The
execve ();
system call
transforms the calling process into a new process.
The new process is constructed from an ordinary file,
whose name is pointed to by
Fa 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
elf(5)
and
a.out5.
An interpreter file begins with a line of the form:
#!interpreter
Bq Em arg
When an interpreter file is
execve Ap d
the system actually
execve Ap s
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 is set to the specified
interpreter
The argument
Fa argv
is a pointer to a null-terminated array of
character pointers to null-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
Fa path ) .
The argument
Fa envp
is also a pointer to a null-terminated array of
character pointers to null-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 (.);
If any of the standard descriptors (0, 1, and/or 2) are closed at the
time
execve ();
is called, and the process will gain privilege as a result of set-id
semantics, those descriptors will be re-opened automatically.
No programs, whether privileged or not, should assume that these descriptors
will remain closed across a call to
execve (.);
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-user-ID and set-group-ID processing,
the effective user ID is recorded as the saved set-user-ID,
and the effective group ID is recorded as the saved set-group-ID.
These values may be used in changing the effective IDs later (see
setuid(2)).
The set-ID bits are not honored if the respective file system has the
nosuid
option enabled or if the new process file is an interpreter file.
Syscall
tracing is disabled if effective IDs are changed.
The new process also inherits the following attributes from
the calling process:
When a program is executed as a result of an
execve ();
system call, it is entered as follows:
main(argc, argv, envp)
int argc;
char **argv, **envp;
where
Fa argc
is the number of elements in
Fa argv
(the ``arg count'')
and
Fa argv
points to the array of character pointers
to the arguments themselves.
RETURN VALUES
As the
execve ();
system call 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.
ERRORS
The
execve ();
system call
will fail and return to the calling process if:
Bq Er ENOTDIR
A component of the path prefix is not a directory.
Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
Bq Er ENAMETOOLONG
When invoking an interpreted script, the interpreter name
exceeds
MAXSHELLCMDLEN
characters.
Bq Er ENOENT
The new process file does not exist.
Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
Bq Er EACCES
Search permission is denied for a component of the path prefix.
Bq Er EACCES
The new process file is not an ordinary file.
Bq Er EACCES
The new process file mode denies execute permission.
Bq Er ENOEXEC
The new process file has the appropriate access
permission, but has an invalid magic number in its header.
Bq Er ETXTBSY
The new process file is a pure procedure (shared text)
file that is currently open for writing or reading by some process.
Bq Er ENOMEM
The new process requires more virtual memory than
is allowed by the imposed maximum
(getrlimit(2)
)
Bq Er E2BIG
The number of bytes in the new process' argument list
is larger than the system-imposed limit.
This limit is specified by the
sysctl(3)
MIB variable
KERN_ARGMAX
Bq Er EFAULT
The new process file is not as long as indicated by
the size values in its header.
Bq Er EFAULT
The
Fa path ,
Fa argv ,
or
Fa envp
arguments
point
to an illegal address.
Bq Er EIO
An I/O error occurred while reading from the file system.
CAVEAT
If a program is
setuid
to a non-super-user, but is executed when
the real
uid
is ``root'', then the program has some of the powers
of a super-user as well.
The
execve ();
system call conforms to
St -p1003.1-2001 ,
with the exception of reopening descriptors 0, 1, and/or 2 in certain
circumstances.
A future update of the Standard is expected to require this behavior,
and it may become the default for non-privileged processes as well.
The support for executing interpreted programs is an extension.
