Question

I want to change one value of a file, but the follow code doesn’t work at all. It works properly when i remove the if statement.
Code follows:

#include <stdio.h>

int main()
{
    int b;
    FILE *fp;

    fp=fopen("new", "r+");
    fseek(fp, 0x30, 0);

    b = fgetc(fp);
    if(b == 0x63)
        fputc(0x64, fp);

    fclose(fp);
    return 0;
}

file “new”:
c111111111c1111111111111111111111111111111111111c1111111111111111111

Is there some details that I don’t know？

Answer 1

You’ve written one past b if it is 0x63, because fgetc advances the file stream:

b = fgetc(fp);
if(b == 0x63) {
    fseek(fp, -1, SEEK_CUR);
    fputc(0x64, fp);
}