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EXEC(3P)                   POSIX Programmer's Manual                  EXEC(3P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

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

SYNOPSIS
       #include <unistd.h>

       extern char **environ;
       int execl(const char *path, const char *arg0, ... /*, (char *)0 */);
       int execv(const char *path, char *const argv[]);
       int execle(const char *path, const char *arg0, ... /*,
              (char *)0, char *const envp[]*/);
       int execve(const char *path, char *const argv[], char *const envp[]);
       int execlp(const char *file, const char *arg0, ... /*, (char *)0 */);
       int execvp(const char *file, char *const argv[]);


DESCRIPTION
       The  exec  family  of functions shall replace the current process image with a new process image. The new image
       shall be constructed from a regular, executable file called the new process  image  file.  There  shall  be  no
       return from a successful exec, because the calling process image is overlaid by the new process image.

       When  a  C-language  program is executed as a result of this call, it shall be entered as a C-language function
       call as follows:


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

       where argc is the argument count and argv is an array of character pointers to  the  arguments  themselves.  In
       addition, the following variable:


              extern char **environ;

       is  initialized as a pointer to 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.

       Conforming  multi-threaded  applications shall not use the environ variable to access or modify any environment
       variable while any other thread is concurrently modifying any environment variable.  A  call  to  any  function
       dependent on any environment variable shall be considered a use of the environ variable to access that environ-
       ment variable.

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

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

       The argument file is used to construct a pathname that identifies the new process image file. If the file argu-
       ment contains a slash character, the file argument shall be used as the pathname for this file. Otherwise,  the
       path  prefix  for  this file is obtained by a search of the directories passed as the environment variable PATH
       (see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 8, Environment Variables).  If this  environ-
       ment variable is not present, the results of the search are implementation-defined.

       There  are  two  distinct ways in which the contents of the process image file may cause the execution to fail,
       distinguished by the setting of errno to either [ENOEXEC] or [EINVAL] (see the ERRORS section).  In  the  cases
       where the other members of the exec family of functions would fail and set errno to [ENOEXEC], the execlp() and
       execvp() functions shall execute a command interpreter and the environment of the executed command shall be  as
       if the process invoked the sh utility using execl() as follows:


              execl(<shell path>, arg0, file, arg1, ..., (char *)0);

       where  <shell path>  is  an  unspecified  pathname  for the sh utility, file is the process image file, and for
       execvp(), where arg0, arg1, and so on correspond to the values passed to execvp() in argv[0], argv[1],  and  so
       on.

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

       The argument argv is an array of character pointers to null-terminated strings. The  application  shall  ensure
       that  the  last member of this array is a null pointer. These strings shall constitute the argument list avail-
       able 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  argument envp is an array of character pointers to null-terminated strings. These strings shall constitute
       the environment for the new process image. The envp array is terminated by a null pointer.

       For those forms not containing an envp pointer ( execl(), execv(), execlp(), and execvp()), the environment for
       the new process image shall be taken from the external variable environ in the calling process.

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

       File descriptors open in the calling process image shall remain open in the new process image, except for those
       whose close-on- exec flag FD_CLOEXEC is set. For those file descriptors that remain open, all attributes of the
       open  file description remain unchanged. For any file descriptor that is closed for this reason, file locks are
       removed as a result of the close as described in close(). Locks  that  are  not  removed  by  closing  of  file
       descriptors remain unchanged.

       If  file descriptors 0, 1, and 2 would otherwise be closed after a successful call to one of the exec family of
       functions, and the new process image file has the set-user-ID or set-group-ID file  mode  bits  set,   and  the
       ST_NOSUID  bit  is not set for the file system containing the new process image file,  implementations may open
       an unspecified file for each of these file descriptors in the new process image.

       Directory streams open in the calling process image shall be closed in the new process image.

       The state of the floating-point environment in the new process image shall be set to the default.

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


              setlocale(LC_ALL, "C")

       shall be executed at start-up.

       Signals  set to the default action (SIG_DFL) in the calling process image shall be set to the default action in
       the new process image. Except for SIGCHLD, signals set to be ignored (SIG_IGN) by  the  calling  process  image
       shall  be  set  to  be  ignored by the new process image. Signals set to be caught by the calling process image
       shall be set to the default action in the new process image (see <signal.h>). If the SIGCHLD signal is  set  to
       be  ignored  by the calling process image, it is unspecified whether the SIGCHLD signal is set to be ignored or
       to the default action in the new process image.   After a successful call to any of the exec functions,  alter-
       nate signal stacks are not preserved and the SA_ONSTACK flag shall be cleared for all signals.

