exec(3)

NAME

execl, execlp, execle, exect, execv, execvp, execvP - exe

cute 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-termi

nated 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 pro

vide an array
of pointers to null-terminated strings that represent the

argument list
available to the new program. The first argument, by con

vention, 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 environ

ment of the
executed process by following the NULL pointer that termi

nates the list
of arguments in the argument list or the pointer to the argv

array with
an additional argument. This additional argument is an ar

ray of pointers
to null-terminated strings and must be terminated by a NULL

pointer. The
other functions take the environment for the new process im

age from the
external variable environ in the current process.

Some of these functions have special semantics.

The functions execlp(), execvp(), and execvP() will dupli

cate 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 environ

ment by
``PATH'' variable. If this variable is not specified, the

default path
is set according to the _PATH_DEFPATH definition in which is

set to
``/usr/bin:/bin''. For execvP(), the search path is speci

fied 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 continued. If the search completes with

out 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 trac

ing 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 cur

rent 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 specified 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 exis

tence and executability more carefully. In particular, EACCES for inac

cessible directories in the path prefix is no longer confused with EACCES

for files
with unsuitable execute permissions. In 4.4BSD, they re

turned 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 un

usual 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() func

tions may fail
and set errno for any of the errors specified for the li

brary 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() func

tions conform
to IEEE Std 1003.1-1988 (``POSIX.1''). The execvP() func

tion first
appeared in FreeBSD 5.2.

BSD January 24, 1994

BSD