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 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 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 suit-
     able 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 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 exe-
     cute 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 direc-
     tory 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 tra-
     ditionally 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 inaccessible directories 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 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