I am trying to put together and example of coding inline assembly code in a 'C' program. I have not had any success. I am using GCC 4.9.0. As near as I can tell my syntax is correct. However the build blows up with the following errors:
/tmp/ccqC2wtq.s:48: Error: syntax error; found `(', expected `,'
Makefile:51: recipe for target 'all' failed
/tmp/ccqC2wtq.s:48: Error: junk at end of line: `(31)'
/tmp/ccqC2wtq.s:49: Error: syntax error; found `(', expected `,'
/tmp/ccqC2wtq.s:49: Error: junk at end of line: `(9)'
These are related to the input/output/clobbers lines in the code. Anyone have an idea where I went wrong?
asm volatile("li 7, %0\n" // load destination address
"li 8, %1\n" // load source address
"li 9, 100\n" // load count
// save source address for return
"mr 3,7\n"
// prepare for loop
"subi 8,8,1\n"
"subi 9,9,1\n"
// perform copy
"1:\n"
"lbzu 10,2(8)\n"
"stbu 10,2(7)\n"
"subi 9,9,1\n" // Decrement the count
"bne 1b\n" // If zero, we've finished
"blr\n"
: // no outputs
: "m" (array), "m" (stringArray)
: "r7", "r8"
);
It's not clear what you are trying to do with the initial instructions:
li 7, %0
li 8, %1
Do you want to load the address of the variables into those registers? In general, this is not possible because the address is not representable in an immediate operand. The easiest way out is to avoid using r7 and r8, and instead use %0 and %1 directly as the register names. It seems that you want to use these registers as base addresses, so you should use the b constraint:
: "b" (array), "b" (stringArray)
Then GCC will take care of the details of materializing the address in a suitable fashion.
You cannot return using blr from inline assembler because it's not possible to tear down the stack frame GCC created. You also need to double-check the clobbers and make sure that you list all the things you clobber (including condition codes, memory, and overwritten input operands).
I was able to get this working by declaring a pair of pointers and initializing them with the addresses of the arrays. I didn't realize that the addresses wouldn't be available directly. I've used inline assembly very sparsely in the past, usually just
to raise or lower interrupt masks, not to do anything that references variables. I just
had some folks who wanted an example. And the "blr" was leftover when I copied a
snipet of a pure assembly routine to use as a starting point. Thanks for the responses.
The final code piece looks like this:
int main()
{
char * stringArrayPtr;
unsigned char * myArrayPtr;
unsigned char myArray[100];
stringArrayPtr = (char *)&stringArray;
myArrayPtr = myArray;
asm volatile(
"lwz 7,%[myArrayPtr]\n" // load destination address
"lwz 8, %[stringArrayPtr]\n" // load source address
"li 9, 100\n" // load count
"mr 3,7\n" // save source address for return
"subi 8,8,1\n" // prepare for loop
"subi 9,9,1\n"
"1:\n" // perform copy
"lbzu 10,1(8)\n"
"stbu 10,1(7)\n"
"subic. 9,9,1\n" // Decrement the count
"bne 1b\n" // If zero, we've finished
// The outputs / inputs / clobbered list
// No output variables are specified here
// See GCC documentation on extended assembly for details.
:
: [stringArrayPtr] "m" (stringArrayPtr), [myArrayPtr]"m"(myArrayPtr)
: "7","8","9","10"
);
}
I was trying to debug a Python 2 script running on Raspbian (Raspberry Pi flavoured Debian Linux) which had code like
euid = os.geteuid()
if euid != 0:
print("you must be root!")
exit(1)
It seemed like, in the user's environment, euid would sometimes be nonzero even if the script was called with sudo.
To investigate whether this was actually the case, I tried to figure out what os.geteuid() is actually doing.
Since the os module is pretty OS-specific by its nature, the source doesn't actually have a clear definition for os.geteuid().
I also tried hg cloneing the source and compiling it, then using inspect.findsource(os.geteuid), but:
TypeError: <built-in function geteuid> is not a module, class, method, function, traceback, frame, or code object
It's... a builtin? Then "geteuid" in dir(__import__("__builtin__")) should be True, but it isn't.
Is geteuid's definition hidden because it could be spoofed into returning the wrong thing (and that would be bad)? Where can I see these sorts of functions' actual source?
