting Permissions.

  * Process signal mask; see Process Signal Mask.

  * Pending signals; see Blocking Signals.

  * Elapsed processor time associated with the process; see Processor Time.

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 User/Group IDs of a Process.

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 Signal Handling.

File descriptors open in the existing process image remain open in the new process image, unless they have the 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 Low-Level Input/Output.

Streams, by contrast, cannot survive through 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-exec process image has a descriptor inside it, and these descriptors do survive through exec (provided that they do not have FD_CLOEXEC set.  The new process image can reconnect these to new streams using fdopen.

Process Completion
------------------

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 sys/wait.h.

 -- Function: pid_t waitpid (pid_t PID, int *STATUS_PTR, int OPTIONS)
     The waitpid function is used to request status information from a child process whose process ID is PID.  Normally, the calling process is suspended until the child process makes status information available by terminating.

     Other values for the PID argument have special interpretations.  A value of -1 or WAIT_ANY requests status information for any child process; a value of 0 or WAIT_MYPGRP requests information for any child process in the same process group as the calling process; and any other negative value - PGID requests information for any child process whose process group ID is 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 programs, you need to call waitpid again.

     The OPTIONS argument is a bit mask.  Its value should be the bitwise OR (that is, the | operator) of zero or more of the WNOHANG and WUNTRACED flags.  You can use the WNOHANG flag to indicate that the parent process shouldn't wait; and the 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 STATUS_PTR points to, unless STATUS_PTR is a null pointer.

     The return value is normally the process ID of the child process whose status is reported.  If the WNOHANG option was specified and no child process is waiting to be noticed, a value of zero is returned.  A value of -1 is returned in case of error.  The following errno error conditions are defined for this function:

     EINTR
          The function was interrupted by delivery of a signal to the calling process.

     ECHILD
          There are no child processes to wait for, or the specified PID is not a child of the calling process.

     EINVAL
          An invalid value was provided for the OPTIONS argument.

These symbolic constants are defined as values for the PID argument to the waitpid function.

WAIT_ANY
     This constant macro (whose value is -1) specifies that waitpid should return status information about any child process.

WAIT_MYPGRP
     This constant (with value 0) specifies that waitpid should return status information about any child process in the same process group as the calling process.

     These symbolic constants are defined as flags for the OPTIONS argument to the waitpid function.  You can bitwise-OR the flags together to obtain a value to use as the argument.

WNOHANG
     This flag specifies that waitpid should return immediately instead of waiting if there is no child process ready to be noticed.

WUNTRACED
     This macro is used to specify that waitpid should also report the status of any child processes that have been stopped as well as those that have terminated.

 -- Function: pid_t wait (int *STATUS_PTR)
     This is a simplified version of waitpid, and is used to wait until any one child process terminates.

          wait (&status)

     is equivalent to:

          waitpid (-1, &status, 0)

     Here's an example of how to use waitpid to get the status from all child processes that have terminated, without ever waiting.  This function is designed to be used as a handler for SIGCHLD, the signal that indicates that at least one child process has terminated.

          void
          sigchld_handler (int signum)
          {
            int pid;
            int status;
            while (1) {
              pid = waitpid (WAIT_ANY, Estatus, WNOHANG);
              if (pid < 0) {
                perror ("waitpid");
                break;
              }
              if (pid == 0)
                break;
              notice_termination (pid, status);
            }
          }

Process Completion Status
-------------------------

If the exit status value (see Program Termination) of the child process is zero, then the status value reported by waitpid or 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 sys/wait.h.

 -- Macro: int WIFEXITED (int STATUS)
     This macro returns a non-zero value if the child process terminated normally with exit or _exit.

 -- Macro: int WEXITSTATUS (int STATUS)
     If WIFEXITED is true of STATUS, this macro returns the low-order 8 bits of the exit status value from the child process.

 -- Macro: int WIFSIGNALED (int STATUS)
     This macro returns a non-zero value if the child process terminated by receiving a signal that was not handled.

 -- Macro: int WTERMSIG (int STATUS)
     If WIFSIGNALED is true of STATUS, this macro returns the number of the signal that terminated the child process.

 -- Macro: int WCOREDUMP (int STATUS)
     This macro returns a non-zero value if the child process terminated and produced a core dump.

 -- Macro: int WIFSTOPPED (int STATUS)
     This macro returns a non-zero value if the child process is stopped.

 -- Macro: int WSTOPSIG (int STATUS)
     If WIFSTOPPED is true of STATUS, this macro returns the number of the signal that caused the child process to stop.

BSD Process Completion Functions
--------------------------------

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

These functions are declared in sys/wait.h.

 -- union Type: wait
     This data type represents program termination status values.  It has the following members:

     int w_termsig
          This member is equivalent to the WTERMSIG macro.

     int w_coredump
          This member is equivalent to the WCOREDUMP macro.

     int w_retcode
          This member is equivalent to the WEXISTATUS macro.

     int w_stopsig
          This member is equivalent to the WSTOPSIG macro.

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

 -- Function: pid_t wait3 (union wait *STATUS_PTR, int OPTIONS, void * USAGE)
     If USAGE is a null pointer, this function is equivalent to waitpid (-1, STATUS_PTR, OPTIONS).

     The USAGE argument may also be a pointer to a struct rusage object.  Information about system resources used by terminated processes (but not stopped processes) is returned in this structure.

     Incomplete:  The description of the struct rusage structure hasn't been written yet.  Put in a cross-reference here.

 -- Function: pid_t wait4 (pid_t PID, union wait *STATUS_PTR, int OPTIONS, void *USAGE)
     If USAGE is a null pointer, this function is equivalent to waitpid (PID, STATUS_PTR, OPTIONS).

     The USAGE argument may also be a pointer to a struct rusage object.  Information about system resources used by terminated processes (but not stopped processes) is returned in this structure.

     Incomplete:  The description of the struct rusage structure hasn't been written yet.  Put in a cross-reference here.

Process Creation Example
------------------------

Here is an example program showing how you might write a function similar to the built-in system.  It executes its COMMAND argument using the equivalent of sh -c COMMAND.

     #include <stddef.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/wait.h>

     /* Execute the command using this shell program.  */
     #define SHELL "/bin/sh"

     int
     my_system (char *command)
     {
       int status;
       pid_t pid;

       pid =  fork ();
       if (pid == 0) {
         /* This is the child process.  Execute the shell command. */
         execl (SHELL, SHELL, "-c", command, NULL);
         exit (EXIT_FAILURE);
       }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else {
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;
       }
       return status;
     }

There are a couple of things you should pay attention to in this example.

Remember that the first argv argument supplied to the program represents the name of the program being executed.  That is why, in the call to execl, SHELL is supplied once to name the program to execute and a second time to supply a value for argv[0].

The 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 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.  To do this, exit is called with a failure status.