How to Convert String to Lowercase in C

1. Use the tolower Function to Convert String to Lowercase in C
2. Use the Custom Function to Convert String to Lowercase in C

This article will demonstrate multiple methods about how to convert string to lowercase in C.

Use the tolower Function to Convert String to Lowercase in C

The tolower function is part of the C standard library defined in the <ctype.h> header file. tolower takes one int type argument and returns the converted value of the given character if the corresponding lowercase representation exists.

Note that the passed character needs to be either EOF or unsigned char type representable. In this case, we initialize the char pointer with a string literal value and then iterate over each character to convert to lowercase values.

That passing char type arguments to tolower function must be cast to unsigned char.

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

int main() {
  char *str = "THIS STRING LITERAL IS ARBITRARY";

  printf("%s\n", str);
  for (size_t i = 0; i < strlen(str); ++i) {
    printf("%c", tolower((unsigned char)str[i]));
  }
  printf("\n");

  exit(EXIT_SUCCESS);
}

Output:

THIS STRING LITERAL IS ARBITRARY
this string literal is arbitrary

Previous example code overwrites contents of the original string with the converted, lowercase characters. Alternatively, we can allocate another char pointer using calloc, which is similar to malloc except that it zeros out the allocated memory and store the converted string separately.

Notice that the pointer should be freed before the program exits, or if the process is long-running, it should be freed as soon as the string variable is not needed.

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

int main() {
  char *str = "THIS STRING LITERAL IS ARBITRARY";

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

  size_t len = strlen(str);
  char *lower = calloc(len + 1, sizeof(char));

  for (size_t i = 0; i < len; ++i) {
    lower[i] = tolower((unsigned char)str[i]);
  }
  printf("%s", lower);
  free(lower);

  exit(EXIT_SUCCESS);
}

Output:

THIS STRING LITERAL IS ARBITRARY
this string literal is arbitrary

Use the Custom Function to Convert String to Lowercase in C

A more flexible solution would be to implement a custom function that takes the string variable as the argument and returns the converted lowercase string at a separate memory location. This method is essentially the decoupling of the previous example from the main function.

In this case, we created a function toLower that takes char*, where a null-terminated string is stored and a size_t type integer denoting the string’s length. The function allocates the memory on the heap using the calloc function; thus the caller is responsible for deallocating the memory before the program exits.

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

char *toLower(char *str, size_t len) {
  char *str_l = calloc(len + 1, sizeof(char));

  for (size_t i = 0; i < len; ++i) {
    str_l[i] = tolower((unsigned char)str[i]);
  }
  return str_l;
}

int main() {
  char *str = "THIS STRING LITERAL IS ARBITRARY";

  printf("%s\n", str);
  size_t len = strlen(str);

  char *lower = toLower(str, len);
  printf("%s", lower);
  free(lower);

  exit(EXIT_SUCCESS);
}

Output:

THIS STRING LITERAL IS ARBITRARY
this string literal is arbitrary