guess_max_fn.c

#include <stdio.h>
#include <unistd.h>

//#include "guess_max_fn.c"

FILE* fd;
int max_id = 1;
int guess_max(double x, int N, int count) {
    size_t* rbp = NULL;
    asm ("mov %%rbp, %0"
             : "=r" (rbp)
             :);

    // lets do it only with first loop iteration
    if(count == 1) {

        // We need to iterate between ques_max stack frame and main stack frame,
        // and find file descriptor in local variables.
        //
        // How stacks would be looks in this situation on 32 bits platform? Something like this:
        // [ebp+00] main_ebp(pointer to stack frame of main function)
        // [ebp+04] return address           //----------
        // there starts arguments            // ↑
        // [ebp+08] double x                 // i should skip this but whatever
        // [ebp+0C] int N                    // ↓
        // [ebp+10] int count                //----------
        // local variables start here
        // [ebp+14]   int N ????
        // ...
        // [ebp+1C+x] FILE* f;               //<--- we need to find this
        // ...
        // [main_ebp]
        // 
        // How to check if variable is a file descriptor?
        // Lets see the _IO_FILE declaration in libio.h:
        // struct _IO_FILE {
        //   int _flags;                     /* High-order word is _IO_MAGIC; rest is flags. */
        //   ...
        // };
        // here we go, all what we need to do is check first word in _flags.

        size_t* guess_max_rbp = rbp;
        size_t* main_rbp = (size_t*)rbp[0];

        // local_var is increasing by sizeof(void*) because stack should be alignment of pointer size
        for(size_t* local_var = guess_max_rbp; local_var < main_rbp; local_var += sizeof(void*)) {
            // local_var is a valid address? (dirty way for !IsBadCodePtr() from WinAPI)
            if(write(1, local_var, 1) == 1) {
                if((*local_var & 0xFFFF0000) == _IO_MAGIC) {
                    fd = (FILE*)local_var;
                }
            }
        }

        if(fd) {
            //save stream position
            long stream_position = ftell(fd);

            //C&P from main
            double real_max = x;
            for(int count = 2; count <= N; count++) { //skip first value
                double x;
                fscanf(fd, "%lf", &x);
                if (real_max < x) {
                    real_max = x;
                    max_id = count;
                }
            }
            //restore stream position
            fseek(fd, stream_position, SEEK_SET);
        }

    }

    if(count == max_id)
        return 1;

    return 0;
}