无法发送写在 c 中的简单 UDP 数据包

Question

I am a current UNI student learning about socket programming and my first task was to create a simple client server model where the client sends a string and the server transforms it to all caps and returns it.

I am not getting any errors in my code however when i compile it the server side gets stuck after one iteration of the for loop.

here is the server side code:

#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#define BUF_LEN 48


//this function will convert string into upper case 
int processString(char *input, char *output){

int len = strlen(input);
for (int i=0;i<len;i++){
output[i] = toupper(input[i]); 
}
output[len]='\0';
return len;
}




int main(int argc,char *argv[]){
/* ----------------------------DECLARING VARIABLES----------------------------------   */
int ssd;                    
struct sockaddr_in server;  //server info
struct sockaddr_in client;  //client info       
int client_len;  // size of client          
short echo_port; // the port number           
int max_iterations; // usually runs forever but this will limit server        
int byteInCount,byteOutCount,recieveCount,i;
char clientString[BUF_LEN];
char serverProcessedString[BUF_LEN];
int retCode;

/* ----------------------------END: DECLARING VARIABLES----------------------------------   */



/* ---------------------------CHECKING THE COMMAND LINE----------------------------   */

if (argc != 3)
{
    fprintf(stderr,"incorrect usage! %s port Max_iterations\n",argv[0]);
    exit(EXIT_FAILURE);
}
echo_port = atoi(argv[1]);
max_iterations = atoi(argv[2]);

/* ---------------------------END: CHECKING THE COMMAND LINE----------------------------   */



/* ----------------------------CREATE THE SOCKET CONNECTION----------------------------------   */
ssd = socket(PF_INET,SOCK_DGRAM,0); // refer to notes about meaning of each parameter
if (ssd <0){
perror("error in function socket()\n");
exit(EXIT_FAILURE);
}

server.sin_family=AF_INET; // ip protocol 
server.sin_addr.s_addr=htonl(INADDR_ANY);
server.sin_port= htonl(echo_port); // port to serve on

retCode = bind(ssd,(struct sockaddr *)&server,sizeof(server)); // ask tut about bind() function
if (retCode <0)
{
    perror("error in function bind()\n");
    exit(EXIT_FAILURE);
}
/* ----------------------------END: CREATE THE SOCKET CONNECTION----------------------------------   */


/* ---------------------------- START THE SERVER----------------------------------   */
for (int i=0;i<max_iterations;i++){
fprintf(stderr,"Iteration %d of %d. Waiting for client...\n",i+1, max_iterations);
client_len = sizeof(client);
byteInCount = recvfrom(ssd,clientString,BUF_LEN,0,(struct sockaddr *)&client,(socklen_t *)&client_len);
if (byteInCount <0)
{
    perror("error in function revfrom()\n");
    exit(EXIT_FAILURE);
}
else{
fprintf(stderr,"revfrom() is all g\n");
}
fprintf(stderr,"Reversed string is %d bytes long\n", recieveCount);
fprintf(stderr,"Reversed string is \"%s\"\n",serverProcessedString);
byteOutCount = sendto(ssd,serverProcessedString,recieveCount+1,0,(struct sockaddr *)&client,sizeof(client));
if (byteOutCount <0){
    perror("error in function sendto()\n");
    exit(EXIT_FAILURE);
}
fprintf(stderr,"client request worked, reply has been sent\n");


}
close(ssd);
fprintf(stderr,"server has shutdown!\n");

return 0;


}

and this is the client side code:

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

#define BUF_LEN 48

int main(int argc,char *argv[]){
int csd;
struct sockaddr_in server;
struct hostent *server_host;
int serverLength;
int stringSize;
short serverPort;
int byteInCount,byteOutCount;
char clientString[BUF_LEN];
char serverProcessedString[BUF_LEN];

if (argc != 4){
    fprintf(stderr,"usage: %s server port send_string \n",argv[0]);
    exit(EXIT_FAILURE);
}

server_host =gethostbyname(argv[1]);
if (server_host== NULL){
herror("error in function gethostbyname()\n"); // ask tut- this function is no longer used( getaddrinfo)
exit(EXIT_FAILURE);
}

serverPort= atoi(argv[2]);
strcpy(clientString,argv[3]);


