C socket client

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 

void error(const char *msg) {
    perror(msg);
    exit(0);
}

int main(int argc, char *argv[]) {
    int 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");

    while (1) {
        printf("Please enter the message: ");
        bzero(buffer, 256);
        fgets(buffer, 255, stdin);
        // n = write(sockfd, buffer, strlen(buffer));
        n = send(sockfd, buffer, strlen(buffer), 0);
        if (n < 0) error("ERROR writing to socket");

        bzero(buffer, 256);
        // n = read(sockfd, buffer, 255);
        n = recv(sockfd, buffer, 255, 0);
        if (n < 0) error("ERROR reading from socket");

        printf("%s\n", buffer);
    }
    close(sockfd);

    return 0;
}

The code shown above works only at the first iteration. However, At second iteration it blocks at recv() or read().

Could someone please explain the reason for this behaviour?

Update: here is the server code (not the one I'm using, but the same principles) and it works like the one I mentioned above:

/* A simple server in the internet domain using TCP
   The port number is passed as an argument */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h> 
#include <sys/socket.h>
#include <netinet/in.h>

void error(const char *msg) {
    perror(msg);
    exit(1);
}

int main(int argc, char *argv[]) {
    int sockfd, newsockfd, portno;
    socklen_t clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;
    int n;
    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");
    memset((char*) &serv_addr, 0, sizeof(serv_addr));
    portno = atoi(argv[1]);
    serv_addr.sin_family = AF_INET;
    serv_addr.sin_开发者_JAVA百科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);
    while (1) {
        newsockfd = accept(sockfd,
                (struct sockaddr *) &cli_addr,
                &clilen);
        if (newsockfd < 0)
            error("ERROR on accept");
        memset(buffer, 0, sizeof(buffer));
        n = read(newsockfd, buffer, 255);
        if (n < 0) error("ERROR reading from socket");
        printf("Here is the message: %s\n", buffer);
        n = write(newsockfd, "I got your message", 18);
        if (n < 0) error("ERROR writing to socket");
    }
    close(newsockfd);
    close(sockfd);
    return 0;
}

Your while loop in the server code is at the wrong place. It should be changed to this:

        memset(buffer, 0, sizeof(buffer));
        n = read(newsockfd, buffer, 255);
        if (n < 0) error("ERROR reading from socket");
        printf("Here is the message: %s\n", buffer);
        n = write(newsockfd, "I got your message", 18);
        if (n < 0) error("ERROR writing to socket");

In your code, you accept() a new connection in each iteration then listen() for a read() on it. The client on the other hand does not establish a new connection, it performs a write() to the same socket.

Either your client needs to close the connection and reconnect each time, or the server needs to allow multiple read()/write() for the communication.

This is one of those things with sockets I don't truly understand, but IF you do it like this, it should work. At least for me it does.

while(recv(sockfd, buffer, 255, 0) > 0) {
    ...
}

If anyone knows why this work, please leave me a comment below :)