Pointers and Strings

char str[20] = "Hello";
char str[] = "Hello"; // Size is determined automatically (6 bytes including the null character).

// Declare a pointer to a string literal (not modifiable).
const char *p = "Hello";

// Common standard string functions (requires #include <string.h>).
strlen(str) // Returns the length of the string (excluding the null character).
strcpy(dest, src) // Copies a string.
strncpy(dest, src, n) // Copies up to n characters.
strcat(dest, src) // Concatenates two strings.
strcmp(str1, str2) // Compares two strings.
strchr(str, ch) // Searches for a character and returns its address.
strstr(str, sub) // Searches for a substring and returns its address.

sample_pointer_string.c

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

int main(void) {
    // A char array string can be modified.
    char greeting[20] = "Hello";
    greeting[0] = 'h'; // Overwrite the first character.
    printf("%s\n", greeting); // Prints 'hello'.

    // const char * is a pointer to a string literal (not modifiable).
    const char *lang = "C language";
    printf("%s\n", lang);

    printf("Length: %zu\n", strlen(greeting)); // Prints 'Length: 5'.

    // Copy a string.
    char copy[20];
    strncpy(copy, greeting, sizeof(copy) - 1);
    copy[sizeof(copy) - 1] = '\0'; // Ensure null termination for safety.
    printf("Copy: %s\n", copy); // Prints 'Copy: hello'.

    // Compare strings (you cannot use == for this).
    char s1[] = "sample_data_x";
    char s2[] = "sample_data_x";
    if (strcmp(s1, s2) == 0) {
        printf("The strings are equal.\n");
    }

    // Search for a character.
    char sentence[] = "Hello, World!";
    char *found = strchr(sentence, 'W');
    if (found != NULL) {
        printf("Position of 'W': %s\n", found); // Prints 'World!'.
    }

    // Iterate over characters one by one using a pointer.
    char *p = sentence;
    int vowels = 0;
    while (*p != '\0') {
        if (*p == 'a' || *p == 'e' || *p == 'i' || *p == 'o' || *p == 'u') {
            vowels++;
        }
        p++;
    }
    printf("Vowel count: %d\n", vowels);

    return 0;
}

Run the following command:

gcc pointer_string.c -o pointer_string
./pointer_string
hello
C language
Length: 5
Copy: hello
The strings are equal.
Position of 'W': World!
Vowel count: 3

Common Mistake: Using == to Compare Strings

Using == to compare strings compares the addresses (pointer values), not the contents. Even if two strings have the same content, they may not match. Always use strcmp() to compare string contents.

string_eq_ng.c

#include <stdio.h>

int main(void) {
    char s1[] = "item_x";
    char s2[] = "item_x";

    /* NG: == compares addresses (may evaluate as not equal) */
    if (s1 == s2) {
        printf("Equal\n");
    } else {
        printf("Not equal (different addresses).\n");
    }
    return 0;
}

Run the following command:

gcc string_eq_ng.c -o string_eq_ng
./string_eq_ng
Not equal (different addresses).

string_eq_ok.c

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

int main(void) {
    char s1[] = "item_x";
    char s2[] = "item_x";

    /* OK: use strcmp() to compare contents */
    if (strcmp(s1, s2) == 0) {
        printf("Contents are equal.\n");
    }
    return 0;
}

Run the following command:

gcc string_eq_ok.c -o string_eq_ok
./string_eq_ok
Contents are equal.

C has no dedicated string type — all strings are represented as char arrays terminated by a null character. Using == to compare strings compares their addresses, not their contents. Always use strcmp() to compare string values.

strcpy() and strcat() do not check buffer sizes, which can lead to buffer overflows if the destination is too small. For security, it is recommended to use the size-limited variants strncpy() and strncat().

For safe string input, use fgets(). For printf/scanf format specifiers, see printf() / fprintf().