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

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

     Standard C Library (libc, -lc)

     #include <unistd.h>

     extern char **environ;

     execl(const char *path, const char *arg0, ... /*, (char *)0 */);

     execle(const char *path, const char *arg0, ...
         /*, (char *)0, char *const envp[] */);

     execlp(const char *file, const char *arg0, ... /*, (char *)0 */);

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

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

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

     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

     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. program.
     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 pointers
     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 arguments
     ments in the argument list or the pointer to the argv array with an addi-tional additional
     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 isn't 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 spe-cially. 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 execute
     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 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.)

     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.

     /bin/sh  The shell.

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

     The execv() function may fail and set errno for any of the errors speci-fied specified
     fied for the library function execve(2).

     sh(1), execve(2), fork(2), ptrace(2), environ(7)

     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 traditionally
     tionally been documented and is not specified by the POSIX standard.

     Traditionally, the execlp() and execvp() functions 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 executability
     cutability more carefully.  In particular, EACCES for inaccessible direc-tories directories
     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).

     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