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

       If the ST_NOSUID bit is set for the file system containing the new process image file, then the effective  user
       ID,  effective group ID, saved set-user-ID, and saved set-group-ID are unchanged in the new process image. Oth-
       erwise,  if the set-user-ID mode bit of the new process image file is set, the effective user  ID  of  the  new
       process  image  shall  be set to the user 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 shall be set  to  the
       group ID of the new process image file. The real user ID, real group ID, and supplementary group IDs of the new
       process image shall remain the same as those of the calling process image. The effective user ID and  effective
       group  ID of the new process image shall be saved (as the saved set-user-ID and the saved set-group-ID) for use
       by setuid().

       Any shared memory segments attached to the calling process image shall not  be  attached  to  the  new  process
       image.

       Any named semaphores open in the calling process shall be closed as if by appropriate calls to sem_close().

       Any  blocks of typed memory that were mapped in the calling process are unmapped, as if munmap() was implicitly
       called to unmap them.

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

       Memory  mappings  created  in  the process are unmapped before the address space is rebuilt for the new process
       image.

       For the SCHED_FIFO and SCHED_RR scheduling policies, the policy and priority settings shall not be changed by a
       call to an exec function. For other scheduling policies, the policy and priority settings on exec are implemen-
       tation-defined.

       Per-process timers created by the calling process shall be deleted before replacing the current  process  image
       with the new process image.

       All open message queue descriptors in the calling process shall be closed, as described in mq_close().

       Any  outstanding  asynchronous  I/O  operations may be canceled. Those asynchronous I/O operations that are not
       canceled shall complete as if the exec function had not yet occurred, but any associated  signal  notifications
       shall  be  suppressed.  It is unspecified whether the exec function itself blocks awaiting such I/O completion.
       In no event, however, shall the new process image created by the exec function be affected by the  presence  of
       outstanding  asynchronous  I/O operations at the time the exec function is called. Whether any I/O is canceled,
       and which I/O may be canceled upon exec, is implementation-defined.

       The new process image shall inherit the CPU-time clock of the calling process  image.  This  inheritance  means
       that  the process CPU-time clock of the process being exec-ed shall not be reinitialized or altered as a result
       of the exec function other than to reflect the time spent by the process executing the exec function itself.

       The initial value of the CPU-time clock of the initial thread of the new process image shall be set to zero.

       If the calling process is being traced, the new process image shall continue to be traced into the  same  trace
       stream  as  the  original process image, but the new process image shall not inherit the mapping of trace event
       names to trace event type identifiers that was defined  by  calls  to  the  posix_trace_eventid_open()  or  the
       posix_trace_trid_eventid_open() functions in the calling process image.

       If  the  calling process is a trace controller process, any trace streams that were created by the calling pro-
       cess shall be shut down as described in the posix_trace_shutdown() function.

       The new process shall inherit at least the following attributes from the calling process image:

        * Nice value (see nice())


        * semadj values (see semop())


        * Process ID


        * Parent process ID


        * Process group ID


        * Session membership


        * Real user ID


        * Real group ID


        * Supplementary group IDs


        * Time left until an alarm clock signal (see alarm())


        * Current working directory


        * Root directory


        * File mode creation mask (see umask())


        * File size limit (see ulimit())


        * Process signal mask (see sigprocmask())


        * Pending signal (see sigpending())


        * tms_utime, tms_stime, tms_cutime, and tms_cstime (see times())


        * Resource limits


        * Controlling terminal


        * Interval timers


       All other process attributes defined in this volume of IEEE Std 1003.1-2001 shall be the same in  the  new  and
       old process images. The inheritance of process attributes not defined by this volume of IEEE Std 1003.1-2001 is
       implementation-defined.

       A call to any exec function from a process with more than one thread shall result in all threads  being  termi-
       nated and the new executable image being loaded and executed. No destructor functions shall be called.

       Upon successful completion, the exec functions shall mark for update the st_atime field of the file. If an exec
       function failed but was able to locate the process image file, whether the st_atime field is marked for  update
       is  unspecified.  Should  the  exec  function  succeed, the process image file shall be considered to have been
       opened with open().  The corresponding close() shall be 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,
       posix_spawn(), or posix_spawnp().  The argv[] and envp[] arrays of pointers and  the  strings  to  which  those
       arrays point shall not be modified by a call to one of the exec functions, except as a consequence of replacing
       the process image.

