EXEC(3)

NAME
execl, execlp, execle, execv, execvp, execvpe - execute a file

SYNOPSIS
#include <unistd.h>
extern char **environ;
int
execl(const char *path, const char *arg, ...);
int
execlp(const char *file, const char *arg, ...);
int
execle(const char *path, const char *arg, ..., char *const envp[]);
int
execv(const char *path, char *const argv[]);
int
execvp(const char *file, char *const argv[]);
int
execvpe(const char *file, char *const argv[], char *const envp[]);

DESCRIPTION
The exec family of functions replace 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 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 NUL-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 execv(), execvp() and execvpe() functions provide an array of pointers to NUL-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 itself.

The execle() function also specifies 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 pointers to NUL-terminated strings and must be terminated by a null pointer itself. 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 execvpe() 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, _PATH_DEFPATH from <paths.h> is used instead, its value being: /usr/bin:/bin:/usr/sbin:/sbin:/usr/X11R6/bin:/usr/local/bin

In addition, certain errors are treated specially.

If permission is denied for a file (the attempted execve returned EACCES), these functions will continue searching the rest of the search path. If no other file is found, however, they will return with the global variable errno set to EACCES.

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.)

RETURN VALUES
If any of the exec functions return, an error has occurred. The return value is -1, and the global variable errno will be set to indicate the error.

FILES
/bin/sh
    default shell program

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

execv() 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
Historically, the default path for the execlp() and execvp() functions was .:/bin:/usr/bin. This was changed to improve security and behaviour.

The behavior of execlp() and execvp() when errors occur while attempting to execute the file is historic practice, but 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 ENOMEM and E2BIG, upon which they returned. They now return if any error other than the ones described above occurs.

execl(), execv(), execle(), execlp() and execvp() conform to p1003.1-88. execvpe() is a GNU extension.

HISTORY
The functions execl() and execv() first appeared in AT&T UNIX. A predecessor to execvp(), pexec(), first appeared in the Programmer's Workbench (PWB/UNIX). The functions execle(), execlp(), execve(), and execvp() first appeared in AT&T UNIX.