exec(2) exec(2)
NAME
exec: execl, execv, execle, execve, execlp, 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), const char *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
exec in all its forms overlays a new process image on an old process.
The new process image is constructed from an ordinary, executable
file. This file is either an executable object file, or a file of data
for an interpreter. There can be no return from a successful exec
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 exec'd, the system execs 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 remain-
ing arguments to the interpreter are arg0 through argn of the origi-
nally exec'd file.
When a C program is executed, it is called as follows:
int main(int argc, char *argv[], char *envp[]);
where argc is the argument count, argv is an array of character poin-
ters to the arguments themselves, and envp is an array of character
pointers to the environment strings. As indicated, argc is at least
one, and the first member of the array points to a string containing
the name of the file.
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exec(2) exec(2)
path points to a pathname that identifies the new process file.
file points to the new process file. If file contains a slash charac-
ter, this argument is used as a pathname for this file. If file does
not contain a slash character, 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 [see sh(1)].
If the new process file is not an executable object file, execlp() and
execvp() use the contents of that file as standard input to sh(1).
The arguments arg0, ..., argn point to null-terminated character
strings. These strings constitute the argument list available to the
new process image. Minimally, arg0 must be present. It will become the
name of the process, as displayed by the ps command. arg0 points to a
string that is the same as path (or the last component of path). The
list of argument strings is terminated by a (char *)0 argument.
argv is an array of character pointers to null-terminated strings.
These strings constitute the argument list available to the new pro-
cess image. argv must have at least one member, and it should point to
a string that is the same as path (or its last component). argv is
terminated by a null pointer.
envp is an array of character pointers to null-terminated strings.
These strings constitute the environment for the new process image.
envp is terminated by a null pointer. For execl(), execv(), execvp(),
and execlp(), the C 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 pro-
cess to the new process.
File descriptors open in the calling process remain open in the new
process, except for those whose close-on-exec flag is set; [see
fcntl(2)]. For those file descriptors that remain open, the file
pointer is unchanged.
Signals that are being caught by the calling process are set to the
default disposition in the new process image [see signal(2)]. Other-
wise, the new process image inherits the signal dispositions of the
calling process.
After a successful call to any of the exec functions, any functions
previously registered by the atexit() function are no longer
registered.
If the set-user-ID mode bit of the new process file is set [see
chmod(2)], exec sets the effective user ID of the new process to the
user ID of the new process file. Similarly, if the set-group-ID mode
bit of the new process file is set, the effective group ID of the new
process is set to the group ID of the new process file. The real user
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exec(2) exec(2)
ID and real group ID of the new process remain the same as those of
the calling process. Moreover, if the set-user-ID mode bit of the new
process file is set and the real user ID is not identical with the
effective user ID, the file size limit will be set to the system
default value.
If the effective user-ID is root or superuser, the set-user-ID and
setgroup-ID bits will be honored when the process is being controlled
by ptrace().
The shared memory segments attached to the calling process will not be
attached to the new process [see shmop(2)].
Profiling is disabled for the new process [see profil(2)].
The new process also inherits the following attributes from the cal-
ling process:
- nice value [see nice(2)]
- scheduler class and priority [see priocntl(2)]
- process ID
- parent process ID
- process group ID
- real user ID
- real group ID
- supplementary group IDs
- semadj values [see semop(2)]
- session ID [see exit(2) and signal(2)]
- trace flag [see ptrace(2), request 0)
- time left until an alarm clock signal [see alarm(2)]
- current working directory
- root directory
- file mode creation mask [see umask(2)]
- file size limit [see ulimit(2) and getrlimit(2)]
- resource limits [see getrlimit(2)]
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exec(2) exec(2)
- tmsutime(), tmsstime(), tmscutime(), and tmscstime [see
times(2)]
- file-locks [see fcntl(2) and lockf(3C)]
- controlling terminal
- process signal mask [see sigprocmask(2)]
- pending signals [see sigpending(2)]
Upon successful completion, exec marks for update the statime field
of the file. Should the exec succeed, the process image file is con-
sidered to have been open()-ed. The corresponding close() is con-
sidered to occur at a time after this open, but before process termi-
nation or successful completion of a subsequent call to exec.
ERRORS
The following error code descriptions are function-specific. You will
find a general description in introprm2(2) or in errno(5).
exec will fail and return to the calling process if one or more of the
following apply:
EACCES Search permission is denied for a directory listed in
the new process file's path prefix.
E2BIG The number of bytes in the new process' argument list is
greater than the system-imposed limit of 20480 bytes.
The argument list limit is the sum of the size of the
argument list plus the size of the environment's
exported shell variables.
EACCES The new process file is not an ordinary file.
EACCES The new process file mode denies execution permission.
EAGAIN Total amount of system memory available when reading via
raw I/O is temporarily insufficient.
EFAULT Required hardware is not present.
EFAULT An a.out that was compiled with the MAU or 32B flag is
running on a machine without a MAU or 32B.
EFAULT An argument points to an illegal address.
EINTR A signal was caught during the exec system call.
ELIBACC Required shared library does not have execute permis-
sion.
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exec(2) exec(2)
ELIBEXEC Trying to exec a shared library directly.
ELOOP Too many symbolic links were encountered in translating
path or file.
EMULTIHOP Components of path require hopping to multiple remote
machines and the file system type does not allow it.
ENAMETOOLONG The length of the file or path arguments, or an element
of the environment variable PATH prefixed to a file,
exceeds PATHMAX, or the length of a file or path com-
ponent exceeds NAMEMAX.
ENOENT One or more components of the new process pathname of
the file do not exist or is a null pathname.
ENOTDIR A component of the new process path of the file prefix
is not a directory.
ENOEXEC The exec is not an execlp() or execvp(), and the new
process file has the appropriate access permission but
is not in the proper format.
ETXTBSY The new process file is a pure procedure (shared text)
file that is currently open for writing by some process.
ENOMEM The new process requires more memory than is allowed by
the system-imposed maximum MAXMEM.
ENOLINK path points to a remote machine and the link to that
machine is no longer active.
RESULT
If exec returns to the calling process, an error has occurred; the
result is -1 and errno is set to indicate the error.
NOTES
If you do not use the default C locale in your application, you must
call setlocale(3C) with the appropriate arguments to set the locale of
the new process.
SEE ALSO
ps(1), sh(1), alarm(2), exit(2), fcntl(2), fork(2), getrlimit(2),
nice(2), priocntl(2), ptrace(2), semop(2), signal(2), sigpending(2),
sigprocmask(2), times(2), umask(2), atexit(3C), lockf(3C), system(3S),
a.out(4), unistd(4), environ(5), lfs(5).
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