EXEC(2) 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 successful
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 argu-
ments 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.
EXEC(2) EXEC(2)
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 indi-
cated, 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 point-
ers 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 conven-
tionally 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 sub-
programs executed by the current program. The exec routines
use lower-level routines as follows to pass an environment
explicitly:
execle(file, arg0, arg1, . . . , argn, 0, environ);
execve(file, argv, 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
EXEC(2) EXEC(2)
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 exe-
cuted.
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
(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
EXEC(2) EXEC(2)
...
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 con-
ventions.