How to Find Substring in C: Simple Guide with Examples

In C, you can find a substring inside a string using the strstr function from string.h. It returns a pointer to the first occurrence of the substring or NULL if not found.

Syntax

The strstr function is declared in string.h and used like this:

• char *strstr(const char *haystack, const char *needle);

Here, haystack is the main string to search in, and needle is the substring you want to find.

The function returns a pointer to the first occurrence of needle in haystack, or NULL if needle is not found.

c

char *strstr(const char *haystack, const char *needle);

Example

This example shows how to use strstr to find a substring and print its position or a message if not found.

c

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

int main() {
    const char *text = "Hello, welcome to C programming.";
    const char *sub = "welcome";

    char *pos = strstr(text, sub);

    if (pos != NULL) {
        printf("Substring found at position: %ld\n", pos - text);
    } else {
        printf("Substring not found.\n");
    }

    return 0;
}

Output

Substring found at position: 7

Common Pitfalls

Common mistakes when using strstr include:

• Not checking if the return value is NULL before using it.
• Confusing the returned pointer with an index; you must subtract the original string pointer to get the position.
• Passing NULL or uninitialized strings to strstr, which causes undefined behavior.

c

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

int main() {
    const char *text = "example text";
    const char *sub = "text";

    // Wrong: Using strstr return value without NULL check
    // printf("Position: %ld\n", strstr(text, sub) - text); // Unsafe if substring not found

    // Right: Check for NULL before using
    char *pos = strstr(text, sub);
    if (pos) {
        printf("Position: %ld\n", pos - text);
    } else {
        printf("Substring not found.\n");
    }

    return 0;
}

Output

Position: 8

Quick Reference

Function	Description
strstr(haystack, needle)	Returns pointer to first occurrence of needle in haystack or NULL if not found
Return value	Pointer to substring start or NULL
Include header
Check for NULL	Always check before using the returned pointer
Position calculation	Use pointer subtraction: pos - haystack

Key Takeaways

Use strstr from string.h to find substrings in C strings.

Always check if strstr returns NULL before using the result.

Calculate substring position by subtracting the original string pointer from the returned pointer.

Passing uninitialized or NULL strings to strstr causes errors.

Remember strstr finds the first occurrence only.