Question

I have a weird issue with printing data out. I use printf to print a char* string and then after that print another one. However part of the first string doesn’t get printed and when I print the second string the missing part of the first one is prepended to that one. What is happening here?

I’m writting a simple libpcap implimentation. Here is a sample callback function that will produce the same results. I tried removing buffering and adding a putchar(‘\n’) after printing but it didn’t help.

void ParseData(u_char* useless, const struct pcap_pkthdr* pkthdr, const u_char* packet){
   int packetLen, i;
   packetLen = pkthdr->len;
   for (i = 0; i < packetLen; i++){
      putchar(packet[i]);
   }
}

Answer 1

stdio buffers characters. Unless you tell it otherwise, usually it will only actually issue a write when it sees a newline character. If you want a different behavior, you can remedy it with some of these:

• After your first printf, call fflush(stdout); to flush the buffer.

• Alternatively, call setbuf(stdout, NULL); to disable buffering. Do this before you do any printfs.

• Bypass stdio by coding to platform specific APIs like write (POSIX) or WriteFile (Windows). Usually I would recommend against this, especially for something like stdout..