Put 25 characters from a file into a char array

Greets all,

I am messing around with files in C. I'll try to explain fully so you are able to help/understand me better. I'm trying to get a c program to open up a text file(with text in it already), and put the first 25 or so characters into a variable as i want to compare a defined string to the string of 25 characters from the file.

Would anybody lend me a hand?

Thanks.

Code:

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#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
void test(FILE*);

int main(){
        const char *compare= "this is 25 charaCTERS    ";
        FILE *in;
        char tmp[512];
        test(in=fopen("myfile.txt","r"));
        while(!feof(in) ){
                memset(tmp,0x00,sizeof(tmp));
                if(fgets(tmp,511,in)!=NULL){
                  tmp[25]=0x00;  /* tmp is now 25 characters long */
                  if(!strncmp(tmp,compare,25)){
                          printf("Equal\n");
                  }else{
                          printf("Not equal \n");
                  }
                }
        }
        fclose(in);
        return 0;
}

void test(FILE *tst){
        if(tst==NULL){
              perror("Error on input file");
              exit(EXIT_FAILURE);
              }
}

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Explain to me why you get 512 bytes when you want only 25? Because of the page size, the sector size? What optimization is that?
On windows you should explicitly state whether to open the file in text or binary format.