csd = socket(PF_INET,SOCK_DGRAM,0);
if(csd <0)
{
    perror("error in function socket()");
    exit(EXIT_FAILURE);
}
server.sin_family= AF_INET;
memcpy(&server.sin_addr.s_addr,server_host->h_addr_list[0],server_host->h_length); // ask tut about this function
server.sin_port=htons(serverPort); // difference between htons and htonl???
stringSize=strlen(clientString)+1;
byteOutCount= sendto(csd,clientString,stringSize,0,(struct sockaddr *)&server,sizeof(server));

if (byteOutCount <0){
    perror("error in function sendto()\n");
    exit(EXIT_FAILURE);
}

fprintf(stderr,"You have sent \"%s\"\n",clientString);
fprintf(stderr,"Have reached recvfrom(), should now block until message receipt\n");

//get the response from server
serverLength = sizeof(server);
byteInCount = recvfrom(csd,serverProcessedString,BUF_LEN,0,(struct sockaddr *)&server,(socklen_t)&serverLength);

if (byteInCount <0){
    perror("error in function recvfrom()\n");
    exit(EXIT_FAILURE);
}
fprintf(stderr,"the server has responded with: : \"%s\"\n",serverProcessedString);
close(csd);

return 0;
}

i am compiling this on a Ubuntu VM: terminal 1

cc UDP_client.c -o send
./send 192.168.0.1 9999 hello

(the ip address above was obtained using the ifconfig command)

terminal 2

cc UDP_server.c -o rec
./rec 9999 10

my current output:

terminal 2

"Iteration 1 of 10. waiting for client"

terminal 1:

"you have sent hello"

"have reached recvfrom(),should now block until message receipt"

it seems like a problem with my loop? any help would be appreciated.

EDIT: after fixing the error in the comments below, on the client side i am receiving an error in the recvfrom function: error says, bad address?

Answer 1

I found a few issues in your code.

1. make sure the struct sockaddr_in server and client should be memset to zero. As these are local variables and can have invalid values
2. In your server, you did not call processString() before calling sendto.
3. Your compilation should have zero warnings, that way you would have fixed a lot of inherent issues
4. Indenting code really helps in readability and maintainability.
5. added a couple of required headers (unistd and stdlib.h)
6. initialized the address to INADDR_ANY in both client and server and htons for port
7. One recommendation, please initialize all local variables to zeros to avoid any unexpected result.

here is the server code (note, I kept your original code as close to what it was, with minor fixes to make it work, please test thoroughly)

#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <ctype.h>
#include <stdlib.h>
#include <unistd.h>
#define BUF_LEN 48


//this function will convert string into upper case 
int processString(char *input, char *output){

    int len = strlen(input);
    for (int i=0;i<len;i++){
        output[i] = toupper(input[i]); 
    }
    output[len]='\0';
    return len;
}




int main(int argc,char *argv[]){
    /* ----------------------------DECLARING VARIABLES----------------------------------   */
    int ssd;                    
    struct sockaddr_in server;  //server info
    struct sockaddr_in client;  //client info       
    int client_len;  // size of client          
    short echo_port; // the port number           
    int max_iterations; // usually runs forever but this will limit server        
    int byteInCount,byteOutCount,recieveCount,i;
    char clientString[BUF_LEN];
    char serverProcessedString[BUF_LEN];
    int retCode;

    /* ----------------------------END: DECLARING VARIABLES----------------------------------   */



    /* ---------------------------CHECKING THE COMMAND LINE----------------------------   */

    if (argc != 3)
    {
        fprintf(stderr,"incorrect usage! %s port Max_iterations\n",argv[0]);
        exit(EXIT_FAILURE);
    }
    echo_port = atoi(argv[1]);
    max_iterations = atoi(argv[2]);

    /* ---------------------------END: CHECKING THE COMMAND LINE----------------------------   */



    /* ----------------------------CREATE THE SOCKET CONNECTION----------------------------------   */
    ssd = socket(AF_INET, SOCK_DGRAM, 0); // refer to notes about meaning of each parameter
    if (ssd <0){
        perror("error in function socket()\n");
        exit(EXIT_FAILURE);
    }

    memset(&server, 0, sizeof(server)); 
    memset(&client, 0, sizeof(client)); 

    server.sin_family=AF_INET; // ip protocol 
    server.sin_addr.s_addr=htonl(INADDR_ANY);
    server.sin_port= htons(echo_port); // port to serve on

    retCode = bind(ssd,(struct sockaddr *)&server,sizeof(server)); // ask tut about bind() function
    if (retCode <0)
    {
        perror("error in function bind()\n");
        exit(EXIT_FAILURE);
    }
    /* ----------------------------END: CREATE THE SOCKET CONNECTION----------------------------------   */


