simple trim function in C

trim.c

// cc -Wall -Wextra -Wpedantic -Wfatal-errors -O3 /tmp/trim.c -o /tmp/trim
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdint.h>

#define FAIL() { \
  ++totalres; \
  fprintf(stderr, "%s failed at %s(%i)\n", __func__, __FILE__, __LINE__); \
}

/**
 * trim the string s of leading and trailing whitespaces
 * \params s the string to trim
 * \params output optional pointer to which the result is copied
 * \returns a pointer to the result
 * if output is NULL, the result is copied in s and s is returned.
 * if output is not NULL, the result is copied in output and output is returned.
 */
char *trim(char *s, char *output) {
  if (*s == 0) return s;
  char *beg = s;
  char *end = s;
  while (*beg && isspace(*beg)) ++beg;
  while (*end) ++end;
  while ((end - 1) > beg && isspace(*(end - 1))) --end;
  if (output != NULL) s = output;
  memmove(s, beg, end - beg + 1);
  s[end - beg] = 0;
  return s;
}

int main() {
  char test[255];
  uint8_t totalres = 0;
  if (strcmp("toto",      trim(" toto ",        test))) FAIL();
  if (strcmp("tot    o",  trim("   tot    o",   test))) FAIL();
  if (strcmp("tot    o",  trim("   tot    o  ", test))) FAIL();
  if (strcmp("toto",      trim("toto",          test))) FAIL();
  if (strcmp("o",         trim("o ",            test))) FAIL();
  if (strcmp("o",         trim(" o",            test))) FAIL();
  if (strcmp("",          trim("",              test))) FAIL();
  if (strcmp("",          trim(" ",             test))) FAIL();
  if (strcmp("",          trim("     ",         test))) FAIL();

  memcpy(test, " toto ", strnlen(" toto ", 255));
  test[strnlen(" toto ", 255)] = 0;
  if (strcmp("toto",      trim(test,            NULL))) FAIL();

  return totalres;
}