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 functions replace 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. There is 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 is 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 initialised 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 will 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 is considered a use of the environ variable to access that environment variable.

The arguments specified by a program with one of the exec functions are 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 is 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 (see XBD specification, Environment Variables  ). If this environment variable is not present, the results of the search are implementation-dependent.

If the process image file is not a valid executable object, execlp() and execvp() use the contents of that file as standard input to a command interpreter conforming to system(). In this case, the command interpreter becomes the new process image.

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 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 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 argument envp is an array of character pointers to null-terminated strings. These strings 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 (.Fn execl , execv(), execlp() and execvp()), the environment for the new process image is 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-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. For those file descriptors that remain open, all attributes of the open file description, including file locks remain unchanged.

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 process image. 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.h>).  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() 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 is 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 is 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 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().

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

Any mappings established through mmap() are not preserved across an exec.

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().

If the Process Memory Locking option is supported, memory locks established by the calling process via calls to mlockall() or mlock() are 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 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.

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

If the Process Scheduling option is supported, for the SCHED_FIFO and SCHED_RR scheduling policies, the policy and priority settings are not changed by a call to an exec function. For other scheduling policies, the policy and priority settings on exec are implementation-dependent.

If the Timers option is supported, per-process timers created by the calling process are deleted before replacing the current process image with the new process image.

If the Message Passing option is supported, all open message queue descriptors in the calling process are closed, as described in mq_close().

If the Asynchronous Input and Output option is supported, any outstanding asynchronous I/O operations may be canceled. Those asynchronous I/O operations that are not canceled 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 asynchronous I/O operations at the time the exec function is called. Whether any I/O is cancelled, and which I/O may be cancelled upon exec, is implementation-dependent.

The new process also inherits 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 document will be the same in the new and old process images. The inheritance of process attributes not defined by this specification is implementation-dependent.

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 destructor functions will be called.

Upon successful completion, the exec functions 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 is considered to have been opened with open(). The corresponding close() 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 consequence 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 VALUE

If one of the exec functions returns to the calling process 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 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.

[ELOOP]

Too many symbolic links were encountered in resolving path.

[ENAMETOOLONG]

The length of the path or file arguments, or an element of the environment variable prefixed to a file, 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(), 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 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.

 EXAMPLES

None.

 APPLICATION 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() with the appropriate parameters to establish the locale of the new process.

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

 FUTURE DIRECTIONS

None.

 SEE ALSO

alarm(), atexit(), chmod(), exit(), fcntl(), fork(), fstatvfs(), getenv(), getitimer(), getrlimit(), mmap(), nice(), putenv(), semop(), setlocale(), shmat(), sigaction(), sigaltstack(), sigpending(), sigprocmask(), system(), times(), ulimit(), umask(), <unistd.h>, XBD specification, General Terminal Interface .

DERIVATION

Derived from Issue 1 of the SVID.