EXEC(2) System Calls EXEC(2)


NAME


exec, execl, execle, execlp, execv, execve, execvp - execute a file

SYNOPSIS


#include <unistd.h>

int execl(const char *path, const char *arg0, ...
/* const char *argn, (char *)0 */);


int execv(const char *path, char *const argv[]);


int execle(const char *path, const char *arg0, ...
/* const char *argn, (char *)0,char *const envp[]*/);


int execve(const char *path, char *const argv[],
char *const envp[]);


int execlp(const char *file, const char *arg0, ...
/* const char *argn, (char *)0 */);


int execvp(const char *file, char *const argv[]);


DESCRIPTION


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 may also be an interpreter
file. There can be up to four nested interpreter files before the final
interpreter. The setid bits on nested interpreters are silently ignored.


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 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.


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. The standard to which the caller conforms determines
which shell is used. See standards(5).


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 translation 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.h(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).


The privilege sets are changed according to the following rules:

1. The inheritable set, I, is intersected with the limit set, L.
This mechanism enforces the limit set for processes.

2. The effective set, E, and the permitted set, P, are made equal
to the new inheritable set.


The system attempts to set the privilege-aware state to non-PA both
before performing any modifications to the process IDs and privilege sets
as well as after completing the transition to new UIDs and privilege
sets, following the rules outlined in privileges(5).


If the {PRIV_PROC_OWNER} privilege is asserted in the effective set, the
set-user-ID and set-group-ID bits will be honored when the process is
being controlled by ptrace(3C). Additional restriction can apply when the
traced process has an effective UID of 0. See privileges(5).


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(3C)


Profiling is disabled for the new process; see profil(2).


Timers created by the calling process with timer_create(3C) 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(3C).


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.


All active contract templates are cleared (see contract(4)).


The new process also inherits the following attributes from the calling
process:

o controlling terminal

o current working directory

o file-locks (see fcntl(2) and lockf(3C))

o file mode creation mask (see umask(2))

o file size limit (see ulimit(2))

o limit privilege set

o nice value (see nice(2))

o parent process ID

o pending signals (see sigpending(2))

o privilege debugging flag (see privileges(5) and getpflags(2))

o process ID

o process contract (see contract(4) and process(4))

o process group ID

o process signal mask (see sigprocmask(2))

o processor bindings (see processor_bind(2))

o processor set bindings (see pset_bind(2))

o project ID

o real group ID

o real user ID

o resource limits (see getrlimit(2))

o root directory

o scheduler class and priority (see priocntl(2))

o semadj values (see semop(2))

o session membership (see exit(2) and signal(3C))

o supplementary group IDs

o task ID

o time left until an alarm clock signal (see alarm(2))

o tms_utime, tms_stime, tms_cutime, and tms_cstime (see
times(2))

o trace flag (see ptrace(3C) request 0)


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.

RETURN VALUES


If a function in the exec family 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 in the new process's argument list is
greater than the system-imposed limit of {ARG_MAX} bytes.
The argument list limit is sum of the size of the
argument list plus the size of the environment's exported
shell variables.


EACCES
Search permission is denied for a directory listed in the
new process file's path prefix.

The new process file is not an ordinary file.

The new process file mode denies execute permission.

The {FILE_DAC_SEARCH} privilege overrides the restriction
on directory searches.

The {FILE_DAC_EXECUTE} privilege overrides the lack of
execute permission.


EAGAIN
Total amount of system memory available when reading
using raw I/O is temporarily insufficient.


EFAULT
An argument points to an illegal address.


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.


EINTR
A signal was caught during the execution of one of the
functions in the exec family.


ELOOP
Too many symbolic links were encountered in translating
path or file, or too many nested interpreter files.


ENAMETOOLONG
The length of the file or path argument exceeds
{PATH_MAX}, or the length of a file or path component
exceeds {NAME_MAX} while {_POSIX_NO_TRUNC} is in effect.


ENOENT
One or more components of the new process path name of
the file do not exist or is a null pathname.


ENOLINK
The path argument points to a remote machine and the link
to that machine is no longer active.


ENOTDIR
A component of the new process path of the file 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 by memory
management constraints. See brk(2).


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


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(3C) with the appropriate parameters to establish the locale of
thenew process.


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

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | See below. |
+--------------------+-------------------+
|Standard | See standards(5). |
+--------------------+-------------------+


The execle() and execve() fucntions are Async-Signal-Safe.

SEE ALSO


ksh(1), ps(1), sh(1), alarm(2), brk(2), chmod(2), exit(2), fcntl(2),
fork(2), getpflags(2), getrlimit(2), memcntl(2), mmap(2), nice(2),
priocntl(2), profil(2), semop(2), shmop(2), sigpending(2),
sigprocmask(2), times(2), umask(2), lockf(3C), ptrace(3C), setlocale(3C),
signal(3C), system(3C), timer_create(3C), a.out(4), contract(4),
process(4), attributes(5), environ(5), privileges(5), standards(5)

WARNINGS


If a program is setuid to a user ID other than the superuser, and the
program is executed when the real user ID is super-user, then the program
has some of the powers of a super-user as well.


October 27, 2015 EXEC(2)