execl, execv, execle, execve, execlp, execvp, exec, exece, environ - execute a file
execl(name, arg0, arg1, ..., argn, 0)
char *name, *arg0, *arg1, ..., *argn;
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;
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 sig-
nal(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 privi-
leges. 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, arg ... address null-terminated strings. Conventionally arg is the name of
From C, two interfaces are available. Execl is useful when a known file with known argu-
ments 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 argu-
ments 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)
char **argv, **envp;
where argc is the argument count and argv is an array of character pointers to the argu-
ments 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 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 direc-
tory list is obtained from the environment.
/bin/sh shell, invoked if command file found by execlp or execvp
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 exe-
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 and argv[-1] will be modified
(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.
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.
arg0: byte ...
argp0: int arg0
envp0: int env0
%2-> space 40
This arrangement happens to conform well to C calling conventions.