Linux C socket server printf problem - c

Question

I have a simple socket server here, and have what seems to me should be a simple question. Before the socket accepts connections I wish to have it print its process id. But it won't print anything no matter what it is, until there has been a connection. At first I thought this was because the accept call was blocking the print somehow so I tried adding this in various places:
int fdflags = fcntl(sockfd, F_GETFL, 0);
fcntl(sockfd, F_SETFL, fdflags | O_NONBLOCK);
But this had no effect on the code other than making the accept non blocking. So I was hoping someone here might be able to tell me what is going on. The server otherwise works. I'll go ahead and post the client code too.
server.c:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <fcntl.h>
static void error(char *msg) {
perror(msg);
exit(1);
}
static void SIGCHLD_Handler(int sig) {
waitpid(-1, NULL, WNOHANG);
}
int main(int argc, char *argv[]) {
int num,sockfd, newsockfd, portno, clilen;
char buffer[256];
struct sockaddr_in serv_addr, cli_addr;
struct sigaction sigact;
sigact.sa_handler = SIGCHLD_Handler;
if (sigaction(SIGCHLD, &sigact, 0)) error("sighandle def");
int n, childPid;
if (argc < 2) {
fprintf(stderr,"ERROR, no port provided\n");
exit(1);
}
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) error("ERROR opening socket");
bzero((char *) &serv_addr, sizeof(serv_addr));
portno = atoi(argv[1]);
serv_addr.sin_family = AF_INET;
serv_addr.sin_addr.s_addr = INADDR_ANY;
serv_addr.sin_port = htons(portno);
if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) < 0) error("ERROR on binding");
listen(sockfd,5);
clilen = sizeof(cli_addr);
printf("I am the Knock Knock server and my pid number is %d\n", getpid());
while (1) {
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);
if (newsockfd < 0) else error("ERROR on accept");
bzero(buffer,256);
childPid = fork();
if (childPid < 0) error ("ERROR on fork");
else if (childPid == 0) {
close(sockfd);
while(1) {
// read an int from the client that says how big the message will be
n = read(newsockfd, &num, sizeof(int));
// if client sends just Enter, then quit
if(num==2) break;
// read num bytes from client
n = read(newsockfd,buffer,num);
if (n < 0) error("ERROR reading from socket");
// display the message from the client
printf("Here is the message: %s\n",buffer);
num=19;
// Tell the client to expect 19 bytes
n = write(newsockfd, &num, sizeof(int));
// Send client 19 bytes
n = write(newsockfd,"I got your message",num);
if (n < 0) error("ERROR writing to socket");
}
exit(0);
} else close(newsockfd);
}
return 0;
}
client.c:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
void error(char *msg) {
perror(msg);
exit(0);
}
int main(int argc, char *argv[]) {
int num, sockfd, portno, n;
struct sockaddr_in serv_addr;
struct hostent *server;
char buffer[256];
if (argc < 3) {
fprintf(stderr,"usage %s hostname port\n", argv[0]);
exit(0);
}
portno = atoi(argv[2]);
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0)
error("ERROR opening socket");
server = gethostbyname(argv[1]);
if (server == NULL) {
fprintf(stderr,"ERROR, no such host\n");
exit(0);
}
bzero((char *) &serv_addr, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
bcopy((char *)server->h_addr,
(char *)&serv_addr.sin_addr.s_addr,
server->h_length);
serv_addr.sin_port = htons(portno);
if (connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
error("ERROR connecting");
do{
printf("Please enter the message: ");
bzero(buffer,256);
fgets(buffer,255,stdin);
num = strlen(buffer)+1;
// send server an int that says how long the coming message will be
n = write(sockfd, &num, sizeof(int));
// num=2 when user just presses Enter. No message = quit
if(num==2) break;
// send server the message (num bytes long)
n = write(sockfd,buffer,num);
if (n < 0)
error("ERROR writing to socket");
bzero(buffer,256);
// read how many bytes are coming from the server
n = read(sockfd, &num, sizeof(int));
// read num bytes from the server
n = read(sockfd,buffer,num);
if (n < 0)
error("ERROR reading from socket");
// display the message from the server
printf("%s\n",buffer);
}while(1);
return 0;
}

Answer 1

Add a call to fflush(stdout); after the call to printf, or use setvbuf(stdout, NULL, _IOLBF, 0); to set stdout to line-buffered.

Answer 2

Did you try some:
pid_t pid = getpid();
and try to use gdb to print pid value?
Nevertheless, your pid should print. What if you try :
printf("pid = %ld\n", getpid());

Answer 3

accept() blocks, but it doesn't interfere with printf(), and making it non-blocking won't help that, and will destroy the correct working of your program unless you really know what you're doing with non-blocking network code. This must be a buffering problem: fflush(stout) should fix it.