C slow buffer access

Question

I have two code snippets with an similar task.

Following code needs four seconds:

while(1)
{
    fputs(erz_aint2hex(buffer[write_helper++]), fp);
    if(write_helper > _size) { if(enable_output) erz_output(white, LOCAL_FUNCTION_NAME, "Write data to file\r\n"); break; }
}

The second code needs above 15 minutes:

while(1)
{
    strcat(buf_ret, (erz_aint2hex(buffer[write_helper++])));            
    if(write_helper > _size) { if(enable_output) erz_output(white, LOCAL_FUNCTION_NAME, "Write data to buffer\r\n"); break; }
}

buf_ret and buffer are malloc char arrays. The first code stores any input file convertet to a new file in hex format. The second code do the same but stores the content in a buffer. The input file have 7mb and converted 15mb. Why is the second code so slow? And what can i do to make it faster?

I also tried snprintf(hexhelper, sizeof(hexhelper), "%X", buffer[write_helper++]); and strncat(). Is strcat() so slow?

Answer 1

strcat will need to look for the end of the string contained in buf_ret (that is, the trailing null byte) on every call, so yeah, it's going to be slower and slower the longer buf_ret grows.

Something like

char *buf_w = buf_ret;
while (1) {
  char *hex = erz_aint2hex(buffer[write_helper++]);
  *buf_w++ = hex[0];  // assumes hex is 
  *buf_w++ = hex[1];  // 2 bytes long

  if (write_helper > _size) {
    if (enable_output)
      erz_output(white, LOCAL_FUNCTION_NAME, "Write data to buffer\r\n");
    break;
  }
}

would probably be a lot faster. (YMMV, beware, no bounds checks here.)