The Basic Program/System Interface
**********************************

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.  Though it may have multiple threads of control within the same program and a program may be composed of multiple logically separate modules, a process always executes exactly one program.

Note that we are using a specific definition of ``program'' for the purposes of this manual, which corresponds to a common definition in the context of Unix system.  In popular usage, ``program'' enjoys a much broader definition; it can refer for example to a system's kernel, an editor macro, a complex package of software, or a discrete section of code executing within a process.

Writing the program is what this manual is all about.  This chapter explains the most basic interface between your program and the system that runs, or calls, it.  This includes passing of parameters (arguments and environment) from the system, requesting basic services from the system, and telling the system the program is done.

A program starts another program with the exec family of system calls. This chapter looks at program startup from the execee's point of view.  To see the event from the execor's point of view, see Executing a File.

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

The system starts a C program by calling the function main.  It is up to you to write a function named main---otherwise, you won't even be able to link your program without errors.

In ISO C 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 given in the shell command used to invoke the program; thus, in cat foo bar, the arguments are foo and bar.  The only way a program can look at its command line arguments is via the arguments of main.  If main doesn't take arguments, then you cannot get at the command line.

The value of the ARGC argument is the number of command line arguments.  The ARGV argument is a vector of C strings; its elements are the individual command line argument strings.  The file name of the program being run is also included in the vector as the first element; the value of ARGC counts this element.  A null pointer always follows the last element: ARGV[ARGC] is this null pointer.

For the command cat foo bar, ARGC is 3 and ARGV has three elements, "cat", "foo" and "bar".

In Unix systems you can define main a third way, using three arguments:

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

The first two arguments are just the same.  The third argument ENVP gives the program's environment; it is the same as the value of environ.  See Environment Variables.  POSIX.1 does not allow this three-argument form, so to be portable it is best to write main to take two arguments, and use the value of environ.

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

POSIX recommends these conventions for command line arguments. getopt (see Getopt) and argp_parse (see Argp) make it easy to implement them.

  * 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 characters (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 not 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 implementations of getopt and argp_parse in theglibc normally make 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.  They do 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.

  * 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.

GNU adds long options to these conventions.  Long options consist of -- followed by a name made of alphanumeric characters and dashes.  Option names are typically one to three words long, with hyphens to separate words.  Users can abbreviate the option names as long as the abbreviations are unique.

To specify an argument for a long option, write --NAME=VALUE.  This syntax enables a long option to accept an argument that is itself optional.

Eventually, gnusystems will provide completion for long option names in the shell.

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

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 (see Getopt) or argp_parse (see Argp) to do the parsing.

getopt is more standard (the short-option only version of it is a part of the POSIX standard), but using argp_parse is often easier, both for very simple and very complex option structures, because it does more of the dirty work for you.

     [file getopt.texi not included]
     [file argp.texi not included]

Parsing of Suboptions
.....................

Having a single level of options is sometimes not enough.  There might be too many options which have to be available or a set of options is closely related.

For this case some programs use suboptions.  One of the most prominent programs is certainly mount(8).  The -o option take one argument which itself is a comma separated list of options.  To ease the programming of code like this the function getsubopt is available.

 -- Function: int getsubopt (char **OPTIONP, char *const *TOKENS, char **VALUEP)

     The OPTIONP parameter must be a pointer to a variable containing the address of the string to process.  When the function returns the reference is updated to point to the next suboption or to the terminating \0 character if there is no more suboption available.

     The TOKENS parameter references an array of strings containing the known suboptions.  All strings must be \0 terminated and to mark the end a null pointer must be stored.  When getsubopt finds a possible legal suboption it compares it with all strings available in the TOKENS array and returns the index in the string as the indicator.

     In case the suboption has an associated value introduced by a = character, a pointer to the value is returned in VALUEP.  The string is \0 terminated.  If no argument is available VALUEP is set to the null pointer.  By doing this the caller can check whether a necessary value is given or whether no unexpected value is present.

     In case the next suboption in the string is not mentioned in the TOKENS array the starting address of the suboption including a possible value is returned in VALUEP and the return value of the function is -1.

Parsing of Suboptions Example
-----------------------------

The code which might appear in the mount(8) program is a perfect example of the use of getsubopt:

          [file subopt.c.texi not included]

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 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 used.

The environment variables discussed in this section are the same environment variables that you set using 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.

Libraries should use secure_getenv instead of getenv, so that they do not accidentally use untrusted environment variables. Modifications of environment variables are not allowed in multi-threaded programs.  The getenv and secure_getenv functions can be safely used in multi-threaded programs.

