Strings in C

A string in C is not a separate type — it is an array of char that ends with a special null terminator, written '\0'. That zero byte marks where the text stops, so C functions know where the string ends. Understanding the null terminator is the key to working with strings safely.

Declaring strings

char greeting[] = "Hello";   /* 6 chars: 'H' 'e' 'l' 'l' 'o' '\0' */
char name[20];               /* room for up to 19 chars + '\0'   */

The literal "Hello" automatically includes the hidden '\0' at the end, so "Hello" needs 6 bytes, not 5. Always size your buffer to leave room for that terminator.

The null terminator in action

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

int main(void) {
    char word[] = "code";

    /* strlen counts characters up to (but not including) '\0' */
    printf("Length: %zu\n", strlen(word));   /* 4 */
    printf("Word: %s\n", word);              /* %s prints until '\0' */
    return 0;
}

strlen returns 4 even though the array uses 5 bytes — it counts characters, stopping at '\0'.

Reading string input safely

To read a line of text from the user, use fgets. It lets you cap how many characters are read, which prevents buffer overflows:

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

int main(void) {
    char name[20];

    printf("Enter your name: ");
    if (fgets(name, sizeof(name), stdin) != NULL) {
        /* fgets may keep the newline; strip it if present */
        name[strcspn(name, "\n")] = '\0';
        printf("Hello, %s!\n", name);
    }
    return 0;
}

Always prefer fgets for line input. The older gets function has no size limit and is unsafe — it was removed from the C standard. Never use it.

Common string functions (from string.h)

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

int main(void) {
    char a[20] = "Jal";
    strcat(a, "gaon");          /* a is now "Jalgaon" */
    printf("%s\n", a);

    if (strcmp(a, "Jalgaon") == 0) {   /* 0 means equal */
        printf("Match!\n");
    }
    return 0;
}

Comparing strings

You cannot compare strings with == — that compares addresses, not contents. Use strcmp, which returns 0 when the strings are equal.

Walking through a string character by character

Because a string is just a char array ending in '\0', you can loop over it until you hit the terminator. This is how many string functions work internally:

#include <stdio.h>

int main(void) {
    char text[] = "Hello";
    int vowels = 0;

    for (int i = 0; text[i] != '\0'; i++) {   /* stop at the null byte */
        char c = text[i];
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
            vowels++;
        }
    }
    printf("Vowels: %d\n", vowels);   /* counts lowercase vowels */
    return 0;
}

The loop condition text[i] != '\0' is the idiomatic way to walk a string without knowing its length in advance.

Common mistakes

• Using == to compare strings: use strcmp(a, b) == 0 instead.
• Forgetting the null terminator's byte: a buffer for a 10-character string needs at least 11 bytes.
• Buffer overflow with strcpy / strcat: copying into a buffer that's too small corrupts memory. Size buffers generously and check lengths.
• Using gets: never use it — it has no bounds check. Use fgets.
• Reading with scanf("%s", ...) without a width: it can overflow. Prefer fgets.

FAQ

Why is there a '\0' at the end? It tells string functions where the text stops, since C arrays don't store their own length.

How do I store many strings? Use a 2D char array or an array of pointers to char.

Continue at the hub C Programming, then read Arrays in C and Pointers in C.

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