A Guide to using strstr() in C/C++

In this article, we’ll take a look at using the strstr() function in C / C++.

The strstr() function is very useful if you want to find out whether one string is a sub-string of another. It also gives us the pointer to the first match!

Due to this convenience, strstr() is used somewhat frequently in C, since it is a part of the C standard.

Basic Syntax of strstr() in C / C++

This function takes in two input strings search_string and target. Here, we will search the string search_string for the target pattern target.

NOTE: This function ignores the terminal ‘\0’ for both the strings

It returns a char* pointer to the first character of the first match, if found.

If target is not a sub-string of search_string, the returned value is NULL.

Also, it is defined in the <string.h> header file, so make sure that you include this header file first!

#include <string.h>

char* strstr(const char* search_string, const char* target);

Now, this description must be pretty clear to you. Let’s now look at some examples, to exactly see what happens.

Using strstr() in C / C++ – Some Examples

Let’s take a simple string “Hello from JournalDev”, and search for “JournalDev”. If this function works as expected, we’ll get a pointer to the first match character (“J”).

Let’s write our program first.

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

int main() {
    char* search_string = "Hello from JournalDev";
    char* target = "JournalDev";

    printf("Search String: %s\n", search_string);
    printf("Target Pattern: %s\n", target);

    char* result = strstr(search_string, target);

    if (result == NULL) {
        printf("Target pattern is not a substring!\n");
    }
    else {
        printf("Target pattern is a substring!\n");
        printf("First character of result = %c\n", *result);
        printf("The complete result string (result) = %s\n", result);
    }

    return 0;
}

Output

Search String: Hello from JournalDev
Target Pattern: JournalDev
Target pattern is a substring!
First character of result = J
The complete result string (result) = JournalDev

Indeed, we get what we expected! strstr() finds out that “JournalDev” is a substring of “Hello from JournalDev”, and returns a pointer to the first match (“J”).

From there, we can simply print the rest of the string, which is a sub-string of the original (“JournalDev”)!

Now, let’s take another string, which is not a sub-string of the input.

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

int main() {
    char* search_string = "Hello from JournalDev";
    char* target = "Hi JournalDev";

    printf("Search String: %s\n", search_string);
    printf("Target Pattern: %s\n", target);

    char* result = strstr(search_string, target);

    if (result == NULL) {
        printf("Target pattern is not a substring!\n");
    }
    else {
        printf("Target pattern is a substring!\n");
        printf("First character of result = %c\n", *result);
        printf("The complete result string (result) = %s\n", result);
    }

    return 0;
}

Output

Search String: Hello from JournalDev
Target Pattern: Hi JournalDev
Target pattern is not a substring!

Since our target pattern is not a sub-string of the input, result will be NULL!

Hopefully, these examples make it pretty clear about what strstr() does.

Conclusion

In this article, we learned about using the strstr() function in C / C++. This function makes it easy to check if a string is a sub-string of another string, and also gives a pointer to the first match, if it exists!

References

• The Linux manual page documentation on strstr() in C