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.
The fexecve() function shall be equivalent to the execve()
function except that the file to be executed is determined by the
file descriptor fd instead of a pathname. The file offset of fd
is ignored.
When a C-language program is executed as a result of a call to
one of the exec family of functions, 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, which must be declared by the user if it is
to be used directly:
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.
Applications can change the entire environment in a single
operation by assigning the environ variable to point to an array
of character pointers to the new environment strings. After
assigning a new value to environ, applications should not rely on
the new environment strings remaining part of the environment, as
a call to getenv(), putenv(), setenv(), unsetenv(), or any
function that is dependent on an environment variable may, on
noticing that environ has changed, copy the environment strings
to a new array and assign environ to point to it.
Any application that directly modifies the pointers to which the
environ variable points has undefined behavior.
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
POSIX.1‐2017, 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 string 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 string 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 descriptor 0, 1, or 2 would otherwise be closed after a
successful call to one of the exec family of functions,
implementations may open an unspecified file for the file
descriptor in the new process image. If a standard utility or a
conforming application is executed with file descriptor 0 not
open for reading or with file descriptor 1 or 2 not open for
writing, the environment in which the utility or application is
executed shall be deemed non-conforming, and consequently the
utility or application might not behave as described in this
standard.
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 initial thread
of 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 the atexit() or
pthread_atfork() functions 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.
When the calling process image does not use the SCHED_FIFO,
SCHED_RR, or SCHED_SPORADIC scheduling policies, the scheduling
policy and parameters of the new process image and the initial
thread in that new process image are implementation-defined.
When the calling process image uses the SCHED_FIFO, SCHED_RR, or
SCHED_SPORADIC scheduling policies, the process policy and
scheduling parameter settings shall not be changed by a call to
an exec function. The initial thread in the new process image
shall inherit the process scheduling policy and parameters. It
shall have the default system contention scope, but shall inherit
its allocation domain from the calling process image.
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 thread ID of the initial thread in the new process image is
unspecified.
The size and location of the stack on which the initial thread in
the new process image runs is unspecified.
The initial thread in the new process image shall have its
cancellation type set to PTHREAD_CANCEL_DEFERRED and its
cancellation state set to PTHREAD_CANCEL_ENABLED.
The initial thread in the new process image shall have all
thread-specific data values set to NULL and all thread-specific
data keys shall be removed by the call to exec without running
destructors.
The initial thread in the new process image shall be joinable, as
if created with the detachstate attribute set to
PTHREAD_CREATE_JOINABLE.
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 getrlimit() and setrlimit())
* Process signal mask (see pthread_sigmask())
* Pending signal (see sigpending())
* tms_utime, tms_stime, tms_cutime, and tms_cstime (see
times())
* Resource limits
* Controlling terminal
* Interval timers
The initial thread of the new process shall inherit at least the
following attributes from the calling thread:
* Signal mask (see sigprocmask() and pthread_sigmask())
* Pending signals (see sigpending())
All other process attributes defined in this volume of
POSIX.1‐2017 shall be inherited in the new process image from the
old process image. All other thread attributes defined in this
volume of POSIX.1‐2017 shall be inherited in the initial thread
in the new process image from the calling thread in the old
process image. The inheritance of process or thread attributes
not defined by this volume of POSIX.1‐2017 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 or cleanup handlers shall be called.
Upon successful completion, the exec functions shall mark for
update the last data access timestamp of the file. If an exec
function failed but was able to locate the process image file,
whether the last data access timestamp 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.
