C strstr Function: Complete Guide & Examples

The function strstr is used to locate the first occurrence of one string within another string. The prototype of the function is as follows:

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

In this case, haystack represents the string to be searched, and needle represents the substring to search for.

Here is how to use:

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

int main() {
    const char *haystack = "Hello, world!";
    const char *needle = "world";
    
    char *result = strstr(haystack, needle);
    
    if (result) {
        printf("'%s' found at position %ld\n", needle, result - haystack);
    } else {
        printf("'%s' not found\n", needle);
    }
    
    return 0;
}

The code above searches for the “needle” in the haystack and outputs its position if found, otherwise it outputs a message indicating it was not found.