C recv function behavior

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.