 -- 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 non-Unix systems not using theglibc, 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: char * secure_getenv (const char *NAME)
     This function is similar to getenv, but it returns a null pointer if the environment is untrusted.  This happens when the program file has SUID or SGID bits set.  General-purpose libraries should always prefer this function over getenv to avoid vulnerabilities if the library is referenced from a SUID/SGID program.

     This function is a GNU extension.

 -- Function: int putenv (char *STRING)
     prelim:mtsenv 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.

     If the function is successful it returns 0.  Otherwise the return value is nonzero and errno is set to indicate the error.

     The difference to the setenv function is that the exact string given as the parameter STRING is put into the environment.  If the user should change the string after the putenv call this will reflect automatically in the environment.  This also requires that STRING not be an automatic variable whose scope is left before the variable is removed from the environment.  The same applies of course to dynamically allocated variables which are freed later.

     This function is part of the extended Unix interface.  You should define _XOPEN_SOURCE before including any header.

 -- Function: int setenv (const char *NAME, const char *VALUE, int REPLACE)
     prelim:mtsenv The setenv function can be used to add a new definition to the environment.  The entry with the name NAME is replaced by the value NAME=VALUE.  Please note that this is also true if VALUE is the empty string.  To do this a new string is created and the strings NAME and VALUE are copied.  A null pointer for the VALUE parameter is illegal.  If the environment already contains an entry with key NAME the REPLACE parameter controls the action.  If replace is zero, nothing happens.  Otherwise the old entry is replaced by the new one.

     Please note that you cannot remove an entry completely using this function.

     If the function is successful it returns 0.  Otherwise the environment is unchanged and the return value is -1 and errno is set.

     This function was originally part of the BSD library but is now part of the Unix standard.

 -- Function: int unsetenv (const char *NAME)
     prelim:mtsenv Using this function one can remove an entry completely from the environment.  If the environment contains an entry with the key NAME this whole entry is removed.  A call to this function is equivalent to a call to putenv when the VALUE part of the string is empty.

     The function return -1 if NAME is a null pointer, points to an empty string, or points to a string containing a = character. It returns 0 if the call succeeded.

     This function was originally part of the BSD library but is now part of the Unix standard.  The BSD version had no return value, though.

There is one more function to modify the whole environment.  This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1.  But this never happened.  But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.

 -- Function: int clearenv (void)
     prelim:mtsenv The clearenv function removes all entries from the environment. Using putenv and setenv new entries can be added again later.

     If the function is successful it returns 0.  Otherwise the return value is nonzero.

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 declared in the header file unistd.h.

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

Unix systems, and gnusystems, pass the initial value of environ as the third argument to main. See Program Arguments.

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

These environment variables have standard meanings.  This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings.  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.

     The user can set HOME to any value. If you need to make sure to obtain the proper home directory for a particular user, you should not use HOME; instead, look up the user's name in the user database (see User Database).

     For most purposes, it is better to use HOME, precisely because this lets the user specify the value.

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 program; a function like getlogin (see Who Logged In) is better for that purpose.

     For most purposes, it is better to use LOGNAME, precisely because this lets the user specify the value.

PATH

     A path is a sequence of directory names which is used for searching for a file.  The variable PATH holds a path used for searching for programs to be run.

     The execlp and execvp functions (see Executing a File) use 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/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 TZ Variable, 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

     If this environment variable is set it overrides the selection for all the locales done using the other LC_* environment variables.  The value of the other LC_* environment variables is simply ignored in this case.

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_MESSAGES

     This specifies what locale to use for printing messages and to parse responses.

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.

NLSPATH

     This specifies the directories in which the catopen function looks for message translation catalogs.

_POSIX_OPTION_ORDER

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

Auxiliary Vector
================

When a program is executed, it receives information from the operating system about the environment in which it is operating.  The form of this information is a table of key-value pairs, where the keys are from the set of AT_ values in elf.h.  Some of the data is provided by the kernel for libc consumption, and may be obtained by ordinary interfaces, such as sysconf.  However, on a platform-by-platform basis there may be information that is not available any other way.

Definition of getauxval
-----------------------

 -- Function: unsigned long int getauxval (unsigned long int TYPE)
     This function is used to inquire about the entries in the auxiliary vector.  The TYPE argument should be one of the AT_ symbols defined in elf.h.  If a matching entry is found, the value is returned; if the entry is not found, zero is returned and errno is set to ENOENT.

