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

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     extern char **environ;

     int
     execl(const char *path, const char *arg, ... /*, (char *)0 */);

     int
     execlp(const char *file, const char *arg, ... /*, (char *)0 */);

     int
     execle(const char *path, const char *arg, ... /*, (char *)0, char *const envp[] */);

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

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

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

     int
     execvP(const char *file, const char *search_path, char *const argv[]);

DESCRIPTION
     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
     process.)

     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 exe-
     cuted 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 exe-
     cuted.  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 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 ``/usr/bin:/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 execute 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 contin-
     ued.  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
     according to whether at least one file with suitable execute permissions was found.

     If the header of a file is not 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)).

RETURN VALUES
     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.

FILES
     /bin/sh  The shell.

COMPATIBILITY
     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 traditionally been documented and is not speci-
     fied by the POSIX standard.

     Traditionally, the functions execlp() and execvp() ignored all errors except 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 executability more carefully.  In particular, EACCES for inaccessible directo-
     ries 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).

ERRORS
     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).

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

STANDARDS
     The execl(), execv(), execle(), execlp() and execvp() functions conform to IEEE Std
     1003.1-1988 (``POSIX.1'').  The execvP() function first appeared in FreeBSD 5.2.

BSD					 January 24, 1994				      BSD
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