UDP客户端仅在特定情况下才会卡在recvfrom中

Question

I'm writing a C++ code that implements an UDP server and client.

The code works fine when I write two codes, one for the server and another for the client, as in this example : https://www.geeksforgeeks.org/udp-server-client-implementation-c/ .

What I'm trying to do is to write a client function and a server function in the same code. The ideia is that I select how the program is going to work with the command lines argument.

The problem is that, implementing this way and testing in two terminals running the same code, with different command line arguments, one for server and another for client, the client stucks in the recvfrom, when receiving the server response.

#include <unistd.h> 
#include <stdio.h> 
#include <sys/socket.h> 
#include <stdlib.h> 
#include <netinet/in.h> 
#include <string.h> 
#include <arpa/inet.h>
#include <unistd.h>
#define MAXLINE 1024
#define PORT 32000

int send(){
    int sockfd; 
    char buffer[MAXLINE]; 
    char *hello = "Hello from server"; 
    struct sockaddr_in servaddr, cliaddr; 

    // Creating socket file descriptor 
    if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
        perror("socket creation failed"); 
        exit(EXIT_FAILURE); 
    } 

    memset(&servaddr, 0, sizeof(servaddr)); 
    memset(&cliaddr, 0, sizeof(cliaddr)); 

    // Filling server information 
    servaddr.sin_family = AF_INET; // IPv4 
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 
    servaddr.sin_port = htons(PORT); 

    // Bind the socket with the server address 
    if ( bind(sockfd, (const struct sockaddr *)&servaddr, 
            sizeof(servaddr)) < 0 ) 
    { 
        perror("bind failed"); 
        exit(EXIT_FAILURE); 
    } 

    socklen_t len;
    int n; 
    n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
                MSG_WAITALL, ( struct sockaddr *) &cliaddr, 
                &len); 
    buffer[n] = '\0'; 
    printf("Client : %s\n", buffer); 
    sendto(sockfd, (const char *)hello, strlen(hello), 
        MSG_CONFIRM, (const struct sockaddr *) &cliaddr, 
            len); 
    printf("Hello message sent.\n"); 

    return 0; 
}

int receive(){
    int sockfd; 
    char buffer[MAXLINE]; 
    char *hello = "Hello from client"; 
    struct sockaddr_in   servaddr; 

    // Creating socket file descriptor 
    if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
        perror("socket creation failed"); 
        exit(EXIT_FAILURE); 
    } 

    memset(&servaddr, 0, sizeof(servaddr)); 

    // Filling server information 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 

    int n;
    socklen_t len; 

    sendto(sockfd, (const char *)hello, strlen(hello), 
        MSG_CONFIRM, (const struct sockaddr *) &servaddr, 
            sizeof(servaddr)); 
    printf("Hello message sent.\n"); 

    n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
                MSG_WAITALL, (struct sockaddr *) &servaddr, 
                &len); 
    buffer[n] = '\0'; 
    printf("Server : %s\n", buffer); 

    return 0; 
}

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

    int command = atoi(argv[1]);

    if(command == 0){
        send();

    }
    if(command == 1){
        receive();
    }


    return 0;
}

The expected results is something like this, that i get when running the client and the server on separated codes:

Server side:

Hello from client

Hello message sent

Client side:

Hello message sent

Hello from server

But what I get when running the code above is

Server side:

Hello from client

Hello message sent

Client side:

Hello message sent

---gets stucked here---

What am i doing wrong?

Answer 1

In your send() function, you are not initializing len to the length of the buffer where recvfrom can store the client address.

According to the man page for recvfrom :

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen);

If src_addr is not NULL, and the underlying protocol provides the source address of the message, that source address is placed in the buffer pointed to by src_addr. In this case, addrlen is a value-result argument. Before the call, it should be initialized to the size of the buffer associated with src_addr. Upon return, addrlen is updated to contain the actual size of the source address.

It's not working because the client address isn't being properly received so the response message is being sent to the wrong address. To resolve your problem, you just need to initialize len before the call to recvfrom :

socklen_t len = sizeof(cliaddr); // The size of the buffer you're passing to store the client address