execve, exect - execute a file
#include <unistd.h>
int
execve(const char *path, char *const argv[], char *const
envp[]);
int
exect(const char *path, char *const argv[], char *const
envp[]);
execve() transforms the calling process into a new process.
The new process
is constructed from an ordinary file, whose name is
pointed to by
path, 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, followed by pages of
data representing
the initial program (text) and initialized data
pages. Additional
pages may be specified by the header to be initialized
with zero data;
see a.out(5) and elf(5).
An interpreter file begins with a line of the form:
#! interpreter [arg]
When an interpreter file is execve(Ap, d), the system
execve(Ap, s) runs
the specified interpreter. If the optional arg is specified, it becomes
the first argument to the interpreter, and the name of the
originally
execve(Ap, d) file becomes the second argument; otherwise,
the name of
the originally execve(Ap, d) file becomes the first argument. The original
arguments are shifted over to become the subsequent arguments. The
zeroth argument, normally the name of the execve(Ap, d)
file, is left unchanged.
The argument argv is a pointer to a null-terminated array of
character
pointers to nul-terminated character strings. These strings
construct
the argument list to be made available to the new process.
At least one
argument must be present in the array; by custom, the first
element
should be the name of the executed program (for example, the
last component
of path).
The argument envp is also a pointer to a null-terminated array of character
pointers to nul-terminated strings. A pointer to this
array is normally
stored in the global variable environ. These strings
pass information
to the new process that is not directly an argument to
the command
(see environ(7)).
File descriptors open in the calling process image remain
open in the new
process image, except for those for which the close-on-exec
flag is set
(see close(2) and fcntl(2)). Descriptors that remain open
are unaffected
by execve(). In the case of a new setuid or setgid executable being executed,
if file descriptors 0, 1, or 2 (representing stdin,
stdout, and
stderr) are currently unallocated, these descriptors will be
opened to
point to some system file like /dev/null. The intent is to
ensure these
descriptors are not unallocated, since many libraries make
assumptions
about the use of these 3 file descriptors.
Signals set to be ignored in the calling process are set to
be ignored in
the new process. Signals which are set to be caught in the
calling process
image are set to default action in the new process image. Blocked
signals remain blocked regardless of changes to the signal
action. The
signal stack is reset to be undefined (see sigaction(2) for
more information).
If the set-user-ID mode bit of the new process image file is
set (see
chmod(2)), the effective user ID of the new process image is
set to the
owner ID of the new process image file. If the set-group-ID
mode bit of
the new process image file is set, the effective group ID of
the new process
image is set to the group ID of the new process image
file. (The
effective group ID is the first element of the group list.)
The real user
ID, real group ID and other group IDs of the new process
image remain
the same as the calling process image. After any set-userID and setgroup-ID
processing, the effective user ID is recorded as
the saved setuser-ID,
and the effective group ID is recorded as the saved
set-groupID.
These values may be used in changing the effective IDs
later (see
setuid(2)). The set-user-ID and set-group-ID bits have no
effect if the
new process image file is located on a file system mounted
with the nosuid
flag. The process will be started without the new permissions.
The new process also inherits the following attributes from
the calling
process:
process ID see getpid(2)
parent process ID see getppid(2)
process group ID see getpgrp(2)
session ID see getsid(2)
access groups see getgroups(2)
working directory see chdir(2)
root directory see chroot(2)
control terminal see termios(4)
resource usages see getrusage(2)
interval timers see getitimer(2) (unless process
image file is
setuid or setgid, in which case
all timers are
disabled)
resource limits see getrlimit(2)
file mode mask see umask(2)
signal mask see sigaction(2), sigsetmask(3)
When a program is executed as a result of an execve() call,
it is entered
as follows:
main(int argc, char **argv, char **envp)
where argc is the number of elements in argv (the ``arg
count'') and argv
points to the array of character pointers to the arguments
themselves.
The exect() function is equivalent to execve() with the additional property
that it executes the file with the program tracing facilities enabled
(see ptrace(2)).
As the execve() function overlays the current process image
with a new
process image the successful call has no process to return
to. If
execve() does return to the calling process an error has occurred; the
return value will be -1 and the global variable errno is set
to indicate
the error.
execve() will fail and return to the calling process if:
[ENOTDIR] A component of the path prefix is not a directory.
[ENAMETOOLONG]
A component of a pathname exceeded {NAME_MAX}
characters,
or an entire path name exceeded {PATH_MAX}
characters.
[ENOENT] The new process file does not exist.
[ELOOP] Too many symbolic links were encountered in
translating the
pathname.
[EACCES] Search permission is denied for a component of
the path
prefix.
[EACCES] The new process file is not an ordinary file.
[EACCES] The new process file mode denies execute permission.
[EACCES] The new process file is on a filesystem mounted with execution
disabled (MNT_NOEXEC in <sys/mount.h>).
[ENOEXEC] The new process file has the appropriate access permission,
but has 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 or reading
by some process.
[ENOMEM] The new process requires more virtual memory
than is allowed
by the imposed maximum (getrlimit(2)).
[E2BIG] The number of bytes in the new process's argument list is
larger than the system-imposed limit. The
limit in the
system as released is 262144 bytes (NCARGS in
<sys/param.h>).
[EFAULT] The new process file is not as long as indicated by the
size values in its header.
[EFAULT] path, argv, or envp point to an illegal address.
[EIO] An I/O error occurred while reading from the
file system.
[ENFILE] During startup of an interpreter, the system
file table was
found to be full.
_exit(2), fork(2), execl(3), exit(3), a.out(5), elf(5), environ(7)
The exect() function should not be used in portable applications.
The execve() function call appeared in 4.2BSD.
If a program is setuid to a non-superuser, but is executed
when the real
uid is ``root'', then the program has some of the powers of
a superuser
as well.
OpenBSD 3.6 January 24, 1994
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