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I have this code in Assembly that I want to open a C program, passing a parameter and after the execution of this C program, a value is returned to my Assembly program.

More specifically, this:

  • Open a C program passing a String as parameter.
  • The C program will convert this String into a float and return it
  • I'll catch this float and use it as I need in my assembly program.
[ASM]                 // Executing Assembly code
     \------------->  // Run C Program passing a value as parameter
      [C Program]     // Do Stuff
     /------------->  // Return value that I can catch back in my Assembly code
[ASM]                 // Goes back to executing Assembly code using value the 
                      // value received by the C program.

Important info: I'm on Linux, and am not sure of what is my C binary. *.out maybe? Could use a few pointers on that too.

P.S: I'm aware of all the floating point instructions and the countless other options I have for doing that all in Assembly, but it's for my Computer Architecture and Organization class and that's how I need it to be.

Thanks everyone.

EDIT: Let me explain it a little better.

I have compiled this C code into a file called Convert and placed it into the same folder as my Assembly (.s) code:

//#includes
int main(int argc, char* argv[]){

    float fVal;
    sscanf(argv[1], "%f", &fVal);

    return fVal;
}

Via Assembly, I want to execute Convert passing a string as "3.1415" as parameter. Convert will convert it to float and return.

Via Assembly, I'll then POP that from the stack and use the Float 3.1415.

All I have in mind is something like a System Call:

push   parameter
push   Executable's name
sys                     ! to wake the system to run my executable

pop    eax              ! pop the Float returned by Convert to EAX

Hope it's clearer now.

Again, thanks!

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6
  • The usual approach is to write the code in C first and use the assembly that the compiler generates. Converting to assembly is quite optional. Commented May 31, 2011 at 2:22
  • Are you sure it's a "program", i.e. a complete binary, and not just a separately compiled C function? Commented May 31, 2011 at 2:23
  • Yes, I wrote a C program (just the main() function, actually), compiled it and I want to run it from my Assembly code, for example: "Program parameter" and my Program will run, catch argv[1]("parameter") and return a value that I'll catch back in my Assembly code with POP to get the return from the stack. Commented May 31, 2011 at 2:25
  • What was the result of using sscanf in your last question? If your c library doesn't even contain sscanf it will most likely not contain execve too. Thats the name of the function you are looking for, BTW Commented May 31, 2011 at 7:50
  • I'm calling sscanf from my external C program which includes the Standard Library, but my Assembler, which implements itself the System Calls to only a few functions of the Std. Lib. doesn't have all the functions I need. I could implement them myself, though, and even tried, but against a bad formatted code with few comments which are in Dutch, I just can't win... Commented May 31, 2011 at 15:36

3 Answers 3

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You can certainly achieve your overall goal of not having to write floating-point conversion code by hand in assembly by delegating to C instead, but you're thinking about the problem the wrong way. You don't need to call an entire C program from your assembly program, you just need to call one function. That one function is called strtod, and you don't have to implement it, because it's already in the C library.

You appear to be on x86-32, so this should do what you want:

push    $0x0   ; endptr
push    parameter
call    strtod
addl    %esp, $8
; double return value is in %st(0)
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4 Comments

Thanks very much for the tip, even though that doesn't help me in this specific situation, it was good to learn of this possible method for the future. But unfortunately, I do need to call the C program, as I'm using an Assembler called as88 which does not include the functions from C's standard library, only some very specific ones that are implemented in its main.c file... Yeah, I do need to open this external program. :/
So get as88 to produce an object file file rather than an executable, then use the C compiler to link it. That way, you can use the regular C library. (You need to make your assembly program implement main like a C program would, rather than _start or whatever, for this to work.)
Nikolai's answer, by the way, hints at the complexity of "opening an external program". It takes a minimum of four system calls to do that, with nontrivial calling conventions, and you can't return a float from main in the C program, it simply won't work.
Exactly, that's what I just realized. I'll try some new things after having all this feedback as soon as I get back home and post the results later. Thank you.
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If you are sure you want to execute the whole program and not just call a function, look into how to perform system calls (here's a link for Linux, though I haven't read this one), and then look at fork(2) and wait(2). You will have to come up with a scheme to pass the result back, something like a pipe(2).

Edit 0:

You cannot return float from main() like that. First, it will be cast to integer, so you lose the fractional part. Second, only part of the word is actually used by the OS to return the program exit status. Check out the wait(2) manual page for the macros you'd use in C to extract that status, and look into you libc headers to see what they do.

You need to use some other facility to return the floating point value from the process, maybe something like Unix pipeline.

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2 Comments

Sounds like a good solution for another kind of problem. Although it doesn't help me right now, as pipe sends an array of integers and there is no way I can use it to return my float. I'm still thinking of something, though, and will definitely keep this solution in mind for another day. Thanks Nikolai.
Hmm, pipe(2) gives you a file-like connection between two processes, i.e. it returns two file descriptors, which you can use to send whatever. Read the manual pages, they are oh-so-useful.
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This depends on the how the C Compiler generates the assembly/machine code, and what cpu it is running on.

Many 'C' compilers will pass the function arguments on the stack, which means you would have understand a stack frame, create one in the assembly code and then jump to your 'C' function/label.

So maybe the easy way to do this would be to create a Test 'C' application that calls your function, compile it and look at the assembly the compiler generates. Then use this assembly as example for how to call from your assembly code.

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I'm running on x86 and I understand how Stack works. I just want to call a C compiled program from my Assembly. Then I'll just pop the return off the stack and use it. I just can't figure out how to call this program in Assembly. :/

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