EXEC(2) GENIX Programmer's Manual 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/held signals remain ignored/held 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 exe-
cuted. 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.
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EXEC(2) GENIX Programmer's Manual EXEC(2)
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 conven-
tionally 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 sub-
programs 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 execut-
able 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), csh(1)
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 con-
stitutes 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
Plain exec is obsoleted by exece, but remains for historical rea-
sons.
When the called file starts execution on the NS16000, 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,
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EXEC(2) GENIX Programmer's Manual EXEC(2)
followed by a null pointer, followed by the pointers to the environ-
ment 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
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