I had an earlier question about integrating Mathematica with functions written in C++.
This is a follow-up question:
If the computation takes too long I'd like to be able to abort it using Evaluation > Abort Evaluation. Which of the technologies suggested in the answers make it possible to have an interruptible C-based extension function? How can "interruptibility" be implemented on the C side?
I need to make my function interruptible in a way which will corrupt neither it, nor the Mathematica kernel (i.e. it should be possible to call the function again from Mathematica after it has been interrupted)
For MathLink - based functions, you will have to do two things (On Windows): use MLAbort to check for aborts, and call MLCallYieldFunction, to yield the processor temporarily. Both are described in the MathLink tutorial by Todd Gayley from way back, available here.
Using the bits from my previous answer, here is an example code to compute the prime numbers (in an inefficient manner, but this is what we need here for an illustration):
code =
"
#include <stdlib.h>
extern void primes(int n);
static void yield(){
MLCallYieldFunction(
MLYieldFunction(stdlink),
stdlink,
(MLYieldParameters)0 );
}
static void abort(){
MLPutFunction(stdlink,\" Abort \",0);
}
void primes(int n){
int i = 0, j=0,prime = 1, *d = (int *)malloc(n*sizeof(int)),ctr = 0;
if(!d) {
abort();
return;
}
for(i=2;!MLAbort && i<=n;i++){
j=2;
prime = 1;
while (!MLAbort && j*j <=i){
if(i % j == 0){
prime = 0;
break;
}
j++;
}
if(prime) d[ctr++] = i;
yield();
}
if(MLAbort){
abort();
goto R1;
}
MLPutFunction(stdlink,\"List\",ctr);
for(i=0; !MLAbort && i < ctr; i++ ){
MLPutInteger(stdlink,d[i]);
yield();
}
if(MLAbort) abort();
R1: free(d);
}
";
and the template:
template =
"
void primes P((int ));
:Begin:
:Function: primes
:Pattern: primes[n_Integer]
:Arguments: { n }
:ArgumentTypes: { Integer }
:ReturnType: Manual
:End:
";
Here is the code to create the program (taken from the previous answer, slightly modified):
Needs["CCompilerDriver`"];
fullCCode = makeMLinkCodeF[code];
projectDir = "C:\\Temp\\MLProject1";
If[! FileExistsQ[projectDir], CreateDirectory[projectDir]]
pname = "primes";
files = MapThread[
Export[FileNameJoin[{projectDir, pname <> #2}], #1,
"String"] &, {{fullCCode, template}, {".c", ".tm"}}];
Now, here we create it:
In[461]:= exe=CreateExecutable[files,pname];
Install[exe]
Out[462]= LinkObject["C:\Users\Archie\AppData\Roaming\Mathematica\SystemFiles\LibraryResources\
Windows-x86-64\primes.exe",161,10]
and use it:
In[464]:= primes[20]
Out[464]= {2,3,5,7,11,13,17,19}
In[465]:= primes[10000000]
Out[465]= $Aborted
In the latter case, I used Alt+"." to abort the computation. Note that this won't work correctly if you do not include a call to yield.
The general ideology is that you have to check for MLAbort and call MLCallYieldFunction for every expensive computation, such as large loops etc. Perhaps, doing that for inner loops like I did above is an overkill though. One thing you could try doing is to factor the boilerplate code away by using the C preprocessor (macros).
Without ever having tried it, it looks like the Expression Packet functionality might work in this way - if your C code goes back and asks mathematica for some more work to do periodically, then hopefully aborting execution on the mathematica side will tell the C code that there is no more work to do.
If you are using LibraryLink to link external C code to the Mathematica kernel, you can use the Library callback function AbortQ to check if an abort is in progress.
Related
I recently wrote a parser generator tool that takes a BNF grammar (as a string) and a set of actions (as a function pointer array) and output a parser (= a state automaton, allocated on the heap). I then use another function to use that parser on my input data and generates a abstract syntax tree.
In the initial parser generation, there is quite a lot of steps, and i was wondering if gcc or clang are able to optimize this, given constant inputs to the parser generation function (and never using the pointers values, only dereferencing them) ? Is is possible to run the function at compile time, and embed the result (aka, the allocated memory) in the executable ?
(obviously, that would be using link time optimization, since the compiler would need to be able to check that the whole function does indeed have the same result with the same parameters)
What you could do in this case is have code that generates code.
Have your initial parser generator as a separate piece of code that runs independently. The output of this code would be a header file containing a set of variable definitions initialized to the proper values. You then use this file in your main code.
