How to Get Substring in C

1. Use the strncpy Function to Get Substring in C
2. Use Custom Function to Get Substring in C

This article will introduce multiple methods about how to get substring in C.

Use the strncpy Function to Get Substring in C

strncpy is part of the C string library function defined in the <string.h> header file. It copies the given number of bytes from the source string to the destination. strncpy takes three parameters - destination char*, source pointer, and the integer to denote the number of bytes to copy. If the number of bytes specified is more than the source string contains, then additional null bytes are stored at the destination.

The strncpy function returns the pointer to the destination string; thus, we can chain the call into the printf statement to directly print the substring. The following example demonstrates how to print the first four character substring and then the next 10 character substring.

#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

Use Custom Function to Get Substring in C

Alternatively, we can define a custom function wrapper for strncpy and specify a new interface of four parameters. Namely, the function getSubstring will take destination and source strings plus two integers specifying the starting and end positions of the characters needed as a substring.

Note that this function prototype does not implement additional error checking, but it directly returns the char* pointer passed on from the strncpy call.

Similar to the previous example, getSubstring can also be chained into the printf function as an argument. One caveat regarding the strncpy is that the destination and source strings must not overlap in the memory. Also, the destination pointer should point to a large enough buffer to store the source string.

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

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

char *getSubstring(char *dst, const char *src, size_t start, size_t end) {
  return strncpy(dst, src + start, end);
}

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

  printf("%s\n", getSubstring(str, tmp, 0, 4));
  printf("%s\n", getSubstring(str, tmp, 5, 10));

  free(str);
  exit(EXIT_SUCCESS);
}

Output:

This
string lit