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System Calls                                              exec(2)



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

SYNOPSIS
     #include <unistd.h>

     int execl(const char *path, const  char  *arg0,  ...,  const
     char *argn, char * /*NULL*/);

     int execv(const char *path, char *const argv[]);

     int execle(const char *path, const char  *arg0,  ...,  const
     char *argn, char * /*NULL*/, char *const envp[]);

     int execve(const char *path, char *const argv[], char *const
     envp[]);

     int execlp(const char *file, const char  *arg0,  ...,  const
     char *argn, char * /*NULL*/);

     int execvp(const char *file, char *const argv[]);

DESCRIPTION
     Each of the  functions  in  the  exec  family  replaces  the
     current  process  image  with  a  new process image. The new
     image is constructed from a regular, executable file  called
     the  new  process image file. This file is either an execut-
     able object file or a file of data for an interpreter. There
     is  no  return  from a successful call to one of these func-
     tions because the calling process image is overlaid  by  the
     new process image.

     An interpreter file begins with a line of the form

          #! pathname [arg]


     where pathname is the path of the interpreter, and arg is an
     optional argument. When an interpreter file is executed, the
     system  invokes  the  specified  interpreter.  The  pathname
     specified  in  the interpreter file is passed as arg0 to the
     interpreter. If arg was specified in the  interpreter  file,
     it is passed as arg1 to the interpreter. The remaining argu-
     ments to the interpreter are arg0 through argn of the origi-
     nally  exec'd  file.  The interpreter named by pathname must
     not be an interpreter file.

     When a C-language program is executed as a  result  of  this
     call,  it  is  entered as a C-language function call as fol-
     lows:




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System Calls                                              exec(2)



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


     where argc is the argument count, argv is an array of  char-
     acter  pointers  to the arguments themselves, and envp is an
     array of character pointers to the environment strings.  The
     argv  and  environ  arrays  are  each  terminated  by a null
     pointer. The null pointer terminating the argv array is  not
     counted  in  argc. The value of argc is non-negative, and if
     greater than 0, argv[0] points to a  string  containing  the
     name  of  the file. If argc is 0, argv[0] is a null pointer,
     in which case there are no  arguments.  Applications  should
     verify  that argc is greater than 0 or that argv[0] is not a
     null pointer before dereferencing argv[0].

     The arguments specified by a program with one  of  the  exec
     functions  are  passed  on  to  the new process image in the
     main() arguments.

     The path argument points to a path name that identifies  the
     new process image file.

     The file argument is used to construct a pathname that iden-
     tifies  the  new  process  image file . If the file argument
     contains a slash character, it is used as the  pathname  for
     this  file.  Otherwise,  the  path  prefix  for this file is
     obtained by a search of the directories passed in  the  PATH
     environment  variable  (see  environ(5)). The environment is
     supplied typically by the shell. If the process  image  file
     is not a valid executable object file, execlp() and execvp()
     use the contents of that  file  as  standard  input  to  the
     shell.  In  this  case,  the  shell  becomes the new process
     image.  In  a  standard-conforming  application  (see  stan-
     dards(5)), the exec family of functions use /usr/xpg4/bin/sh
     (see  ksh(1)); otherwise, they use /usr/bin/sh (see  sh(1)).

     The arguments represented by arg0... are pointers  to  null-
     terminated  character  strings. These strings constitute the
     argument list available to the new process image.  The  list
     is  terminated  by  a null pointer. The arg0 argument should
     point to a filename that  is  associated  with  the  process
     being started by one of the exec functions.

     The argv argument is  an  array  of  character  pointers  to
     null-terminated  strings. The last member of this array must
     be a null pointer. These  strings  constitute  the  argument
     list  available  to  the  new  process  image.  The value in
     argv[0] should point to a filename that is  associated  with
     the process being started by one of the exec functions.

     The envp argument is  an  array  of  character  pointers  to
     null-terminated   strings.   These  strings  constitute  the



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System Calls                                              exec(2)



     environment for the new process image.  The  envp  array  is
     terminated   by   a  null  pointer.  For  execl(),  execv(),
     execvp(), and execlp(), the  C-language  run-time  start-off
     routine  places  a pointer to the environment of the calling
     process in the global object extern char **environ,  and  it
     is  used  to  pass the environment of the calling process to
     the new process image.

     The number of bytes available for the new process's combined
     argument   and   environment   lists   is   ARG_MAX.  It  is
     implementation-dependent whether null terminators, pointers,
     and/or any alignment bytes are included in this total.

     File descriptors open in the calling  process  image  remain
     open  in  the  new  process  image,  except  for those whose
     close-on-exec flag FD_CLOEXEC is set; (see  fcntl(2)).   For
     those  file  descriptors that remain open, all attributes of
     the open file  description,  including  file  locks,  remain
     unchanged.