As an example, suppose you have a program that needs to know the number of bits that are set in a given byte. You could do this manually whenever you need:
int count_bits(uint8_t b)
{
int count = 0;
while (b) {
count += b & 1;
b >>= 1;
}
return count;
}
Or you can generate the table in a separate program:
int main()
{
FILE *header = fopen("bitcount.h", "w");
if (!header) {
perror("fopen failed");
exit(1);
}
fprintf(header, "int bit_counts[256] = {\n");
int count;
unsigned v;
for (v=0,count=0; v<256; v++) {
uint8_t b = v;
while (b) {
count += b & 1;
b >>= 1;
}
fprintf(header, " %d,\n" count);
}
fprintf(header, "};\n");
fclose(header);
return 0;
}
This create a file called bitcount.h that looks like this:
int bit_counts[256] = {
0,
1,
1,
2,
...
7,
};
That you can include in your "real" code.
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Closed 10 years ago.
Possible Duplicate:
ANSI C equivalent of try/catch?
Is there a way to skip critical code ? More or less like try-catch in modern programming languages. Just now I'm using this technique to spot errors:
bindSignals();
{
signal(SIGFPE, sigint_handler);
// ...
}
int main(void)
{
bindsignals();
int a = 1 / 0; // division by zero, I want to skip it
return 0;
}
The problem is if I don't exit the program in the handler I get the very same error again and again. If possible I would like to avoid goto. I also heard about "longjump" or something. Is it worth to (learn to) use ?
Well, you can probably accomplish something like that using longjmp(), yes.
Possibly with the "help" of some macros. Note the comment on the manual page, though:
longjmp() and siglongjmp() make programs hard to understand and maintain. If possible an alternative should be used.
I'll throw my two cents in on this. C does not have a mechanism like the try/catch that other languages support. You can build something using setjmp() and longjmp() that will be similar, but nothing exactly the same.
Here's a link showing a nice way of using setjmp and longjmp to do what you were thinking; and a code snippet from the same source:
jmp_buf jumper;
int SomeFunction(int a, int b)
{
if (b == 0) // can't divide by 0
longjmp(jumper, -3);
return a / b;
}
void main(void)
{
if (setjmp(jumper) == 0)
{
int Result = SomeFunction(7, 0);
// continue working with Result
}
else
printf("an error occured\n");
}
I'm currently going from C to Java and I'm having a hard time understanding why you'd use try/catch in the first place. With good error checking you should be fine, always always always use the values that are returned from functions, check the errono values, and validate any user input.
I'm done. That's how my code looks like now. Almost like Java and C#.
#include <setjmp.h>
jmp_buf jumper;
#define try if (setjmp(jumper) == 0)
#define catch else
#define skip_to_catch longjmp(jumper, 0)
static void sigint_handler(int sig)
{
skip_to_catch;
}
int main(void)
{
// init error handling once at the beginning
signal(SIGFPE, sigint_handler);
try
{
int a = 1 / 0;
}
catch
{
printf("hello error\n");
}
return 0;
}
Here is my C code..
void Read(int t,char* string1)
{
int j,i,p,row,count=0;
for(i=0;i<t;++i,string1=strchr(string1,')')+2)
{
sscanf(string1,"(%d,%d)",&p,&row);
CallFunction(p,row);
}
}
Here is how i have to call this function:
Read(2,"(3,5),(7,8)")
Is this a good way to deal with such kind of input parameters? Is it time consuming?
Is there any other good way (optimised way) of reading the same input parameters?
You could use the %n format-specifier for sscanf(), which allows you to omit the strchr() function. The speed improvement is probably marginal.
BTW: dont' call a function "Read", not even if you can assume a case-sensitive compiler and linker.
#include <stdio.h>
#define CallFunction(a,b) fprintf(stderr, "p=%d row=%d\n", a, b)
void do_read(int cnt,char *input)
{
int i,err,p,row,res;
for(i=0; i<cnt ; i++,input += res )
{
err = sscanf(input,"(%d,%d)%n",&p,&row, &res);
if (err < 2) {
fprintf(stderr, "%s:%d: input='%s', err=%d\n"
, __FILE__ , __LINE__, input, err );
break;
}
CallFunction(p,row);
if (input[res] == ',') res++;
}
}
int main(void)
{
do_read(2,"(3,5),(7,8)"); /* this should succeed */
do_read(2,"(3,5)#(7,8)"); /* this must fail ... */
return 0;
}
This code is reasonably fast. But how fast it needs to be depends on your constrainsts which are unknown to me.
I hope that your input data is already checked because string1=strchr(string1,')')+2 (and what follows) is not safe.
Reading your code makes me think that, if you really need bare to the metal speed, you should ditch the function calls and do the job manually (parsing the string yourself).
But given the 'API' you have published, the question of the speed may be defeated ABOVE and BELOW this code snippet.
Reaching the optimal code chain then depends on... all the chain: the whole will not run faster than the slowest function in the chain.
