Programming

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

int main(int argc, char**argv)
{
    FILE *finp, *fref;
    char inp[4096];
    char ref[4096];
    char * pinp = inp+1;
    char * pref = ref+1;

    if (argc != 3)
    {
        printf("Usage: %s <input file> <reference file>", argv[0]);
        return -1;
    }

    if ((finp = fopen(argv[1], "r")) == NULL)
    {
        perror("fopen");
        return -1;
    }
    if ((fref = fopen(argv[2], "r")) == NULL)
    {
        perror("fopen");
        return -1;
    }


    while (fgets(ref, sizeof(ref), fref))
    {
        rewind(finp);
        while (fgets(inp, sizeof(inp), finp)) {
            if (strcmp(pref, pinp) == 0) {
                printf("%s", inp);
                printf("%s", fgets(inp, sizeof(inp), finp));
            }
        }
    }

   fclose(finp);
   fclose(fref);
   return 0;
}

$ man rewind
REWIND(3P)                 POSIX Programmer's Manual                REWIND(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the  corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

NAME
       rewind - reset the file position indicator in a stream

SYNOPSIS
       #include <stdio.h>

       void rewind(FILE *stream);

DESCRIPTION
       The call:


              rewind(stream)

       shall be equivalent to:


              (void) fseek(stream, 0L, SEEK_SET)