How to Check if String Contains Substring in C

1. Use the strstr Function to Check if a String Contains a Substring in C
2. Use the strcasestr Function to Check if a String Contains a Substring
3. Use the strncpy Function to Copy a Substring

This article will explain several methods of how to check if a string contains a given substring in C.

Use the strstr Function to Check if a String Contains a Substring in C

The strstr function is part of the C standard library string facilities, and it’s defined in the <string.h> header. The function takes two char pointer arguments, the first denoting the string to search in and the other denoting the string to search for.

It finds the first starting address of the given substring and returns the corresponding pointer to char. If the substring is not found in the first argument string, the NULL pointer is returned.

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

const char *tmp = "This string literal is arbitrary";

int main(int argc, char *argv[]) {
  char *ret;

  ret = strstr(tmp, "literal");
  if (ret)
    printf("found substring at address %p\n", ret);
  else
    printf("no substring found!\n");

  exit(EXIT_SUCCESS);
}

Output:

found substring at address 0x55edd2ecc014

Use the strcasestr Function to Check if a String Contains a Substring

strcasestr is not part of the standard library features, but it’s implemented as an extension in the GNU C library, which can be indicated with _GNU_SOURCE macro definition.

Once defined, we can call the strcasestr function to find the given substring’s first occurrence. Mind though, that this function ignores the case of both strings.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const char *tmp = "This string literal is arbitrary";

int main(int argc, char *argv[]) {
  char *ret;

  ret = strcasestr(tmp, "LITERAL");
  if (ret)
    printf("found substring at address %p\n", ret);
  else
    printf("no substring found!\n");

  exit(EXIT_SUCCESS);
}

Output:

found substring at address 0x55edd2ecc014

Use the strncpy Function to Copy a Substring

Alternatively, one can copy the given substring to a new buffer using the strncpy function. It takes three arguments, the first of which is the destination char pointer where the copied substring will be stored.

The second argument is the source string, and the last argument denotes the first number of bytes to copy at most. Notice that if the null byte is not found in the source string’s first bytes, the destination string won’t be null-terminated.

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

const char *tmp = "This string literal is arbitrary";

int main(int argc, char *argv[]) {
  char *str = malloc(strlen(tmp));

  printf("%s\n", strncpy(str, tmp, 4));
  printf("%s\n", strncpy(str, tmp + 5, 10));

  free(str);
  exit(EXIT_SUCCESS);
}

Output:

This
string lit