EXECL(3) Library Functions Manual EXECL(3)
execl, execv, execle, execlp, execvp, exec, exece, exect, 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;
exect(name, argv, envp)
char *name, *argv, *envp;
extern char **environ;
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
The name argument is a pointer to the name of the file to be executed.
The pointers arg, arg ... address null-terminated strings. Con-
ventionally arg 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 charac-
ter 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.
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 instruc-
tion 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
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 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 environ-
ment 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(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
/bin/sh shell, invoked if command file found by execlp or execvp
execve(2), fork(2), environ(7), csh(1)
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 consti-
tutes 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
If execvp is called to execute a file that turns out to be a shell com-
mand file, and if it is impossible to execute the shell, the values of
argv and argv[-1] will be modified before return.
4th Berkeley Distribution 1 April 1981 EXECL(3)