execlp

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 differ (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 environment 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 argument 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
       environment 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
       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 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,
       alternate 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. Otherwise,  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
       implementation-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 process 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 terminated 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 substituted pathname string
              exceeded {PATH_MAX}.

       ENOMEM The new process image requires more memory than is allowed by the
              hardware or system-imposed memory management 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 process 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 particular the use of the word should, requires a
       Strictly Conforming POSIX Application to pass at least one argument 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 argument be a filename associated with
       the process being started. Although some existing applications pass a
       pathname 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 interpreter'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 appropriate 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 environment. 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 argument.

       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 language 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
       compatibility 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 combinations 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 historical 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 environ
       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 corrupted. 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
              following 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 functions 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_eventid_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, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications 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 Standard, 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)