execl (3p) - Linux Manuals
execl: execute a
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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult 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,
... /*,
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 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 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, 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 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 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 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, any functions
previously registered 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 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
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 new process shall inherit at least the following attributes from
the calling process image:
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 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.
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.
The exec functions shall fail if:
The exec functions, except for execlp() and execvp(),
shall fail if:
The exec functions may fail if:
The following sections are informative.
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.
The following example is similar to Using execl() . In addition, it
specifies the environment
for the new process image using the env argument.
The following example searches for the location of the ls command
among the
directories specified by the PATH environment variable.
The following example passes arguments to the ls command in
the cmd array.
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.
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.
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.
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
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 ( '-' ) to the actual filename,
which indicates 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 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 implementations, 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:
Applications that do not require to access their arguments may use
the form:
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) 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 compatibility
for dst= src derived from the
ISO C standard summarizes the compatibility:
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 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.
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:
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>
RETURN VALUE
ERRORS
EXAMPLES
Using execl()
#include <unistd.h>
int ret;
...
ret = execl ("/bin/ls", "ls", "-1", (char *)0);
Using execle()
#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()
#include <unistd.h>
int ret;
...
ret = execlp ("ls", "ls", "-l", (char *)0);
Using execv()
#include <unistd.h>
int ret;
char *cmd[] = { "ls", "-l", (char *)0 };
...
ret = execv ("/bin/ls", cmd);
Using execve()
#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()
#include <unistd.h>
int ret;
char *cmd[] = { "ls", "-l", (char *)0 };
...
ret = execvp ("ls", cmd);
APPLICATION USAGE
RATIONALE
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.
main(void)
dst: char *[] const char *[] char *const[] const char *const[] src: char *[] VALID - VALID - const char *[] - VALID - VALID char * const [] - - VALID - const char *const[] - - - VALID
FUTURE DIRECTIONS
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 .
SEE ALSO