Sorry not to be more specific but this is really a more global question than the information you provide lets me address it (I don't have the whole picture).
I am confused by the for(;;) construct. I think it is a form of shorthand for an unlimited for loop but I can't be sure.
Here is the code:
for(;;)
{
//whatever statements
}
Your guess is correct; it's an infinite loop.* This is a common C idiom, although many people (including me) believe the following to be less cryptic:
while (1) { whatever statements; }
* It's infinite assuming there are no break/return/etc. statements inside the loop body.
It's an un-terminated loop. It is sometimes written with a while:
while (1)
or even better:
while (true)
I would expect to see a break or return inside any such loop, no matter whether it is written with for or while. There has to be some abnormal control flow or it really will be an infinite loop.
Yes, that's the for C syntax with blank fields for initialization expression, loop condition and increment expression.
The for statement can also use more than one value, like this sample :
for (i=0, j=100, k=1000; j < 500 || i<50 || k==5000; i++, j+=2, k*=6) {};
Maybe one step beyond in for understanding ? =)
Yes, the expressions in the for loop are just optional. if you omit them, you will get an infinite loop. The way to get out is break or exit or so.
This statement is basically equal to:
while(1) {}
There is no start, no condition and no step statement.
As I understand it, for(;;) creates a deliberate non-exiting loop. Your code is expected to exit the loop based on one or more conditions. It was once provided to me as a purer way to have a do while false loop, which was not considered good syntax. Based on the exit condition, it is easier to dispatch to a function to handle the result, failure, warning, or success, for example.
My explanation may not be the reason someone used that construct, but I'll explain in greater detail what it means to me. This construct may be someone's "Pure C" way of having a loop in which you can serially perform multiple steps, whose completion mean something like your application has performed all steps of initialization.
#define GEN_FAILURE -99
#define SUCCESS 0
/* perform_init_step1() and perform_init_step2() are dummy
place-holder functions that provide a complete example.
You could at least have one of them return non-zero
for testing. */
int perform_init_step1();
int perform_init_step2();
int perform_init_step1()
{
return 0;
}
int perform_init_step2()
{
return 0;
}
int ret_code = GEN_FAILURE;
for(;;)
{
if(SUCCESS != perform_init_step1())
{
ret_code = -1;
break;
}
if(SUCCESS != perform_init_step2())
{
ret_code = -2;
break;
}
break;
}
If part of the initialization fails, the loop bails out with a specific error code.
I arrived at using C having done a lot of firmware work, writing in assembly language. Good assembly language programmers taught me to have a single entry point and single exit. I took their advice to heart, because their creed helped them and me immensely when debugging.
Personally, I never liked the for(;;) construct, because you can have an infinite loop if you forget to break; out at the end.
Someone I worked with came up with do..until(FALSE), but the amount of proper C furvor this caused was not to be believed.
#define GEN_FAILURE -99
#define SUCCESS 0
/* perform_init_step1() and perform_init_step2() are dummy
place-holder functions that provide a complete example.
You could at least have one of them return non-zero
for testing. */
int perform_init_step1();
int perform_init_step2();
int perform_init_step1()
{
return 0;
}
int perform_init_step2()
{
return 0;
}
int ret_code = GEN_FAILURE;
do
{
if(SUCCESS != perform_init_step1())
{
ret_code = -1;
break;
}
if(SUCCESS != perform_init_step2())
{
ret_code = -2;
break;
}
}
until (FALSE);
This runs once, no matter what.
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I'd love some references, or tips, possibly an e-book or two. I'm not looking to write a compiler, just looking for a tutorial I could follow along and modify as I go. Thank you for being understanding!
BTW: It must be C.
Any more replies would be appreciated.
A great way to get started writing an interpreter is to write a simple machine simulator. Here's a simple language you can write an interpreter for:
The language has a stack and 6 instructions:
push <num> # push a number on to the stack
pop # pop off the first number on the stack
add # pop off the top 2 items on the stack and push their sum on to the stack. (remember you can add negative numbers, so you have subtraction covered too). You can also get multiplication my creating a loop using some of the other instructions with this one.
ifeq <address> # examine the top of the stack, if it's 0, continue, else, jump to <address> where <address> is a line number
jump <address> # jump to a line number
print # print the value at the top of the stack
dup # push a copy of what's at the top of the stack back onto the stack.
Once you've written a program that can take these instructions and execute them, you've essentially created a very simple stack based virtual machine. Since this is a very low level language, you won't need to understand what an AST is, how to parse a grammar into an AST, and translate it to machine code, etc. That's too complicated for a tutorial project. Start with this, and once you've created this little VM, you can start thinking about how you can translate some common constructs into this machine. e.g. you might want to think about how you might translate a C if/else statement or while loop into this language.
