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EXEC(P) EXEC(P)
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 environ-
ment 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, Environ-
ment 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 mem-
bers 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 util-
ity 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 pro-
cess 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 pro-
cess 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 exter-
nal variable environ in the calling process.
The number of bytes available for the new process’ combined argument and environ-
ment 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 descrip-
tion 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 pro-
cess 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 pro-
cess 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 reg-
istered 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 appropri-
ate 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 pro-
cesses, 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 set-
tings 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 pro-
cess 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 exe-
cuted. 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 pro-
cess’ 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 envi-
ronment 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 permis-
sion, or the new process image file is not a regular file and the implemen-
tation does not support execution of files of its type.
EINVAL The new process image file has the appropriate permission and has a recog-
nized 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 argu-
ment, 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 exe-
cutable ( /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 direc-
tories 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 direc-
tories 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 setlo-
cale() 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 argu-
ment 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 ( â€â€™-â€â€™ ) to the actual filename, which indi-
cates 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 exe-
cutable, 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 exe-
cutable file. Because of these implementations, the [ENOEXEC] error is not men-
tioned for execlp() or execvp(), although implementations can still give it.
Another way that some historical implementations handle shell scripts is by recog-
nizing 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 func-
tions. 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 rec-
ognized:
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 any-
where 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 function-
ality, 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 stan-
dard, 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 compati-
bility 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 argu-
ments. It is unfortunate that the fourth column cannot be used, because the decla-
ration 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 con-
flict. As long as environ is treated in the same way as an entry point (for exam-
ple, 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 implemen-
tation 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 exe-
cute 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 prob-
lem by returning [EINVAL] when a valid executable for a different architec-
ture 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 require-
ments 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_even-
tid_open() , putenv() , semop() , setlocale() , shmat() , sigaction() , sigalt-
stack() , 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 .
POSIX 2003 EXEC(P)
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