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

int main(int argc, char* argv[]) {
    FILE* fp = fopen("o.txt", "w");
    size_t position_in_file = ftell(fp);  // file position indicator (starts at 0)
    printf("ftell(fp) == %zd  # just opened file\n", position_in_file);
    int n = 0x0a216f59;  // changing first two bytes of the int to 0x0a ('\n') so that end of string will be '\n'.
    int* a_n = &n;
    fwrite(a_n, sizeof(*a_n), 1, fp);
    printf("ftell(fp) == %zd  # wrote one int\n", ftell(fp));
    fclose(fp); // RULE:  If you open a file, you must close it (once).

    fp = fopen("o.txt", "r");
    for(char ch = fgetc(fp); ! feof(fp); ch = fgetc(fp)) {
        fputc(ch, stdout);  // print just one character
    }

    // File position indicator goes forward once for every call to fputc(…) or fgetc(…).
    fseek(fp, 0, SEEK_SET);
    for(char ch = fgetc(fp); ! feof(fp); ch = fgetc(fp)) {
        fputc(ch, stdout);  // print just one character
    }

    fclose(fp);

    return EXIT_SUCCESS;
}
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