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

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

     Standard C	Library	(libc, -lc)

     #include <unistd.h>

     extern char **environ;

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

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

     execle(const char *path, const char *arg, ..., NULL, char *const envp[]);

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

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

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

     execvP(const char *file, const char *search_path, 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(), execvp(), 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 ar-
     ray 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 argument list or the pointer to the argv array	with
     an	additional argument.  This additional argument is an array of pointers
     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(), execvp(), 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.  For execlp()	and execvp(),
     search path is the	path specified in the environment by "PATH" variable.
     If	this variable is not specified,	the default path is set	according to
     the _PATH_DEFPATH definition in <paths.h>,	which is set to
     "/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin".  For
     execvP(), the search path is specified as an argument to the function.
     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 is	not recognized (the attempted execve() re-
     turned 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.

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

     The execl(), execle(), execlp(), execvp() and execvP() functions may fail
     and set errno for any of the errors specified for the library functions
     execve(2) and malloc(3).

     The exect() and execv() functions may fail	and set	errno for any of the
     errors specified for the library function execve(2).

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

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

     The exec()	function appeared in Version 1 AT&T UNIX.  The execl() and
     execv() functions appeared	in Version 2 AT&T UNIX.	 The execlp(),
     execle(), execve(), and execvp() functions	appeared in Version 7 AT&T
     UNIX.  The	execvP() function first	appeared in FreeBSD 5.2.

     The type of the argv and envp parameters to execle(), exect(), execv(),
     execvp(), and execvP() is a historical accident and no sane implementa-
     tion should modify	the provided strings.  The bogus parameter types trig-
     ger false positives from const correctness	analyzers.  On FreeBSD,	the
     __DECONST() macro may be used to work around this limitation.

     Due to a fluke of the C standard, on platforms other than FreeBSD the
     definition	of NULL	may be the untyped number zero,	rather than a (void
     *)0 expression.  To distinguish the concepts, they	are referred to	as a
     "null pointer constant" and a "null pointer", respectively.  On exotic
     computer architectures that FreeBSD does not support, the null pointer
     constant and null pointer may have	a different representation.  In	gen-
     eral, where this document and others reference a NULL value, they actu-
     ally imply	a null pointer.	 E.g., for portability to non-FreeBSD operat-
     ing systems on exotic computer architectures, one may use (char *)NULL in
     place of NULL when	invoking execl(), execle(), and	execlp().

BSD				March 22, 2020				   BSD


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