ASCII stupid question, get a stupid ANSI.
I did try full-text searching the source, but apparently I used the wrong command the first time and gave up.
$ grep -rnw '.' -e "geteuid"
./Misc/setuid-prog.c:129: uid_t euid = geteuid();
./Lib/site.py:209: if hasattr(os, "getuid") and hasattr(os, "geteuid"):
./Lib/site.py:211: if os.geteuid() != os.getuid():
./Lib/test/test_shutil.py:84: #unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
./Lib/test/test_httpservers.py:339: if os.name == 'posix' and os.geteuid() != 0:
./Lib/test/test_httpservers.py:395:#unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
./Lib/test/test_spwd.py:8:#unittest.skipUnless(hasattr(os, 'geteuid') and os.geteuid() == 0,
./Lib/test/test_posix.py:44: "getegid", "geteuid", "getgid", "getgroups",
./Lib/test/test_argparse.py:1532:#unittest.skipIf(hasattr(os, 'geteuid') and os.geteuid() == 0,
Binary file ./Lib/tarfile.py matches
./Lib/rexec.py:148: 'getcwd', 'getuid', 'getgid', 'geteuid', 'getegid')
Binary file ./Modules/posixmodule.o matches
./Modules/posixmodule.c:4047:"geteuid() -> euid\n\n\
./Modules/posixmodule.c:4053: return _PyInt_FromUid(geteuid());
./Modules/posixmodule.c:8944: {"geteuid", posix_geteuid, METH_NOARGS, posix_geteuid__doc__},
./Doc/library/rexec.rst:234: 'times', 'uname', 'getpid', 'getppid', 'getcwd', 'getuid', 'getgid', 'geteuid',
./Doc/library/os.rst:136:.. function:: geteuid()
Binary file ./python matches
Binary file ./libpython2.7.a matches
./Modules/posixmodule.c:4053, indeed:
#ifdef HAVE_GETEUID
PyDoc_STRVAR(posix_geteuid__doc__,
"geteuid() -> euid\n\n\
Return the current process's effective user id.");
static PyObject *
posix_geteuid(PyObject *self, PyObject *noargs)
{
return _PyInt_FromUid(geteuid());
}
#endif
I don't know what I expected, subprocess.check_output(["id"])?
It uses the C standard library, it's never wrong.
I'm running into a problem with the MNP package which I've traced to an unfortunate call to deparse (whose maximum width is limited to 500 characters).
Background (easily skippable if you're bored)
Because mnp uses a somewhat idiosyncratic syntax to allow for varying choice sets (you include cbind(choiceA,choiceB,...) in the formula definition), the left hand side of my formula call is 1700 characters or so when model.matrix.default calls deparse on it. Since deparse supports a maximum width.cutoff of 500 characters, the sapply(attr(t, "variables"), deparse, width.cutoff = 500)[-1L] line in model.matrix.default has as its first element:
[1] "cbind(plan1, plan2, plan3, plan4, plan5, plan6, plan7, plan8, plan9, plan10, plan11, plan12, plan13, plan14, plan15, plan16, plan17, plan18, plan19, plan20, plan21, plan22, plan23, plan24, plan25, plan26, plan27, plan28, plan29, plan30, plan31, plan32, plan33, plan34, plan35, plan36, plan37, plan38, plan39, plan40, plan41, plan42, plan43, plan44, plan45, plan46, plan47, plan48, plan49, plan50, plan51, plan52, plan53, plan54, plan55, plan56, plan57, plan58, plan59, plan60, plan61, plan62, plan63, "
[2] " plan64, plan65, plan66, plan67, plan68, plan69, plan70, plan71, plan72, plan73, plan74, plan75, plan76, plan77, plan78, plan79, plan80, plan81, plan82, plan83, plan84, plan85, plan86, plan87, plan88, plan89, plan90, plan91, plan92, plan93, plan94, plan95, plan96, plan97, plan98, plan99, plan100, plan101, plan102, plan103, plan104, plan105, plan106, plan107, plan108, plan109, plan110, plan111, plan112, plan113, plan114, plan115, plan116, plan117, plan118, plan119, plan120, plan121, plan122, plan123, "
[3] " plan124, plan125, plan126, plan127, plan128, plan129, plan130, plan131, plan132, plan133, plan134, plan135, plan136, plan137, plan138, plan139, plan140, plan141, plan142, plan143, plan144, plan145, plan146, plan147, plan148, plan149, plan150, plan151, plan152, plan153, plan154, plan155, plan156, plan157, plan158, plan159, plan160, plan161, plan162, plan163, plan164, plan165, plan166, plan167, plan168, plan169, plan170, plan171, plan172, plan173, plan174, plan175, plan176, plan177, plan178, plan179, "