Edit:
From the comments below, it sounds like you need a bit more experience with C before you can tackle this task.
What I would suggest is to first learn about the following topics:
scanf, printf, putchar, getchar - basic C IO functions
struct - the basic data structure in C
malloc - how to allocate memory, and the difference between stack memory and heap memory
linked lists - and how to implement a stack, then perhaps a binary tree (you'll need to
understand structs and malloc first)
Then it'll be good to learn a bit more about the string.h library as well
- strcmp, strdup - a couple useful string functions that will be useful.
In short, C has a much higher learning curve compared to python, just because it's a lower level language and you have to manage your own memory, so it's good to learn a few basic things about C first before trying to write an interpreter, even if you already know how to write one in python.
The only difference between an interpreter and a compiler is that instead of generating code from the AST, you execute it in a VM instead. Once you understand this, almost any compiler book, even the Red Dragon Book (first edition, not second!), is enough.
I see this is a bit of a late reply, however since this thread showed up at second place in the result list when I did a search for writing an interpreter and no one have mentioned anything very concrete I will provide the following example:
Disclaimer: This is just some simple code I wrote in a hurry in order to have a foundation for the explanation below and are therefore not perfect, but it compiles and runs, and seems to give the expected answers.
Read the following C-code from bottom to top:
#include <stdio.h>
#include <stdlib.h>
double expression(void);
double vars[26]; // variables
char get(void) { char c = getchar(); return c; } // get one byte
char peek(void) { char c = getchar(); ungetc(c, stdin); return c; } // peek at next byte
double number(void) { double d; scanf("%lf", &d); return d; } // read one double
void expect(char c) { // expect char c from stream
char d = get();
if (c != d) {
fprintf(stderr, "Error: Expected %c but got %c.\n", c, d);
}
}
double factor(void) { // read a factor
double f;
char c = peek();
if (c == '(') { // an expression inside parantesis?
expect('(');
f = expression();
expect(')');
} else if (c >= 'A' && c <= 'Z') { // a variable ?
expect(c);
f = vars[c - 'A'];
} else { // or, a number?
f = number();
}
return f;
}
double term(void) { // read a term
double t = factor();
while (peek() == '*' || peek() == '/') { // * or / more factors
char c = get();
if (c == '*') {
t = t * factor();
} else {
t = t / factor();
}
}
return t;
}
double expression(void) { // read an expression
double e = term();
while (peek() == '+' || peek() == '-') { // + or - more terms
char c = get();
if (c == '+') {
e = e + term();
} else {
e = e - term();
}
}
return e;
}
double statement(void) { // read a statement
double ret;
char c = peek();
if (c >= 'A' && c <= 'Z') { // variable ?
expect(c);
if (peek() == '=') { // assignment ?
expect('=');
double val = expression();
vars[c - 'A'] = val;
ret = val;
} else {
ungetc(c, stdin);
ret = expression();
}
} else {
ret = expression();
}
expect('\n');
return ret;
}
int main(void) {
printf("> "); fflush(stdout);
for (;;) {
double v = statement();
printf(" = %lf\n> ", v); fflush(stdout);
}
return EXIT_SUCCESS;
}
This is an simple recursive descend parser for basic mathematical expressions supporting one letter variables. Running it and typing some statements yields the following results:
> (1+2)*3
= 9.000000
> A=1
= 1.000000
> B=2
= 2.000000
> C=3
= 3.000000
> (A+B)*C
= 9.000000
You can alter the get(), peek() and number() to read from a file or list of code lines. Also you should make a function to read identifiers (basically words). Then you expand the statement() function to be able to alter which line it runs next in order to do branching. Last you add the branch operations you want to the statement function, like
if "condition" then
"statements"
else
"statements"
endif.
while "condition" do
"statements"
endwhile
function fac(x)
if x = 0 then
return 1
else
return x*fac(x-1)
endif
endfunction
Obviously you can decide the syntax to be as you like. You need to think about ways of define functions and how to handle arguments/parameter variables, local variables and global variables. If preferable arrays and data structures. References∕pointers. Input/output?
In order to handle recursive function calls you probably need to use a stack.
In my opinion this would be easier to do all this with C++ and STL. Where for example one std::map could be used to hold local variables, and another map could be used for globals...
It is of course possible to write a compiler that build an abstract syntax tree out of the code. Then travels this tree in order to produce either machine code or some kind of byte code which executed on a virtual machine (like Java and .Net). This gives better performance than naively parse line by line and executing them, but in my opinion that is NOT writing an interpreter. That is writing both a compiler and its targeted virtual machine.
If someone wants to learn to write an interpreter, they should try making the most basic simple and practical working interpreter.