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

int main()
{
    // We have name of the source file
    char *somename = "testfile";

    // Set file name as our name
    const int SIZE_filename = strlen(somename) + 10;
    char filename[SIZE_filename];
    strcpy(filename, somename);

    // Set output name as file name with "_out" appended
    const int SIZE_outfilename = strlen(filename) + 10;
    char outfilename[SIZE_outfilename];
    strcpy(outfilename, filename);
    strcat(outfilename, "_out");

    FILE *file;
    FILE *out;

    // Open files with their names
    file = fopen(filename, "r");
    out = fopen(outfilename, "w");

    // Read from source and write to the output file
    char c;
    while ((c = fgetc(file)) != EOF) {
        fputc(c, out);
    }

    fclose(file);
    fclose(out);

    return 0;
}