     The  preferred  hardware  address   tranlation   size   (see
     memcntl(2))  for the stack and heap of the new process image
     are set to the default system page size.

     Directory streams open in  the  calling  process  image  are
     closed in the new process image.

     The state of conversion descriptors  and  message  catalogue
     descriptors  in  the new process image is undefined. For the
     new process, the equivalent of:

          setlocale(LC_ALL, "C")

     is executed at startup.

     Signals set to the default action (SIG_DFL) in  the  calling
     process  image are set to the default action in the new pro-
     cess image (see signal(3C)).   Signals  set  to  be  ignored
     (SIG_IGN) by the calling process image are set to be ignored
     by the new process image. Signals set to be  caught  by  the
     calling  process  image are set to the default action in the
     new process image (see signal(3HEAD)).  After  a  successful
     call  to  any of the exec functions, alternate signal stacks
     are not preserved and the SA_ONSTACK flag is cleared for all
     signals.

     After a successful call to any of the  exec  functions,  any
     functions  previously registered by atexit(3C) are no longer
     registered.

     The saved resource limits in the new process image  are  set
     to  be  a  copy of the process's corresponding hard and soft



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System Calls                                              exec(2)



     resource limits.

     If the ST_NOSUID bit is set for the file  system  containing
     the  new  process image file, then the effective user ID and
     effective group ID are unchanged in the new  process  image.
     If the set-user-ID mode bit of the new process image file is
     set (see chmod(2)), the effective user ID of the new process
     image  is set to the owner ID of the new process image file.
     Similarly, if the set-group-ID mode bit of the  new  process
     image file is set, the effective group ID of the new process
     image is set to the group ID of the new process image  file.
     The  real user ID and real group ID of the new process image
     remain the same as those of the calling process  image.  The
     effective  user ID and effective group ID of the new process
     image are saved (as the  saved  set-user-ID  and  the  saved
     set-group-ID for use by setuid(2).

     If the effective user-ID is  root  or  superuser,  the  set-
     user-ID  and set-group-ID bits will be honored when the pro-
     cess is being controlled by ptrace().

     Any shared memory segments attached to the  calling  process
     image  will  not  be  attached to the new process image (see
     shmop(2)). Any mappings established through mmap()  are  not
     preserved  across  an  exec.  Memory mappings created in the
     process are unmapped before the address space is rebuilt for
     the new process image. See mmap(2).

     Memory locks established by the calling process via calls to
     mlockall(3C)  or  mlock(3C)  are removed. If locked pages in
     the address space of the calling  process  are  also  mapped
     into  the  address spaces the locks established by the other
     processes will be unaffected by the call by this process  to
     the exec function. If the exec function fails, the effect on
     memory locks is unspecified.

     If _XOPEN_REALTIME is defined and has a value other than -1,
     any  named semaphores open in the calling process are closed
     as if by appropriate calls to sem_close(3RT)

     Profiling is disabled for the new process; see profil(2).

     Timers created by the calling process with timer_create(3RT)
     are  deleted before replacing the current process image with
     the new process image.

     For the SCHED_FIFO and  SCHED_RR  scheduling  policies,  the
     policy and priority settings are not changed by a call to an
     exec function.

     All open message queue descriptors in  the  calling  process
     are closed, as described in mq_close(3RT).



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System Calls                                              exec(2)



     Any outstanding asynchronous  I/O  operations  may  be  can-
     celled.  Those asynchronous I/O operations that are not can-
     celed will complete as if the  exec  function  had  not  yet
     occurred,   but  any  associated  signal  notifications  are
     suppressed. It is  unspecified  whether  the  exec  function
     itself  blocks  awaiting  such  I/O completion. In no event,
     however, will the new process  image  created  by  the  exec
     function  be  affected  by the presence of outstanding asyn-
     chronous I/O operations at the time  the  exec  function  is
     called.

     The new process also inherits the following attributes  from
     the calling process:

        o  nice value (see nice(2))

        o  scheduler class and priority (see priocntl(2))

        o  process ID

        o  parent process ID

        o  process group ID

        o  task ID

        o  supplementary group IDs

        o  semadj values (see semop(2))

        o  session membership (see exit(2) and signal(3C))

        o  real user ID

        o  real group ID

        o  project ID

        o  trace flag (see ptrace(2) request 0)

        o  time left until an alarm clock signal (see alarm(2))

        o  current working directory

        o  root directory

        o  file mode creation mask (see umask(2))

        o  file size limit (see ulimit(2))

        o  resource limits (see getrlimit(2))




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System Calls                                              exec(2)



        o  tms_utime, tms_stime, tms_cutime, and tms_cstime  (see
           times(2))

        o  file-locks (see fcntl(2) and lockf(3C))

        o  controlling terminal

        o  process signal mask (see sigprocmask(2))

        o  pending signals (see sigpending(2))

        o  processor bindings (see processor_bind(2))

        o  processor set bindings (see pset_bind(2))

     A call to any exec function from a process  with  more  than
     one  thread  results in all threads being terminated and the
     new executable image being loaded and executed. No  destruc-
     tor functions will be called.

