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


     #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;


     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

     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.


     execve(2), fork(2), environ(7), sh(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
     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.