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

RETURN VALUE
       If  one  of  the  exec  functions returns to the calling process image, an error has occurred; the return value
       shall be -1, and errno shall be set to indicate the error.

ERRORS
       The exec functions shall fail if:

       E2BIG  The number of bytes used by the new process image's argument list and environment list is  greater  than
              the system-imposed limit of {ARG_MAX} bytes.

       EACCES Search  permission  is denied for a directory listed in the new process image file's path prefix, or the
              new process image file denies execution permission, or the new process image file is not a regular  file
              and the implementation does not support execution of files of its type.

       EINVAL The new process image file has the appropriate permission and has a recognized executable binary format,
              but the system does not support execution of a file with this format.

       ELOOP  A loop exists in symbolic links encountered during resolution of the path or file argument.

       ENAMETOOLONG
              The length of the path or file arguments exceeds {PATH_MAX} or  a  pathname  component  is  longer  than
              {NAME_MAX}.

       ENOENT A component of path or file does not name an existing file or path or file is an empty string.

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


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

       ENOEXEC
              The new process image file has the appropriate access permission but has an unrecognized format.


       The exec functions may fail if:

       ELOOP  More  than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path or file argument.

       ENAMETOOLONG
              As a result of encountering a symbolic link in resolution of the path argument, the length of  the  sub-
              stituted pathname string exceeded {PATH_MAX}.

       ENOMEM The new process image requires more memory than is allowed by the hardware or system-imposed memory man-
              agement constraints.

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


       The following sections are informative.

EXAMPLES
   Using execl()
       The  following  example executes the ls command, specifying the pathname of the executable ( /bin/ls) and using
       arguments supplied directly to the command to produce single-column output.


              #include <unistd.h>


              int ret;
              ...
              ret = execl ("/bin/ls", "ls", "-1", (char *)0);

   Using execle()
       The following example is similar to Using execl() . In addition, it specifies the environment for the new  pro-
       cess image using the env argument.


              #include <unistd.h>


              int ret;
              char *env[] = { "HOME=/usr/home", "LOGNAME=home", (char *)0 };
              ...
              ret = execle ("/bin/ls", "ls", "-l", (char *)0, env);

   Using execlp()
       The  following  example searches for the location of the ls command among the directories specified by the PATH
       environment variable.


              #include <unistd.h>


              int ret;
              ...
              ret = execlp ("ls", "ls", "-l", (char *)0);

   Using execv()
       The following example passes arguments to the ls command in the cmd array.


              #include <unistd.h>


              int ret;
              char *cmd[] = { "ls", "-l", (char *)0 };
              ...
              ret = execv ("/bin/ls", cmd);

   Using execve()
       The following example passes arguments to the ls command in the cmd array, and specifies  the  environment  for
       the new process image using the env argument.


              #include <unistd.h>


              int ret;
              char *cmd[] = { "ls", "-l", (char *)0 };
              char *env[] = { "HOME=/usr/home", "LOGNAME=home", (char *)0 };
              ...
              ret = execve ("/bin/ls", cmd, env);

   Using execvp()
       The  following  example searches for the location of the ls command among the directories specified by the PATH
       environment variable, and passes arguments to the ls command in the cmd array.


              #include <unistd.h>


              int ret;
              char *cmd[] = { "ls", "-l", (char *)0 };
              ...
              ret = execvp ("ls", cmd);

APPLICATION USAGE
       As the state of conversion descriptors and message catalog descriptors in the new process image  is  undefined,
       conforming  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()  with  the  appropriate
       parameters to establish the locale of the new process.

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

       Applications should not depend on file descriptors 0, 1, and 2 being closed after an exec. A future version may
       allow these file descriptors to be automatically opened for any process.

RATIONALE
       Early proposals required that the value of argc passed to main() be "one or greater". This was  driven  by  the
       same  requirement  in  drafts of the ISO C standard. In fact, historical implementations have passed a value of
       zero when no arguments are supplied to the caller of the exec functions.  This requirement was removed from the
       ISO C  standard and subsequently removed from this volume of IEEE Std 1003.1-2001 as well. The wording, in par-
       ticular the use of the word should, requires a Strictly Conforming POSIX Application to pass at least one argu-
       ment  to  the exec function, thus guaranteeing that argc be one or greater when invoked by such an application.
       In fact, this is good practice, since many existing applications reference argv[0] without first  checking  the
       value of argc.

