0

I'm creating a server in C on Ubuntu, but I've a problem with a printf function which doesn't work. This is the code. I'd like that the terminal prints "dd" as soon as the program starts, but it doesn't do anything. Suggestions?

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <arpa/inet.h> #include <sys/types.h> #include <netinet/in.h> #include <sys/socket.h> #include <netdb.h> void main(){ printf("dd"); int ds_sock; struct sockaddr_in my_addr; ds_sock=socket(AF_INET,SOCK_STREAM,0); memset(&my_addr,0,sizeof(my_addr)); my_addr.sin_family=AF_INET; my_addr.sin_port=htons(25000); my_addr.sin_addr.s_addr=INADDR_ANY; bind(ds_sock,(struct sockaddr *)&my_addr,sizeof(my_addr)); listen(ds_sock,3); printf("dd"); int ds_sock_acc; struct sockaddr_in addr; size_t sin_size = sizeof(struct sockaddr_in); ds_sock_acc = accept(ds_sock,(struct sockaddr *)&addr,&sin_size); while(ds_sock_acc==-1) { printf("connessione non riuscita"); ds_sock_acc = accept(ds_sock,(struct sockaddr *)&addr,&sin_size); } printf("connessione riuscita"); close(ds_sock); close(ds_sock_acc); }

this (client) works as expected:

void main(){ printf("dd"); int ds_sock; ds_sock = socket(AF_INET, SOCK_STREAM,0); int ret; struct sockaddr_in Eaddr; Eaddr.sin_family = AF_INET; Eaddr.sin_port = htons(25000); Eaddr.sin_addr.s_addr=inet_addr("127.0.0.1"); ret = connect(ds_sock,(struct sockaddr *)&Eaddr,sizeof(Eaddr)); while(ret==-1){ printf("Errore nel connect"); ret = connect(ds_sock,(struct sockaddr *)&Eaddr,sizeof(Eaddr)); } printf("connect OK"); close(ds_sock); }
Improve this question
4
  • Add a newline character '\n' after dd, or fprintf(stderr, "dd"); instead. Commented Oct 7, 2014 at 15:27
  • The second example produces output probably only because the program is short lived and output buffers are flushed when the program exits. Otherwise, it would have the same behaviour (try adding a sleep(30) statement after the last close to see for yourself). Commented Oct 7, 2014 at 15:37
  • 1
    You may want to build a program that does not exhibit undefined behaviour by selecting a valid entry point. void main() is not one of them. Commented Oct 7, 2014 at 15:38
  • You will find it much easier if you 1) pay attention to the return value from listen(), 2) make each of the printf("dd") put out unique values, so you know where they are originating. 3) there is no data being communicated, so no indication of the socket worked. 4) skip the while loops for the accept and connect, because if it fails once, it will always fail. Commented Oct 8, 2014 at 3:13

3 Answers 3

Reset to default
5

The output from printf will be buffered.

To get the output immediately, you have two options:

  • Add a newline (\n) to the end of the string (printf("dd\n");). This implicitly flushes the output stream (i.e. writes out the buffered content).

  • Explicitly flush the standard output stream after the printf statement (fflush(stdout);).

As for the second part of the question, the second example produces output probably only because the program is short lived and output buffers are flushed when the program exits. Otherwise, it would have the same behaviour (try adding a sleep(30) statement after the last close to see for yourself).

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Thanks for the reply. Could you explain the difference between the first piece of code and the second one I posted in the first message? Why in the second one there isn't any problem?
See comment to the question.
3

You need to flush your output.

Easiest way:

printf("dd\n");

Better way:

printf("dd"); fflush(stdout);
Improve this answer

3 Comments

Why would you say using fflush is better? Seems to me the newline way would be just fine if you want a newline in your output.
@FredLarson: As far as I am aware, putting a \n in your output is not guaranteed to flush the output, but typically does flush it. If you want guaranteed behavior, use fflush. If you want something that will likely work, use \n.
I see. Thank you. I found more information here: stackoverflow.com/q/5229096/10077
2

You need to flush your output stream. Use fflush(stdout).

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.