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Plan 9 - man page for exec (plan9 section 2)

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EXEC(2) 										  EXEC(2)

       exec, execl, _clock - execute a file

       #include <u.h>
       #include <libc.h>

       int exec(char *name, char* argv[])

       int execl(char *name, ...)

       long *_clock;

       Exec and execl overlay the calling process with the named file, then transfer to the entry
       point of the image of the file.

       Name points to the name of the file to be executed; it must not be a  directory,  and  the
       permissions  must allow the current user to execute it (see stat(2)).  It should also be a
       valid binary image, as defined in the a.out(6) for the current machine architecture, or	a
       shell  script  (see  rc(1)).  The first line of a shell script must begin with followed by
       the name of the program to interpret the file and any initial arguments to  that  program,
       for example

	      ls | mc

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

	      void main(int argc, char *argv[])

       Argv  is a copy of the array of argument pointers passed to exec; that array must end in a
       null pointer, and argc is the number of elements before the null pointer.  By  convention,
       the  first  argument should be the name of the program to be executed.  Execl is like exec
       except that argv will be an array of the parameters that follow name  in  the  call.   The
       last argument to execl must be a null pointer.

       For  a  file  beginning	#!,  the  arguments passed to the program (/bin/rc in the example
       above) will be the name of the file being executed, any arguments on  the  #!   line,  the
       name of the file again, and finally the second and subsequent arguments given to the orig-
       inal exec call.	The result honors the two conventions of a program accepting as  argument
       a file to be interpreted and argv[0] naming the file being executed.

       Most  attributes  of the calling process are carried into the result; in particular, files
       remain open across exec (except those opened with OCEXEC OR'd  into  the  open  mode;  see
       open(2));  and  the  working directory and environment (see env(3)) remain the same.  How-
       ever, a newly exec'ed process has no notification handler (see notify(2)).

       When the new program begins, the global cell _clock is set to the address of a  cell  that
       keeps  approximate  time  expended  by the process at user level.  The time is measured in
       milliseconds but is updated at a system-dependent lower rate.   This  clock  is	typically
       used by the profiler but is available to all programs.

       The above conventions apply to C programs; the raw system interface to the new image is as
       follows: the word pointed to by the stack pointer is argc; the words beyond that  are  the
       zeroth  and  subsequent	elements of argv, followed by a terminating null pointer; and the
       return register (e.g.  R0 on the 68020) contains the address of the clock.

       In Alef, the intent and syntax are the same but the implementation is different.  Exec (or
       execl;  this  description applies to either) may be called only by a proc holding a single
       task, typically the implicit main task of the proc.  First, access(2) is called to see  if
       the  named  file  exists and has execute permission.  If not, exec returns -1 immediately.
       Otherwise, it will never return: it frees resources private to the invoking proc and calls
       the  exec system call.  If this fails, it calls the bare _exits system call (see exits(2))
       with the error string as argument.  Therefore, if the file looks executable,  the  calling
       process is lost, whether the exec succeeds or not.


       intro(2), stat(2)

       If  these  functions fail, they return and set errstr.  There can be no return from a suc-
       cessful exec or execl; the calling image is lost.

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