     Upon successful completion, each of  the  functions  in  the
     exec family marks for update the st_atime field of the file.
     If an exec function failed but was able to locate  the  pro-
     cess  image  file,  whether the st_atime field is marked for
     update is unspecified. Should the function succeed, the pro-
     cess  image  file  is  considered  to  have been opened with
     open(2). The corresponding close(2) is considered  to  occur
     at a time after this open, but before process termination or
     successful completion of a subsequent call  to  one  of  the
     exec functions. The argv[] and envp[] arrays of pointers and
     the strings to which those arrays point will not be modified
     by  a  call to one of the exec functions, except as a conse-
     quence of replacing the process image.

     The saved resource limits in the new process image  are  set
     to  be  a  copy of the process's corresponding hard and soft
     limits.

RETURN VALUES
     If a function in the exec family returns to the calling pro-
     cess  image,  an  error has occurred; the return value is -1
     and errno is set to indicate the error.

ERRORS
     The exec functions will fail if:

     E2BIG The number of bytes in the new process's argument list
           is  greater than the system-imposed limit of {ARG_MAX}
           bytes. The argument list limit is sum of the  size  of
           the  argument  list plus the size of the environment's
           exported shell variables.




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System Calls                                              exec(2)



     EACCES
           Search permission is denied for a directory listed  in
           the  new  process  file's path prefix; the new process
           file is not an ordinary file; or the new process  file
           mode denies execute permission.

     EAGAIN
           Total amount of system memory available  when  reading
           using raw I/O is temporarily insufficient.

     EFAULT
           An argument points to an illegal address.

     EINTR A signal was caught during the execution of one of the
           functions in the exec family.

     ELOOP Too many symbolic links were encountered in  translat-
           ing path or file.

     ENAMETOOLONG
           The length  of  the  file  or  path  argument  exceeds
           {PATH_MAX},  or the length of a file or path component
           exceeds  {NAME_MAX}  while  {_POSIX_NO_TRUNC}  is   in
           effect.

     ENOENT
           One or more components of the new process path name of
           the file do not exist or is a null pathname.

     ENOLINK
           The path argument points to a remote machine  and  the
           link to that machine is no longer active.

     ENOTDIR
           A component of the new process path of the file prefix
           is not a directory.

     The exec functions, except for execlp() and  execvp(),  will
     fail if:

     ENOEXEC
           The new process image file has the appropriate  access
           permission but is not in the proper format.

     The exec functions may fail if:

     ENAMETOOLONG
           Pathname resolution of a  symbolic  link  produced  an
           intermediate result whose length exceeds {PATH_MAX}.

     ENOMEM
           The new process image requires  more  memory  than  is



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System Calls                                              exec(2)



           allowed  by  the  hardware or system-imposed by memory
           management constraints. See brk(2).

     ETXTBSY
           The new process image file is a pure procedure (shared
           text)  file that is currently open for writing by some
           process.

USAGE
     As the state of conversion descriptors and message catalogue
     descriptors  in the new process image is undefined, portable
     applications should not rely on their use and  should  close
     them prior to calling one of the exec functions.

     Applications that  require  other  than  the  default  POSIX
     locale should call setlocale(3C) with the appropriate param-
     eters to establish the locale of thenew process.

     The environ array should not be  accessed  directly  by  the
     application.

ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
   |        ATTRIBUTE TYPE       |        ATTRIBUTE VALUE      |
   | ____________________________|_____________________________|_
   |  Interface Stability        |  Standard                   |
   | ____________________________|_____________________________|_
   |  MT-Level                   |  execle() and  execve()  are|
   |                             |  Async-Signal-Safe          |
   |_____________________________|_____________________________|


SEE ALSO
     ksh(1), ps(1), sh(1), alarm(2), brk(2),  chmod(2),  exit(2),
     fcntl(2),   fork(2),   getrlimit(2),   memcntl(2),  mmap(2),
     nice(2), priocntl(2), profil(2),  semop(2),  shmop(2),  sig-
     pending(2),  sigprocmask(2),  times(2), umask(2), lockf(3C),
     ptrace(2),    setlocale(3C),     signal(3C),     system(3C),
     timer_create(3RT),   a.out(4),   attributes(5),  environ(5),
     standards(5)

WARNINGS
     If a  program  is  setuid  to  a  user  ID  other  than  the
     superuser, and the program is executed when the real user ID
     is super-user, then the program has some of the powers of  a
     super-user as well.






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