EXEC(3) BSD Library Functions Manual EXEC(3)NAME
execl, execle, execlp, 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 *arg0, ... /*, (char *)0 */);
int
execle(const char *path, const char *arg0, ...
/*, (char *)0, char *const envp[] */);
int
execlp(const char *file, const char *arg0, ... /*, (char *)0 */);
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 *arg0 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 pro‐
gram. 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 execv(), execvp(), and execvP() functions provide an array of point‐
ers 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() function also specifies the environment of the executed
process by following the NULL pointer that terminates the list of argu‐
ments in the argument list or the pointer to the argv array with an addi‐
tional 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 argu‐
ment 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
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.)
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 tradi‐
tionally 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 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).
ERRORS
The execl(), execle(), execlp(), execvp(), and execvP() functions may
fail and set errno for any of the errors specified for the library func‐
tions execve(2) and malloc(3).
The execv() function may fail and set errno for any of the errors speci‐
fied for the library function execve(2).
SEE ALSOsh(1), execve(2), fork(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
