EXECL(3) BSD EXECL(3)
NAME
execl, execv, execle, execlp, execvp, exec, execve, exect, 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[];
exect(name, argv, envp)
char *name, *argv[], *envp[];
extern char **environ;
DESCRIPTION
These routines provide various interfaces to the execve system call.
Refer to execve(2) for a description of their properties; only brief
descriptions are provided here.
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.
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.
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 filename (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.
The exect version is used when the executed file is to be manipulated
with ptrace(2). The program is forced to single step a single
instruction, giving the parent an opportunity to manipulate its state.
On the VAX-11 this is done by setting the trace bit in the process status
longword.
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 1 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 equal
sign (=), 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(7) 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.
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
execve(2), fork(2), environ(7), 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
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.