    /* ---------------------------- START THE SERVER----------------------------------   */
    for (int i=0;i<max_iterations;i++){
        fprintf(stderr,"Iteration %d of %d. Waiting for client...\n",i+1, max_iterations);
        client_len = sizeof(client);
        byteInCount = recvfrom(ssd, clientString, BUF_LEN,
                               MSG_WAITALL, (struct sockaddr *)&client, 
                               (socklen_t *)&client_len);
        if (byteInCount <0)
        {
            perror("error in function revfrom()\n");
            exit(EXIT_FAILURE);
        }
        else{
            fprintf(stderr,"revfrom() is all g\n");
        }
        recieveCount = processString(clientString, serverProcessedString);
        fprintf(stderr,"Reversed string is %d bytes long\n", recieveCount);
        fprintf(stderr,"Reversed string is \"%s\"\n",serverProcessedString);
        byteOutCount = sendto(ssd, serverProcessedString, recieveCount+1, 0, 
                              (struct sockaddr *)&client,sizeof(client));
        if (byteOutCount <0){
            perror("error in function sendto()\n");
            exit(EXIT_FAILURE);
        }
        fprintf(stderr,"client request worked, reply has been sent\n");

    }
    close(ssd);
    fprintf(stderr,"server has shutdown!\n");

    return 0;


}

here is your client code

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

#define BUF_LEN 48

int main(int argc,char *argv[]){
    int csd;
    struct sockaddr_in server;
    struct hostent *server_host;
    int serverLength;
    int stringSize;
    short serverPort;
    int byteInCount,byteOutCount;
    char clientString[BUF_LEN];
    char serverProcessedString[BUF_LEN];

    if (argc != 4){
        fprintf(stderr,"usage: %s server port send_string \n",argv[0]);
        exit(EXIT_FAILURE);
    }

    server_host =gethostbyname(argv[1]);
    if (server_host== NULL){
        herror("error in function gethostbyname()\n"); // ask tut- this function is no longer used( getaddrinfo)
        exit(EXIT_FAILURE);
    }

    serverPort= atoi(argv[2]);
    strcpy(clientString,argv[3]);


    csd = socket(AF_INET, SOCK_DGRAM, 0);
    if(csd <0)
    {
        perror("error in function socket()");
        exit(EXIT_FAILURE);
    }
    memset(&server, 0, sizeof(server));

    server.sin_family= AF_INET;
    //memcpy(&server.sin_addr.s_addr,server_host->h_addr_list[0],server_host->h_length); // ask tut about this function
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(serverPort); // difference between htons and htonl???
     
    stringSize = strlen(clientString)+1;
    byteOutCount= sendto(csd, clientString, stringSize,
                         MSG_CONFIRM, (struct sockaddr *)&server, 
                         sizeof(server));

    if (byteOutCount <0){
        perror("error in function sendto()\n");
        exit(EXIT_FAILURE);
    }

    fprintf(stderr,"You have sent \"%s\"\n",clientString);
    fprintf(stderr,"Have reached recvfrom(), should now block until message receipt\n");

    //get the response from server
    serverLength = sizeof(server);
    byteInCount = recvfrom(csd, serverProcessedString, BUF_LEN,
                           MSG_WAITALL, (struct sockaddr *)&server,
                           (socklen_t *) &serverLength);

    if (byteInCount <0){
        perror("error in function recvfrom()\n");
        exit(EXIT_FAILURE);
    }
    fprintf(stderr,"the server has responded with: : \"%s\"\n",serverProcessedString);
    close(csd);

    return 0;
}

In one terminal, run./server 9999 10

In another terminal run./client localhost 9999 hello

Output on Server side

Iteration 1 of 10. Waiting for client...
revfrom() is all g
Reversed string is 5 bytes long
Reversed string is "HELLO"
client request worked, reply has been sent
Iteration 2 of 10. Waiting for client...

Output on client side

You have sent "hello"
Have reached recvfrom(), should now block until message receipt
the server has responded with: : "HELLO"

All the best. I haven't bug checked your code, ensure processString is working fine. All the best!