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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 executable object file or a file of data for an interpreter. There is no return from a successful call to one of these functions 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 arguments to the interpreter are arg0 through argn of the originally 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 follows:
int main (int argc, char *argv[], char *envp[]);
where argc is the argument count, argv is an array of character 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 identifies 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 standards(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 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 process 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 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 process 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).
Any outstanding asynchronous I/O operations may be cancelled. 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.
The new process also inherits the following attributes from the calling process:
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, 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 process image file, whether the st_atime field is marked for update is unspecified. Should the function succeed, the process 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 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.
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