For some platforms, the key AT_HWCAP is the easiest way to inquire about any instruction set extensions available at runtime.  In this case, there will (of necessity) be a platform-specific set of HWCAP_ values masked together that describe the capabilities of the cpu on which the program is being executed.

System Calls
============

A system call is a request for service that a program makes of the kernel.  The service is generally something that only the kernel has the privilege to do, such as doing I/O.  Programmers don't normally need to be concerned with system calls because there are functions in theglibc to do virtually everything that system calls do. These functions work by making system calls themselves.  For example, there is a system call that changes the permissions of a file, but you don't need to know about it because you can just use theglibc's chmod function.

System calls are sometimes called kernel calls.

However, there are times when you want to make a system call explicitly, and for that, theglibc provides the syscall function. syscall is harder to use and less portable than functions like chmod, but easier and more portable than coding the system call in assembler instructions.

syscall is most useful when you are working with a system call which is special to your system or is newer than theglibc you are using.  syscall is implemented in an entirely generic way; the function does not know anything about what a particular system call does or even if it is valid.

The description of syscall in this section assumes a certain protocol for system calls on the various platforms on which theglibc runs.  That protocol is not defined by any strong authority, but we won't describe it here either because anyone who is coding syscall probably won't accept anything less than kernel and C library source code as a specification of the interface between them anyway.

syscall is declared in unistd.h.

 -- Function: long int syscall (long int SYSNO, ...)

     syscall performs a generic system call.

     SYSNO is the system call number.  Each kind of system call is identified by a number.  Macros for all the possible system call numbers are defined in sys/syscall.h

     The remaining arguments are the arguments for the system call, in order, and their meanings depend on the kind of system call.  Each kind of system call has a definite number of arguments, from zero to five. If you code more arguments than the system call takes, the extra ones to the right are ignored.

     The return value is the return value from the system call, unless the system call failed.  In that case, syscall returns -1 and sets errno to an error code that the system call returned.  Note that system calls do not return -1 when they succeed.

     If you specify an invalid SYSNO, syscall returns -1 with errno = ENOSYS.

     Example:

          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>

          ...

          int rc;

          rc = syscall(SYS_chmod, "/etc/passwd", 0444);

          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

     This, if all the compatibility stars are aligned, is equivalent to the following preferable code:

          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>

          ...

          int rc;

          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

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 by 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 signal that kills the program.

Normal Termination
------------------

A process terminates normally when its program signals it is done by calling exit.  Returning from main is equivalent to calling exit, and the value that main returns is used as the argument to exit.

 -- Function: void exit (int STATUS)
     prelim:exit The exit function tells the system that the program is done, which causes it to terminate the process.

     STATUS is the program's exit status, which becomes part of the process' termination status.  This function does not return.

Normal termination causes the following actions:

 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 unlocking locks in shared data bases.

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

 3. _exit is called, terminating the program.  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 failures.  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.

Don't confuse a program's exit status with a process' termination status. There are lots of ways a process can terminate besides having its program finish.  In the event that the process termination is caused by program termination (i.e., exit), though, the program's exit status becomes part of the process' termination status.

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

Your program can arrange to run its own cleanup functions if normal termination happens.  If you are writing a library for use in various application programs, then it is unreliable to insist that all applications call the library's cleanup functions explicitly before exiting.  It is much more robust to make the cleanup invisible to the application, by setting up a cleanup function in the library itself using atexit or on_exit.

 -- Function: int atexit (void (*FUNCTION) (void))
     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 included in theglibc only for compatibility for SunOS, and may not be supported by other implementations.

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

          [file atexit.c.texi not included]

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 (void)
     The abort function causes abnormal program termination.  This does not execute cleanup 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.

Future Change Warning: Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function.  We would be required to say that this is not an acceptable way of terminating a program.

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.

 -- Function: void _Exit (int STATUS)
     The _Exit function is the ISO C equivalent to _exit. The ISO C committee members were not sure whether the definitions of _exit and _Exit were compatible so they have not used the POSIX name.

     This function was introduced in ISO C99 and is declared in stdlib.h.

When a process terminates for any reason---either because the program terminates, or as a result of a signal---the following things happen:

  * All open file descriptors in the process are closed.  See Low-Level I/O. Note that streams are not flushed automatically when the process terminates; see I/O on Streams.

  * A process exit status is saved to be reported back to the parent process via wait or waitpid; see Process Completion.  If the program exited, this status includes as its low-order 8 bits the program exit status.

  * Any child processes of the process being terminated are assigned a new parent process.  (On most systems, including GNU, 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.