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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 *arg_, ... /*, (char *)_ */);

     execle(const char *path, const char *arg_, ... /*, (char *)_, char *const envp[] */);

     execlp(const char *file, const char *arg_, ... /*, (char *)_ */);

     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 func-tions functions
     tions 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 arg_, arg1, ..., argn.  Together they describe a list of one or more pointers to null-terminated nullterminated
     terminated strings that represent the argument list available to the executed program.  The first argu-ment, argument,
     ment, 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 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() function also specifies 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 vari-able variable
     able 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 ambigu-ous ambiguous
     ous 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 without performing a suc-cessful successful
     cessful 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.)

     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.

     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 traditionally been documented and is not specified by the POSIX

     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 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

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

     The execv() function may fail and set errno for any of the errors specified for the library function

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

     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

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