2.11BSD#431 - /usr/man/cat3/execl.0

EXECL(3)	    UNIX Programmer's Manual		 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 sys-
     tem 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 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 instruction giving the parent an opportunity
     to manipulate its state.  On the VAX-11 this is done by set-
     ting the trace bit in the process status longword.  Exect is


Printed 11/26/99	 August 4, 1987                         1


EXECL(3)	    UNIX Programmer's Manual		 EXECL(3)


     not available on the PDP-11.

     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 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 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.

     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 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


Printed 11/26/99	 August 4, 1987                         2


EXECL(3)	    UNIX Programmer's Manual		 EXECL(3)


     shell, the values of argv[0] and argv[-1] will be modified
     before return.


Printed 11/26/99	 August 4, 1987                         3