How to Use the strdup Function in C

1. Use the strdup Function to Duplicate the Given String in C
2. Use the strndup Function to Duplicate the Given String in C
3. Use the strdupa Function to Duplicate the Given String in C

This article will explain several methods of how to use the strdup function in C.

Use the strdup Function to Duplicate the Given String in C

strdup is one of the POSIX compliant functions that’s available on most UNIX based operating systems. It implements string copying functionality but does memory allocation and checking internally. Although a user is responsible for freeing the returned char pointer since the strdup allocates the memory with malloc function call.

strdup takes a single argument - the source string to be duplicated and returns the pointer to a newly copied string. The function returns NULL on failure, namely when there’s an insufficient memory to allocate. In this case, we are retrieving the HOME environment variable using the getenv function and copy its value with strdup.

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

int main(int argc, char *argv[]) {
  char *path = NULL;

  const char *temp = getenv("HOME");

  if (temp != NULL) {
    path = strdup(temp);

    if (path == NULL) {
      perror("strdup");
      exit(EXIT_FAILURE);
    }

  } else {
    fprintf(stderr, "$HOME environment variable is not defined\n");
    exit(EXIT_FAILURE);
  }

  printf("%s\n", path);
  free(path);

  exit(EXIT_SUCCESS);
}

Output:

/home/user

Use the strndup Function to Duplicate the Given String in C

strndup is a similar function that takes an additional argument to specify the number of bytes that need to be copied at most. This version is useful to copy only certain parts of the string. Note though, strndup adds terminating null byte to the copied characters, thus, ensuring that it’s stored as C-style string format and can be manipulated as such.

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

int main(int argc, char *argv[]) {
  char *path = NULL;

  const char *temp = getenv("HOME");

  if (temp != NULL) {
    path = strndup(temp, 5);

    if (path == NULL) {
      perror("strdup");
      exit(EXIT_FAILURE);
    }

  } else {
    fprintf(stderr, "$HOME environment variable is not defined\n");
    exit(EXIT_FAILURE);
  }

  printf("%s\n", path);
  free(path);

  exit(EXIT_SUCCESS);
}

Output:

/home

Use the strdupa Function to Duplicate the Given String in C

strdupa is part of the GNU C library and may not be available in other C compilers. strdupa is similar to the strdup function except that it uses alloca for memory allocation. The alloca function implements memory allocation on the stack region, and the area is freed automatically when the calling function returns. Thus, the pointer returned from strdupa should not be freed explicitly with the free call as it will result in a segmentation fault. Note that _GNU_SOURCE macro should be defined to compile the code successfully.

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

int main(int argc, char *argv[]) {
  char *path = NULL;

  const char *temp = getenv("HOME");

  if (temp != NULL) {
    path = strdupa(temp);

    if (path == NULL) {
      perror("strdup");
      exit(EXIT_FAILURE);
    }

  } else {
    fprintf(stderr, "$HOME environment variable is not defined\n");
    exit(EXIT_FAILURE);
  }

  printf("%s\n", path);

  exit(EXIT_SUCCESS);
}

Output:

/home/user