reading a file from a relative path

bleh.c

static char* read_file(const char* relative_name)
{
  size_t base_offset = strlen(__FILE__) - strlen("offgrid.c");
  size_t abs_length = base_offset + strlen(relative_name);
  char* abs_name = (char*) malloc(sizeof(char) * (abs_length + 1));

  strcpy(abs_name, __FILE__);
  abs_name[base_offset] = '\0';
  strcat(abs_name, relative_name);

  FILE* f = fopen(abs_name, "r");
  free(abs_name);

  if (f == NULL) {
    return NULL;
  }

  fseek(f, 0, SEEK_END);
  long len = ftell(f);
  rewind(f);

  char* buf = (char*) malloc(sizeof(char) * len);
  if (buf == NULL) {
    fclose(f);
    return NULL;
  }

  if (fread(buf, sizeof(char), len, f) != len) {
    fclose(f);
    free(buf);
    return NULL;
  }

  fclose(f);
  return buf;
}