Minix Man Pages

home | help
EXECL(3)                   Library Functions Manual                   EXECL(3)

NAME
       execl, execv, execle, execlp, execvp, exec, environ - execute a file

SYNOPSIS
       #include <unistd.h>

       int execl(const char *name, const char *arg0, ..., (char *) NULL)
       int execv(const char *name, char *const argv[])
       int execle(const char *name, const char *arg0, ..., (char *) NULL, char
            *const envp[])
       int execlp(const char *name, const char *arg0, ..., (char *) NULL)
       int execvp(const char *name, char *const argv[])

       extern char *const *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  file  name  (or  its  last
       component).  A null pointer argument must end the argument list.  (Note
       that the execl* functions are variable argument functions.  This  means
       that the type of the arguments beyond arg0 is not checked.  So the null
       pointer requires an explicit cast to type (char *) if not of that  type
       already.)

       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 null pointer.

       When a C program is executed, it is called as follows:

              int main(int argc, char *const argv[], char *const envp[]);

              exit(main(argc, 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 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 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 variable PATH.  Under standard Minix, if a  file  is  found
       that is executable, but does not have the proper executable header then
       it is assumed to be a shell script.  Execlp and execvp execute  /bin/sh
       to  interpret  the  script.   Under  Minix-vmd  this does not happen, a
       script must begin with #!  and the full path name of the interpreter if
       it is to be an executable script.

SEE ALSO
       execve(2), fork(2), environ(7), sh(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 and errno is set  as
       for  execve.   Even  for  the  super-user, at least one of the execute-
       permission bits must be set for a file to be executed.

4.2 Berkeley Distribution       April 25, 1986                        EXECL(3)
NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | DIAGNOSTICS