EXEC(2)

NAME
execl, execv, execle, execve, execlp, execvp, exec, exece, environ - execute a file

SYNOPSIS
execl(name, arg0, arg1, ..., argn, 0)
char *name, *arg0, *arg1, ..., *argn;

execv(name, argv)
char *name, *argv[ ];

execle(name, arg0, arg1, ..., argn, 0, envp)
char *name, *arg0, *arg1, ..., *argn, *envp[ ];

execve(name, argv, envp);
char *name, *argv[ ], *envp[ ];

extern char **environ;

DESCRIPTION
Exec in all its forms overlays the calling process with the named file, then transfers to the entry point of the core image of the file. There can be no return from a succussful exec; the calling core image is lost.

Files remain open across exec unless explicit arrangement has been made; see ioctl(2). Ignored signals remain ignored across these calls, but signals that are caught (see signal(2)) are reset to their default values.

Each user has a real user ID and group ID and an effective user ID and group ID. The real ID identifies the person using the system; the effective ID determines his access privileges. Exec changes the effective user and group ID to the owner of the executed file if the file has the `set-user-ID' or `set-group-ID' modes. The real user ID is not affected.

The name argument is a pointer to the name of the file to be executed. The pointers arg[0], arg[1] ... address null-terminated strings. Conventionally arg[0] is the name of the file.

From C, two interfaces are available. execl is useful when a known file with known arguments is being called; the arguments to execl are the character strings constituting the file and the arguments; the first argument is conventionally the same as the file name (or its last component). A 0 argument must end the argument list.

The execv version is useful when the number of arguments is unknown in advance; the arguments to execv are the name of the file to be executed and a vector of strings containing the arguments. The last argument string must be followed by a 0 pointer.

When a C program is executed, it is called as follows:

    main(argc, argv, envp)
    int argc;
    char **argv, **envp;

where argc is the argument count and argv is an array of character pointers to the arguments themselves. As indicated, argc is conventionally at least one and the first member of the array points to a string containing the name of the file.

Argv is directly usable in another execv because argv[argc] is 0.

Envp is a pointer to an array of strings that constitute the environment of the process. Each string consists of a name, an ``='', and a null-terminated value. The array of pointers is terminated by a null pointer. The shell sh(1) passes an environment entry for each global shell variable defined when the program is called. See environ(5) for some conventionally used names. The C run-time start-off routine places a copy of envp in the global cell environ, which is used by execv and execl to pass the environment to any subprograms executed by the current program. The exec routines use lower-level routines as follows to pass an environment explicitly:
    execve(file, argv, environ);
    execle(file, arg0, arg1, . . . , argn, 0, environ);

Execlp and execvp are called with the same arguments as execl and execv, but duplicate the shell's actions in searching for an executable file in a list of directories. The directory list is obtained from the environment.

FILES
/bin/sh    shell, invoked if command file found
by execlp or execvp

SEE ALSO
fork(2), environ(5)

DIAGNOSTICS
If the file cannot be found, if it is not executable, if it does not start with a valid magic number (see a.out(5)), if maximum memory is exceeded, or if the arguments require too much space, a return constitutes the diagnostic; the return value is -1. Even for the super-user, at least one of the execute-permission bits must be set for a file to be executed.

BUGS
If execvp is called to execute a file that turns out to be a shell command file, and if it is impossible to execute the shell, the values of argv[0] and argv[-1] will be modified before return.

ASSEMBLER (PDP-11)
(exec = 11.)
sys exec; name; argv

(exece = 59.)
sys exece; name; argv; envp

Plain exec is obsoleted by exece, but remains for historical reasons.

When the called file starts execution on the PDP11, the stack pointer points to a word containing the number of arguments. Just above this number is a list of pointers to the argument strings, followed by a null pointer, followed by the pointers to the environment strings and then another null pointer. The strings themselves follow; a 0 word is left at the very top of memory.

  sp→    nargs
    arg0
    ...
    argn
    0
    env0
    ...
    envm
    0

 arg0:    <arg0\0>
    ...
 env0:    <env0\0>
    0

On the Interdata 8/32, the stack begins at a conventional place (currently 0xD0000) and grows upwards. After exec, the layout of data on the stack is as follows.

	int	0
 arg0:	byte	...
	...
argp0:	int	arg0
	...
	int	0
envp0:	int	env0
	...
	int	0
 %2→	space	40
	int	nargs
	int	argp0
	int	envp0
 %3→

This arrangement happens to conform well to C calling conventions.

On a VAX-11, the stack begins at 0x80000000 and grows towards lower- numbered addresses. After exec, the layout of data on the stack is as follows.

 ap →
 fp →
 sp →	.long nargs
	.long arg0
	...
	.long argn
	.long 0
	.long env0
	...
	.long envn
	.long 0
 arg0:	.byte "arg0\0"
	...
 envn:	.byte "envn\0"
	.long 0