EXEC(3)

NAME
execl, execlp, execle, exect, execv, execvp - 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 exect( const char *path, char *const argv[]);
int execv( const char *path, char *const argv[]);
int execvp( const char *file, 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 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, 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 begin 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 parameter list or the pointer to the argv array with an additional parameter. This additional parameter 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 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. The search path is the path specified in the environment by the PATH variable. If this variable isn't specified, the default path ``/bin:/usr/bin:'' is used (is this true for Linux?). 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.)

If the file is currently busy (the attempted execve returned ETXTBUSY), these functions will sleep for several seconds, periodically re-attempting to execute the file. (Is this true for Linux?)

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

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).

Exect and 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), trace(2), environ(5), ptrace(2)

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 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.

STANDARDS
Execl, execv, execle, execlp and execvp conform to IEEE Std1003.1-88 (``POSIX.1'').