Question

When the following code is compiled with LLVM Compiler, it doesn’t operate correctly.
(i doesn’t increase.)
It operates correctly when compiling with GCC 4.2.
Is this a bug of LLVM Compiler?

#include <stdio.h>
#include <string.h>

void BytesFromHexString(unsigned char *data, const char *string) {
    printf("bytes:%s:", string);
    int len = (int)strlen(string);
    for (int i=0; i<len; i+=2) {
        unsigned char x;
        sscanf((char *)(string + i), "%02x", &x);
        printf("%02x", x);
        data[i] = x;
    }
    printf("\n");
}

int main (int argc, const char * argv[])
{
    // insert code here...
    unsigned char data[64];
    BytesFromHexString(data, "4d4f5cb093fc2d3d6b4120658c2d08b51b3846a39b51b663e7284478570bcef9");
    return 0;
}

Answer 1

For sscanf you’d use %2x instead of %02x. Furthermore, %2x indicates that an extra int* argument will be passed. But you’re passing an unsigned char*. And finally, sscanf takes a const char* as first argument, so there’s no need for that cast.

So give this a try :

int x;
sscanf((string + i), "%2x", &x);

EDIT : to clarify why this change resolves the issue : in your code, sscanf tried to write sizeof(int) bytes in a memory location (&x) that could only hold sizeof(unsigned char) bytes (ie. 1 byte). So, you were overwriting a certain amount of memory. This overwritten memory could very well have been (part of) the i variable.