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EXEC(3P) POSIX Programmer's Manual EXEC(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
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 fol-
lows:
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 envi-
ronment 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 con-
tains 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, 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, distin-
guished 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 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 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 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 envi-
ronment 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 implemen-
tation-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 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 process 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 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 atexit() are no longer reg-
istered.
If the ST_NOSUID bit is set for the file system containing the new process image file, then the effective user ID, effec-
tive 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.
For the SCHED_FIFO and SCHED_RR scheduling policies, the policy and priority settings 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 implemen-
tation-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 func-
tion 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() func-
tions 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 executed. 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 corre-
sponding 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 func-
tions, 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 lim-
its.
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 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.
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.
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 argument, 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 executable ( /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 directories 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 directories 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 setlocale() 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 argument to the exec function, thus guaran-
teeing that argc be one or greater when invoked by such an application. In fact, this is good practice, since many exist-
ing 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 ( '-' ) to the actual filename, which indicates to the com-
mand 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 executable, 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 executable file. Because of these implemen-
tations, the [ENOEXEC] error is not mentioned for execlp() or execvp(), although implementations can still give it.
Another way that some historical implementations handle shell scripts is by recognizing 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 functions. 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 per-
mission) 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 recognized:
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 anywhere 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
functionality, 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 standard, 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) pro-
grams.
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 func-
tions 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 point-
ers is noted as constant. The table of assignment compatibility 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
arguments. It is unfortunate that the fourth column cannot be used, because the declaration 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 conflict. As long as environ is treated in the same way as an entry point (for
example, 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 vol-
ume of IEEE Std 1003.1-2001 is to be used if there is no user-provided environ to take precedence. At least three imple-
mentations 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 implementation to detect executable files gen-
erated for different architectures, and indicate this situation to the application. Historical implementations of
shells, execvp(), and execlp() that encounter an [ENOEXEC] error will execute 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 differ-
ent architecture. An implementation may now choose to avoid this problem by returning [EINVAL] when a valid exe-
cutable for a different architecture 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
requirements 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_eventid_open(), putenv(),
semop(), setlocale(), shmat() , sigaction(), sigaltstack(), 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, Copy-
right (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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 EXEC(3P)