       The requirement on a Strictly Conforming POSIX Application also states that the value passed as the first argu-
       ment be a filename associated with the process being started. Although some existing applications pass a  path-
       name  rather than a filename in some circumstances, a filename is more generally useful, since the common usage
       of argv[0] is in printing diagnostics. In some cases the filename passed is not  the  actual  filename  of  the
       file;  for example, many implementations of the login utility use a convention of prefixing a hyphen ( '-' ) to
       the actual filename, which indicates to the command interpreter being invoked that it is a "login shell".

       Historically there have been two ways that implementations can exec shell scripts.

       One common historical implementation is that the execl(), execv(), execle(), and execve() functions  return  an
       [ENOEXEC]  error  for  any file not recognizable as executable, including a shell script. When the execlp() and
       execvp() functions encounter such a file, they assume the file to be a shell script and invoke a known  command
       interpreter  to  interpret  such files. This is now required by IEEE Std 1003.1-2001.  These implementations of
       execvp() and execlp() only give the [ENOEXEC] error in the rare case of  a  problem  with  the  command  inter-
       preter's  executable  file. Because of these implementations, the [ENOEXEC] error is not mentioned for execlp()
       or execvp(), although implementations can still give it.

       Another way that some historical implementations handle shell scripts is by recognizing the first two bytes  of
       the  file as the character string "#!" and using the remainder of the first line of the file as the name of the
       command interpreter to execute.

       One potential source of confusion noted by the standard developers is over how the contents of a process  image
       file affect the behavior of the exec family of functions. The following is a description of the actions taken:

        1. If the process image file is a valid executable (in a format that is executable and valid and having appro-
           priate permission) for this system, then the system executes the file.


        2. If the process image file has appropriate permission and is in a format that is executable  but  not  valid
           for  this system (such as a recognized binary for another architecture), then this is an error and errno is
           set to [EINVAL] (see later RATIONALE on [EINVAL]).


        3. If the process image file has appropriate permission but is not otherwise recognized:

            a. If this is a call to execlp() or execvp(), then they invoke a command  interpreter  assuming  that  the
               process image file is a shell script.


            b. If this is not a call to execlp() or execvp(), then an error occurs and errno is set to [ENOEXEC].



       Applications that do not require to access their arguments may use the form:


              main(void)
       as  specified in the ISO C standard. However, the implementation will always provide the two arguments argc and
       argv, even if they are not used.

       Some implementations provide a third argument to main() called envp. This is defined as a pointer to the  envi-
       ronment.  The  ISO C  standard  specifies  invoking  main() with two arguments, so implementations must support
       applications written this way. Since this volume of IEEE Std 1003.1-2001 defines the global  variable  environ,
       which is also provided by historical implementations and can be used anywhere that envp could be used, there is
       no functional need for the envp argument. Applications should use the getenv() function rather  than  accessing
       the  environment  directly via either envp or environ. Implementations are required to support the two-argument
       calling sequence, but this does not prohibit an implementation from supporting envp as an optional third  argu-
       ment.

       This  volume  of IEEE Std 1003.1-2001 specifies that signals set to SIG_IGN remain set to SIG_IGN, and that the
       process signal mask be unchanged across an exec. This is consistent with  historical  implementations,  and  it
       permits  some  useful  functionality, such as the nohup command. However, it should be noted that many existing
       applications wrongly assume that they start with certain signals set to the default action and/or unblocked. In
       particular, applications written with a simpler signal model that does not include blocking of signals, such as
       the one in the ISO C standard, may not behave properly if invoked with some signals blocked. Therefore,  it  is
       best  not to block or ignore signals across execs without explicit reason to do so, and especially not to block
       signals across execs of arbitrary (not closely co-operating) programs.

       The exec functions always save the value of the effective user ID and effective group ID of the process at  the
       completion  of  the  exec,  whether or not the set-user-ID or the set-group-ID bit of the process image file is
       set.

