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EXEC(3)			 BSD Library Functions Manual		       EXEC(3)

     execl, execlp, execle, exect, execv, execvp -- execute a file

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     extern char **environ;

     execl(const char *path, const char	*arg, ...);

     execlp(const char *file, const char *arg, ...);

     execle(const char *path, const char *arg, ...);

     exect(const char *path, char *const argv[], char *const envp[]);

     execv(const char *path, char *const argv[]);

     execvp(const char *file, char *const argv[]);

     The exec family of	functions replaces the current process image with a
     new process image.	 The functions described in this manual	page are
     front-ends	for the	function execve(2).  (See the manual page for
     execve(2) for detailed information	about the replacement of the current

     The initial argument for these functions is the pathname of a file	which
     is	to be executed.

     The const char *arg and subsequent	ellipses in the	execl(), execlp(), and
     execle() functions	can be thought of as arg0, arg1, ..., argn.  Together
     they describe a list of one or more pointers to null-terminated strings
     that represent the	argument list available	to the executed	program.  The
     first argument, by	convention, should point to the	file name associated
     with the file being executed.  The	list of	arguments must be terminated
     by	a NULL pointer.

     The exect(), execv(), and execvp()	functions provide an array of pointers
     to	null-terminated	strings	that represent the argument list available to
     the new program.  The first argument, by convention, should point to the
     file name associated with the file	being executed.	 The array of pointers
     must be terminated	by a NULL pointer.

     The execle() and exect() functions	also specify the environment of	the
     executed process by following the NULL pointer that terminates the	list
     of	arguments in the parameter list	or the pointer to the argv array with
     an	additional parameter.  This additional parameter is an array of	point-
     ers to null-terminated strings and	must be	terminated by a	NULL pointer.
     The other functions take the environment for the new process image	from
     the external variable environ in the current process.

     Some of these functions have special semantics.

     The functions execlp() and	execvp() will duplicate	the actions of the
     shell in searching	for an executable file if the specified	file name does
     not contain a slash "/" character.	 The search path is the	path specified
     in	the environment	by "PATH" variable.  If	this variable isn't specified,
     the default path is set according to the _PATH_DEFPATH definition in
     <paths.h>,	which is set to	"/usr/bin:/bin".  In addition, certain errors
     are treated specially.

     If	an error is ambiguous (for simplicity, we shall	consider all errors
     except ENOEXEC as being ambiguous here, although only the critical	error
     EACCES is really ambiguous), then these functions will act	as if they
     stat the file to determine	whether	the file exists	and has	suitable exe-
     cute permissions.	If it does, they will return immediately with the
     global variable errno restored to the value set by	execve().  Otherwise,
     the search	will be	continued.  If the search completes without performing
     a successful execve() or terminating due to an error, these functions
     will return with the global variable errno	set to EACCES or ENOENT	ac-
     cording to	whether	at least one file with suitable	execute	permissions
     was found.

     If	the header of a	file isn't recognized (the attempted execve() returned
     ENOEXEC), these functions will execute the	shell with the path of the
     file as its first argument.  (If this attempt fails, no further searching
     is	done.)

     The function exect() executes a file with the program tracing facilities
     enabled (see ptrace(2)).

     If	any of the exec() functions returns, an	error will have	occurred.  The
     return value is -1, and the global	variable errno will be set to indicate
     the error.

     /bin/sh  The shell.

     Execl(), execle(),	execlp() and execvp() may fail and set errno for any
     of	the errors specified for the library functions execve(2) and

     Exect() and execv() may fail and set errno	for any	of the errors speci-
     fied for the library function execve(2).

     sh(1), execve(2), fork(2),	ktrace(2), ptrace(2), environ(7)

     Historically, the default path for	the execlp() and execvp() functions
     was ":/bin:/usr/bin".  This was changed to	place the current directory
     last to enhance system security.

     The behavior of execlp() and execvp() when	errors occur while attempting
     to	execute	the file is not	quite historic practice, and has not tradi-
     tionally been documented and is not specified by the POSIX	standard.

     Traditionally, the	functions execlp() and execvp()	ignored	all errors ex-
     cept for the ones described above and ETXTBSY, upon which they retried
     after sleeping for	several	seconds, and ENOMEM and	E2BIG, upon which they
     returned.	They now return	for ETXTBSY, and determine existence and exe-
     cutability	more carefully.	 In particular,	EACCES for inaccessible	direc-
     tories in the path	prefix is no longer confused with EACCES for files
     with unsuitable execute permissions.  In 4.4BSD, they returned upon all
     errors except EACCES, ENOENT, ENOEXEC and ETXTBSY.	 This was inferior to
     the traditional error handling, since it breaks the ignoring of errors
     for path prefixes and only	improves the handling of the unusual ambiguous
     error EFAULT and the unusual error	EIO.  The behaviour was	changed	to
     match the behaviour of sh(1).

     Execl(), execv(), execle(), execlp() and execvp() conform to IEEE Std
     1003.1-1988 ("POSIX.1").

BSD			       January 24, 1994				   BSD


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