C recv function behavior - c

Question

This is my two pieces of code:
server.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timeb.h>
#include <string.h>
#include <unistd.h>
#define BYTES_NR 64
#define MSG_NR 512
int main(int argc, char *argv[]) {
char buf[BYTES_NR];
int sock,length;
struct sockaddr_in server,client;
int rval,i;
if(argc !=2) {
fprintf(stderr,"Usage: %s port\n",argv[0]);
exit(-1);
}
sock = socket(AF_INET,SOCK_DGRAM,0);
if(sock<0) {
perror("opening stream socket");
exit(1);
}
server.sin_family = AF_INET;
server.sin_addr.s_addr= INADDR_ANY;
server.sin_port = htons(atoi(argv[1]));
if (bind(sock,(struct sockaddr *)&server,sizeof(server))<0) {
perror("binding stream socket");
exit(1);
}
length = sizeof(server);
if(getsockname(sock,(struct sockaddr *)&server, (socklen_t *)&length)<0){
perror("getting socket name");
exit(1);
}
printf("Socket port #%d\n",ntohs(server.sin_port));
printf("test");
while(1) {
do {
printf("test2");
bzero(buf,sizeof(buf));
rval = recvfrom(sock,buf,sizeof(buf), 0, (struct sockaddr *)&client, (socklen_t *)&length );
if(rval<0)
perror("reading stream message");
i=0;
if(rval==0)
printf("Ending connection\n");
else {
printf("Message received: sending back\n");
strcat(buf,"*");
if (sendto(sock,buf,sizeof(buf),0,(struct sockaddr *)&client,sizeof(client))<0)
perror("writing on stream socket");
}
} while(rval !=0);
}
return 0;
}
client.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <unistd.h>
#define BYTES_NR 64
#define MSG_NR 512
int main(int argc, char *argv[]) {
char buf[BYTES_NR];
char buf2[BYTES_NR];
char msg[MSG_NR][BYTES_NR];
char answ[MSG_NR][BYTES_NR];
struct timeval xstime[MSG_NR];
struct timeval xftime[MSG_NR];
int i,sock,rval,length;
unsigned long delay;
struct sockaddr_in server,client;
struct hostent *hp, *gethostbyname();
if(argc !=3) {
fprintf(stderr,"Usage: %s servername serverport\n",argv[0]);
exit(-1);
}
for(i=0;i<MSG_NR;i++) {
sprintf(&msg[i][0],"%d",i);
}
sock= socket(AF_INET,SOCK_DGRAM,0);
if(sock<0) {
perror("opening stream socket");
exit(1);
}
client.sin_family= AF_INET;
client.sin_addr.s_addr = INADDR_ANY;
client.sin_port = htons(0);
if (bind(sock,(struct sockaddr *)&client,sizeof(client)) <0) {
perror("sending datagram message");
exit(1);
}
length= sizeof(client);
if(getsockname(sock,(struct sockaddr *)&server,(socklen_t *)&length)<0) {
perror("getting socket name");
exit(1);
}
printf("Socket port #%d\n",ntohs(client.sin_port));
hp = gethostbyname(argv[1]);
if (hp == 0) {
fprintf(stderr,"%s :unknow host",argv[1]);
exit(2);
}
bcopy( (char *)hp ->h_addr, (char *)&server.sin_addr,hp ->h_length);
server.sin_family = AF_INET;
server.sin_port = htons(atoi(argv[2]));
for(i=0;i<MSG_NR;i++) {
printf("ciclo-");
strcpy(buf,msg[i]);
gettimeofday(&xstime[i],NULL);
if(sendto(sock, buf, sizeof(buf), 0, (struct sockaddr *)&server, sizeof(server)) < 0)
perror("sendto problem");
if((rval = read(sock,buf2,sizeof(buf2)))<0)
perror("reading stream message");
strcpy(answ[i],buf2);
gettimeofday(&xftime[i],NULL);
}
close(sock);
for (i=0; i<MSG_NR; i++) {
delay = (xftime[i].tv_sec-xstime[i].tv_sec)*1000000.+(xftime[i].tv_usec-xstime[i].tv_usec);
printf("msg %d [%s]: %0.3f ms\n",i,answ[i],delay/1000.);
}
return 0;
}
On the server side, why the printf that prints "test" does not operate before a client arrives with a request to send message? The same is true for the second printf that print "test2".
There's probably something conceptual that escapes me!
If i comment recv , the flow of execution returns normal.

Answer 1

This has nothing to do with recv. Change to:
printf("test\n");
By default, stdout is line-buffered, so you don't see anything until a newline is printed.
If you don't want to print a newline, you can use fflush(stdout); after each printf to print the current buffer. You can also use:
setvbuf(stdout, NULL, _IONBF, BUFSIZ);
to disable output buffering.