       The statement about argv[] and envp[] being constants is included to make explicit to future  writers  of  lan-
       guage bindings that these objects are completely constant. Due to a limitation of the ISO C standard, it is not
       possible to state that idea in standard C. Specifying two levels of const- qualification  for  the  argv[]  and
       envp[]  parameters  for the exec functions may seem to be the natural choice, given that these functions do not
       modify either the array of pointers or the characters to which the function points,  but  this  would  disallow
       existing  correct  code. Instead, only the array of pointers is noted as constant. The table of assignment com-
       patibility for dst= src derived from the ISO C standard summarizes the compatibility:

                        dst:                char *[] const char *[] char *const[] const char *const[]
                        src:
                        char *[]            VALID    -              VALID         -
                        const char *[]      -        VALID          -             VALID
                        char * const []     -        -              VALID         -
                        const char *const[] -        -              -             VALID

       Since all existing code has a source type matching the first row, the column that gives the most valid combina-
       tions  is  the third column. The only other possibility is the fourth column, but using it would require a cast
       on the argv or envp arguments. It is unfortunate that the fourth column cannot be used, because the declaration
       a non-expert would naturally use would be that in the second row.

       The ISO C standard and this volume of IEEE Std 1003.1-2001 do not conflict on the use of environ, but some his-
       torical implementations of environ may cause a conflict.  As long as environ is treated in the same way  as  an
       entry  point  (for example, fork()), it conforms to both standards.  A library can contain fork(), but if there
       is a user-provided fork(), that fork() is given precedence and no problem ensues. The situation is similar  for
       environ: the definition in this volume of IEEE Std 1003.1-2001 is to be used if there is no user-provided envi-
       ron to take precedence.  At least three implementations are known to exist that solve this problem.

       E2BIG  The limit {ARG_MAX} applies not just to the size of the argument list, but to the sum of  that  and  the
              size of the environment list.

       EFAULT Some  historical  systems  return [EFAULT] rather than [ENOEXEC] when the new process image file is cor-
              rupted. They are non-conforming.

       EINVAL This error condition was added to IEEE Std 1003.1-2001 to allow an implementation to  detect  executable
              files  generated for different architectures, and indicate this situation to the application. Historical
              implementations of shells, execvp(), and execlp() that encounter an [ENOEXEC] error will execute a shell
              on  the  assumption  that  the file is a shell script. This will not produce the desired effect when the
              file is a valid executable for a different architecture. An implementation may now choose to avoid  this
              problem  by returning [EINVAL] when a valid executable for a different architecture is encountered. Some
              historical implementations return [EINVAL] to indicate that the path argument contains a character  with
              the  high  order bit set. The standard developers chose to deviate from historical practice for the fol-
              lowing reasons:

               1. The new utilization of [EINVAL] will provide some measure of utility to the user community.


               2. Historical use of [EINVAL] is not acceptable in an internationalized operating environment.


       ENAMETOOLONG
              Since the file pathname may be constructed by taking elements in the  PATH  variable  and  putting  them
              together with the filename, the [ENAMETOOLONG] error condition could also be reached this way.

       ETXTBSY
              System V returns this error when the executable file is currently open for writing by some process. This
              volume of IEEE Std 1003.1-2001 neither requires nor prohibits this behavior.


       Other systems (such as System V) may return [EINTR] from exec.   This  is  not  addressed  by  this  volume  of
       IEEE Std 1003.1-2001, but implementations may have a window between the call to exec and the time that a signal
       could cause one of the exec calls to return with [EINTR].

       An explicit statement regarding the floating-point environment (as defined in the <fenv.h> header) was added to
       make  it  clear  that the floating-point environment is set to its default when a call to one of the exec func-
       tions succeeds.  The requirements for inheritance or setting to the default for other process and thread start-
       up functions is covered by more generic statements in their descriptions and can be summarized as follows:

       posix_spawn()
              Set to default.

       fork() Inherit.

       pthread_create()
              Inherit.


FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm(),  atexit(),  chmod(), close(), exit(), fcntl(), fork(), fstatvfs(), getenv(), getitimer(), getrlimit(),
       mmap(),  nice(),  posix_spawn(),  posix_trace_eventid_open(),  posix_trace_shutdown(),   posix_trace_trid_even-
       tid_open(),  putenv(), semop(), setlocale(), shmat() , sigaction(), sigaltstack(), sigpending(), sigprocmask(),
       system(), times(), ulimit(), umask(), the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 11,  General
       Terminal Interface, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The
       Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Stan-
       dard,  the  original  IEEE  and  The  Open Group Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                             EXEC(3P)