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

int main (int argc, char * argv[]) // taking files
{
    FILE * fp;
    FILE * fp2;
    int c;
.
. // checking possible problems
.
    while ( (c = fgetc(fp)) != EOF){ // copying one file to another one
        fputc (c, stdout);
        fputc (c, fp2);
    }

    fclose(fp);
    fclose(fp2);
    fp = NULL;
    fp2 = NULL;

    return EXIT_SUCCESS;
}

char buffer[4096];
while (fgets(buffer, sizeof(buffer), fp) != 0)
{
    fputs(buffer, stdout);
    fputs(buffer, fp2);
}