Arrays and Strings

After this lesson, you will be able to: Understand arrays in memory, strings as null-terminated char arrays, string.h functions, and why buffer overflows happen.

C arrays + strings are the source of half of CVEs in the wild. Understanding them prevents writing the next one.

Prerequisites:C Fundamentals

Arrays — contiguous memory

Read carefully.

tsx

int main(void) {
    int nums[5] = {1, 2, 3, 4, 5};
    int n = sizeof(nums) / sizeof(nums[0]);   // 5, array length idiom

    for (int i = 0; i < n; i++) {
        printf("%d\n", nums[i]);
    }

    // CRUCIAL: arrays do NOT carry their length around when passed to functions.
    // void print(int arr[]), inside print, sizeof(arr) is the size of a POINTER.
    // Always pass length explicitly: void print(int *arr, size_t n).

    int matrix[2][3] = { {1,2,3}, {4,5,6} };   // 2D array
    return 0;
}

Strings — null-terminated char arrays

Source of countless bugs.

tsx

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

int main(void) {
    char s1[] = "hello";        // 6 bytes: 'h','e','l','l','o','\0'
    char s2[10] = "hi";          // 10 bytes; 'h','i','\0', then 7 unused bytes
    char *s3 = "world";          // pointer to string literal (READ-ONLY)

    size_t len = strlen(s1);     // 5, does NOT count the null terminator
    char copy[20];
    strncpy(copy, s1, sizeof(copy) - 1);   // SAFE: bounded copy
    copy[sizeof(copy) - 1] = '\0';          // ensure null-termination

    // NEVER: strcpy(copy, src) without knowing src length
    //         gets(buf), removed from standard for being unsafe

    if (strcmp(s1, "hello") == 0) printf("match\n");
    return 0;
}

Safe input reading

fgets > gets > scanf for line input.

tsx

char buf[256];
// Reads up to sizeof(buf)-1 chars + null terminator; safe.
if (fgets(buf, sizeof(buf), stdin) != NULL) {
    // strip trailing newline
    buf[strcspn(buf, "\n")] = '\0';
    printf("Hello, %s\n", buf);
}

// NEVER use gets(), removed from C11 for being unsafe.
// scanf("%s", ...) without width specifier is also dangerous.

Common mistakes only experienced C devs avoid

strcpy, strcat without bounds checking. Off-by-one on null terminator. Assuming sizeof(arr) inside a function gives array length (it gives pointer size). Forgetting that string literals are read-only, `char *p = "hi"; p[0] = 'H';` is undefined.

Quick Check

Why is `char *p = "hi"; p[0] = 'H';` undefined behavior?

Pick the C-spec answer.

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