why ? don't you trust fgets(3) manpage ?

test-fgets.c

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

int showChar(char c) {
    switch (c) {
    case '!'...'~':
        fprintf(stderr, "'%c ", c);
        break;
    case 0:
        fprintf(stderr, "\\0 ");
        break;
    case '\n':
        fprintf(stderr, "\\n ");
        break;
    case -1:
        fprintf(stderr, "?? ");
        break;
    default:
        fprintf(stderr, "%02x ", c);
        break;
    }
}

int main() {
    char data[16];

    fprintf(stderr, "%p  ", data);
    for (size_t i = 0; i < sizeof(data); i++) {
        fprintf(stderr, "%02lx-", i);
    }
    fprintf(stderr, "\n");

    while (!feof(stdin)) {
        memset(data, -1, sizeof(data));
        char* ptr = fgets(data, sizeof(data), stdin);
        fprintf(stderr, "%p  ", ptr);
        for (size_t i = 0; i < sizeof(data); i++) {
            showChar(data[i]);
        }
        fprintf(stderr, "\n");
    }
    return 0;
}