exec(2) exec(2)
NAME
execl, execv, execle, execve, execlp, execvp, exect - exe-
cute a file
SYNOPSIS
int execl (path, arg0, arg1, ..., argn, 0)
char *path, *arg0, *arg1, ..., *argn;
int execv (path, argv)
char *path, *argv[ ];
int execle (path, arg0, arg1, ..., argn, 0, envp)
char *path, *arg0, *arg1, ..., *argn, *envp[ ];
int execve (path, argv, envp)
char *path, *argv[ ], *envp[ ];
int execlp (file, arg0, arg1, ..., argn, 0)
char *file, *arg0, *arg1, ..., *argn;
int execvp (file, argv)
char *file, *argv[ ];
SYNOPSIS (4.2BSD)
int exect (path, argv, envp)
char *path, *argv[ ], *envp[ ];
DESCRIPTION
exec in all its forms transforms the calling process into a
new process. The new process is constructed from an ordi-
nary, executable file called the new process file. This
file is either an executable object file, or a file of data
for an interpreter. An executable object file consists of
an identifying header (see a.out(4)), a text segment, and a
data segment. The data segment contains an initialized por-
tion and an uninitialized portion (bss).
An interpreter file begins with a line of the form ``#!
interpreter''. When an interpreter file is exec'd, the sys-
tem execs the specified interpreter, giving it the name of
the originally exec'd file as an argument, shifting over the
rest of the original arguments. This feature is not found
on AT&T SYSTEM V systems.
There can be no return from a successful exec because the
calling process is overlaid by the new process.
Path points to a path name that identifies the new process
file.
File points to the new process file. The path prefix for
this file is obtained by a search of the directories passed
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exec(2) exec(2)
as the environment line "PATH =" (see environ(5)). The
environment is supplied by the shell (see sh(1)).
Arg0, arg1, ..., argn are pointers to null-terminated char-
acter strings. These strings constitute the argument list
available to the new process. By convention, at least arg0
must be present and point to a string that is the same as
path (or its last component).
Argv is an array of character pointers to null-terminated
strings. These strings constitute the argument list avail-
able to the new process. By convention, argv must have at
least one member, and it must 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. Each string consists of a name, an "=", and a
null-terminated value. These strings constitute the
environment for the new process. Envp is terminated by a
null pointer. For execl and execv , the C run-time start-
off routine places a pointer to the environment of the cal-
ling process in the global cell:
extern char **environ;
and it is used to pass the environment of the calling pro-
cess to the new process.
The exect version is used when the executed file is to be
manipulated with ptrace(2). The program is forced to single
step a single instruction giving the parent an opportunity
to manipulate its state.
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 set to terminate the calling process will be set to
terminate the new process. Signals set to be ignored by the
calling process will be set to be ignored by the new pro-
cess. Signals set to be caught by the calling process will
be set to terminate new process; see signal(2).
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 owner 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 ID and real
group ID of the new process remain the same as those of the
calling process. The effective user ID and effective group
ID of the new process are saved for use by the setuid(2)
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exec(2) exec(2)
routine.
When set-group-ID files are executed, the process inherits
the security label of the user and the group of the file.
If the system doesn't have a privilege with this particular
combination of security label and group then execution is
denied.
If the user is operating with an effective user ID of root
and the security level of the file is not 0 then execution
is denied.
If the file is suid-to-root and the security level of the
file is not 0 then execution is denied.
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).
Memory locks established by the calling process via calls to
mlockall() or mlock() are removed. If locked pages in the
calling process' address space are also mapped into other
process' address spaces and are locked by those processes,
the locks established by the other processes are unaffected
by this process's call to the exec function.
Any outstanding asynchronous I/O operations may be can-
celled. An outstanding request will be cancelled if the
actual I/O operation has not occured. Otherwise, it is
allowed to go through I/O completion. However, on exec() all
signals to the process are disabled, which means any signal
generated because of the I/O completion will not affect the
process. Those asynchronous I/O operations which are not
cancelled complete as if the exec function had not yet
occured. 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.
The new process also inherits the following attributes from
the calling process:
nice value (see nice(2))
process ID
parent process ID
process group ID
access groups (see getgroups(2))
semadj values (see semop(2))
tty group ID (see exit(2) and signal(2))
trace flag (see ptrace(2) request PT_TRACE_ME and
PT_TRACE_CHILD)
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exec(2) exec(2)
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))
signal mask (see sigvec(2))
utime, stime, cutime, and cstime (see times(2))
universe (see setuniverse(2))
When the executed program begins, it is called as follows:
main (argc, argv, envp)
int argc;
char **argv, **envp;
where argc is the argument count and argv is an array of
character pointers to the arguments themselves. As indi-
cated, argc is conventionally at least one and the first
member of the array points to a string containing the name
of the file. Envp is a pointer to an array of strings that
constitute the environment of the process, as described ear-
lier.
exec will fail and return to the calling process if one or
more of the following are true:
[ENOENT] One or more components of the new process
path name of the file do not exist.
[ENOTDIR] A component of the new process path of the
file prefix is not a directory.
[EACCES] Search permission is denied for a directory
listed in the new process file's path prefix.
[EPERM] The new process file is set-group-ID to a
group not defined for execution at the cal-
ling process' current classification level.
(B1 only)
[EPERM] The user is operating as root and the secu-
rity level of the file is not 0. (B1 only)
[EPERM] The file is suid-to-root and the security
level of the file is not 0. (B1 only)
[EACCES] The new process file is not an ordinary file.
[EACCES] The new process file mode denies execution
permission.
[ENOEXEC] The exec is not an execlp or execvp, and the
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exec(2) exec(2)
new process file has the appropriate access
permission but an invalid magic number in its
header.
[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.
[E2BIG] The number of bytes in the new process's
argument list is greater than the system-
imposed limit of 5120 bytes.
[EFAULT] The new process file is not as long as indi-
cated by the size values in its header.
[EFAULT] Path, argv, or envp point to an illegal
address.
RETURN VALUE
If exec returns to the calling process an error has
occurred; the return value will be -1 and errno will be set
to indicate the error.
SEE ALSO
alarm(2), exit(2), fork(2), nice(2), ptrace(2), semop(2),
signal(2), times(2), ulimit(2), umask(2), a.out(4),
environ(5).
sh(1) in the CX/UX User's Reference Manual.
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