[4] " plan180, plan181, plan182, plan183, plan184, plan185, plan186, plan187, plan188, plan189, plan190, plan191, plan192, plan193, plan194, plan195, plan196, plan197, plan198, plan199, plan200, plan201, plan202, plan203, plan204, plan205, plan206, plan207, plan208, plan209, plan210, plan211, plan212, plan213, plan214, plan215, plan216, plan217, plan218, plan219, plan220, plan221, plan222, plan223, plan224, plan225, plan226, plan227, plan228, plan229, plan230, plan231, plan232, plan233, plan234, plan235, "
[5] " plan236, plan237, plan238, plan239, plan240, plan241, plan242, plan243, plan244, plan245, plan246, plan247, plan248, plan249, plan250, plan251, plan252, plan253, plan254, plan255, plan256, plan257, plan258, plan259, plan260, plan261, plan262, plan263, plan264, plan265, plan266, plan267, plan268, plan269, plan270, plan271, plan272, plan273, plan274, plan275, plan276, plan277, plan278, plan279, plan280, plan281, plan282, plan283, plan284, plan285, plan286, plan287, plan288, plan289, plan290, plan291, "
[6] " plan292, plan293, plan294, plan295, plan296, plan297, plan298, plan299, plan300, plan301, plan302, plan303, plan304, plan305, plan306, plan307, plan308, plan309, plan310, plan311, plan312, plan313)"
When model.matrix.default tests this against the variables in the data.frame, it returns an error.
The problem
To get around this, I've written a new deparse function:
deparse <- function (expr, width.cutoff = 60L, backtick = mode(expr) %in%
c("call", "expression", "(", "function"), control = c("keepInteger",
"showAttributes", "keepNA"), nlines = -1L) {
ret <- .Internal(deparse(expr, width.cutoff, backtick, .deparseOpts(control), nlines))
paste0(ret,collapse="")
}
However, when I run mnp again and step through, it returns the same error for the same reason (base::deparse is being run, not my deparse).
This is somewhat surprising to me, as what I expect is more typified by this example, where the user-defined function temporarily over-writes the base function:
> print <- function() {
+ cat("user-defined print ran\n")
+ }
> print()
user-defined print ran
I realize the right way to solve this problem is to rewrite model.matrix.default, but as a tool for debugging I'm curious how to force it to use my deparse and why the anticipated (by me) behavior is not happening here.
The functions fixInNamespace and assignInNamespace are provided to allow editing of existing functions. You could try ... but I will not since mucking with deparse looks too dangerous:
assignInNamespace("deparse",
function (expr, width.cutoff = 60L, backtick = mode(expr) %in%
c("call", "expression", "(", "function"), control = c("keepInteger",
"showAttributes", "keepNA"), nlines = -1L) {
ret <- .Internal(deparse(expr, width.cutoff, backtick, .deparseOpts(control), nlines))
paste0(ret,collapse="")
} , "base")
There is an indication on the help page that the use of such functions has restrictions and I would not be surprised that such core function might have additional layers of protection. Since it works via side-effect, you should not need to assign the result.
This is how packages with namespaces search for functions, as described in Section 1.6, Package Namespaces of Writing R Extensions
Namespaces are sealed once they are loaded. Sealing means that imports
and exports cannot be changed and that internal variable bindings
cannot be changed. Sealing allows a simpler implementation strategy
for the namespace mechanism. Sealing also allows code analysis and
compilation tools to accurately identify the definition corresponding
to a global variable reference in a function body.
The namespace controls the search strategy for variables used by
functions in the package. If not found locally, R searches the package
namespace first, then the imports, then the base namespace and then
the normal search path.