Bangladesh Computer Council CA Monitoring System Project Exam: 2021প্রশ্ন ও সমাধান

The null character '\0' (ASCII value 0, also called NUL) is a control character used in C to mark the end of a string. A C "string" is really just an array of char — there is no separate string type — so the language needs some marker to know where the meaningful characters stop; '\0' is that marker (a null-terminated string).

#include <stdio.h>
#include <string.h>
int main() {
    char str[] = "Hi";           // stored as {'H','i','\0'} -> occupies 3 bytes
    printf("Length: %d\n", (int)strlen(str));  // strlen stops counting at '\0'
    printf("Array size: %d\n", (int)sizeof(str));
    return 0;
}

Output:

Length: 2
Array size: 3

Why it matters:

• Functions like strlen(), strcpy(), printf("%s", ...) all scan memory byte-by-byte until they hit '\0' — without it, they would read past the intended data into unrelated memory (undefined behaviour / a buffer over-read).
• sizeof() counts the terminator too (hence 3, not 2, above), while strlen() does not.
• Forgetting to reserve space for '\0' when sizing a char array is a classic C bug (off-by-one buffer overflow).

Exam takeaway: '\0' is the sentinel value that tells C string functions "the string ends here" — every C string must be terminated with it to be used safely.