Brian Douglas

Strings in C. They are wonderfully intuitive and easy to work with cough.

No, strings in C are a nightmare. Doing simple things such as trimming the whitespace off the start or end of a string is complicated. Because a string is not a string it's a char *. Which means that if you want to do interesting things you are going to have to get used to managing memory. Which means thinking, and if there is one thing modern programmers hate, it's thinking.

Anyway, below I have added ltrim_string, rtrim_string, and trim_string methods. I've made the code as readable as I can, and documented it with tests.

Code

The trim methods below will return an pointer to a new char *. So the result of these methods will need freeing. I've opted to return a nullptr when the supplied char * is null, or if memory allocation fails. If the string only contains whitespace, I return an empty string.

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

char *empty_string()
{
  char *str = malloc(1);
  if(!str) return nullptr;
  str[0] = '\0';
  return str;
}

char *ltrim_string(char *str)
{
  if(!str) return nullptr;

  size_t len = strlen(str);
  if(!len) return empty_string();

  size_t start = 0;
  while(start < len && isspace(str[start])) start++;

  if(start >= len) return empty_string();

  size_t new_size = len - start;
  char *result = malloc(new_size);
  if (!result) return nullptr;

  memcpy(result, str + start, new_size);
  result[new_size] = '\0';

  return result;
}

char *rtrim_string(char *str)
{
  if(!str) return nullptr;

  size_t len = strlen(str);
  if(!len) return empty_string();

  size_t end = len - 1;
  while(end && isspace(str[end])) end--;

  if(0 >= end) return empty_string();

  size_t new_size = end + 1;
  char *result = malloc(new_size);
  if (!result) return nullptr;

  memcpy(result, str, new_size);
  result[new_size] = '\0';

  return result;
}

char *trim_string(char *str)
{
  if (!str) return nullptr;

  size_t len = strlen(str);
  if(!len) return empty_string();

  size_t start = 0;
  while(isspace(str[start])) start++;
  
  size_t end = len - 1;
  if(start >= end) return empty_string();
  while(end && isspace(str[end])) end--;

  size_t new_size = end - start + 1;
  char *result = malloc(new_size + 1);
  if(!result) return nullptr;

  memcpy(result, str + start, new_size);
  result[new_size] = '\0';

  return result;
}

int main(void)
{
  // ltrim
  char *test_str = ltrim_string("   hello   ");
  assert(test_str && 0 == strcmp("hello   ", test_str));
  free(test_str);

  test_str = ltrim_string("\r\nhello\n");
  assert(test_str && 0 == strcmp("hello\n", test_str));
  free(test_str);

  // rtrim
  test_str = rtrim_string("   hello   ");
  assert(test_str && 0 == strcmp("   hello", test_str));
  free(test_str);

  test_str = rtrim_string("\r\nhello\n");
  assert(test_str && 0 == strcmp("\r\nhello", test_str));
  free(test_str);

  // trim
  test_str = trim_string("   hello   ");
  assert(test_str && 0 == strcmp("hello", test_str));
  free(test_str);

  test_str = trim_string("\r\nhello\n");
  assert(test_str && 0 == strcmp("hello", test_str));
  free(test_str);

  // empty strings
  test_str = ltrim_string("    ");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  test_str = ltrim_string("");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  test_str = rtrim_string("    ");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  test_str = rtrim_string("");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  test_str = trim_string("    ");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  test_str = trim_string("");
  assert(test_str && 0 == strcmp("", test_str));
  free(test_str);

  assert(trim_string(nullptr) == nullptr);

  return 0;
}