u/Escapshion

Hey everyone,

I was practicing Socket Programming using C and ran into an issue. The recvfrom() function always returns 10 bytes on UDP socket regardless of the input given.

Here is the code which I have written:

udp_server.c:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<sys/socket.h>
#include<sys/types.h>
#include<netinet/in.h>
#include<arpa/inet.h>

#define PORT 6900
#define LINES 128 // QUERY: Changing the value to 4096 breaks the program

int main(){
    int sockfd, numrec, byterec;
    char recvline[LINES + 1]; 

    struct 
sockaddr_in
 ser_addr;
    const struct 
sockaddr_in
 clnt_addr;
    
    
socklen_t
 len = sizeof(clnt_addr);

    if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0){
        perror("Socket Error!!");
        exit(1);
    }

    memset(&ser_addr, '\0', sizeof(ser_addr));
    ser_addr.sin_family =  AF_INET;
    ser_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    ser_addr.sin_port = htons(PORT);

    bind(sockfd, (struct 
sockaddr
*) &ser_addr, sizeof(ser_addr));

    while(1){
        if((numrec = recvfrom(sockfd, recvline, LINES, 0, (struct 
sockaddr
 *)&clnt_addr, &len)) < 0){
            perror("recvfrom error");
            exit(1);
        }

        // QUERY: FROM WHERE byterec IS FETCHING THE VALUE
        fprintf(stdout, "Connection from %s, port %d. Received %d bytes.\n", inet_ntoa(clnt_addr.sin_addr), ntohs(clnt_addr.sin_port), byterec);
        fflush(stdout);

        // recvline[byterec]='\0';
        if(strcmp(recvline, "Terminate\n") == 0){
            fprintf(stdout, "a client want me to terminate\n");
            fflush(stdout);
            break;
        }

        if(sendto(sockfd, recvline, byterec, 0, (struct 
sockaddr
 *)&clnt_addr, len) < 0){
            perror("sendto error");
            exit(1);
        }
    }

    fprintf(stdout, "server normal end\n");
    fflush(stdout); 


    return 0;
}

udp_client.h:

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

#define PORT 6900
#define LINES 20

int main(){
    int sockfd, bytesent, numrec, byterec;
    char sendLine[LINES], recvLine[20];
    
    struct sockaddr_in ser_addr;
    struct sockaddr_in echo_addr;
    socklen_t len = sizeof(echo_addr);
    
    if((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0){
        perror("Socket Error!!");
        exit(1);
    }

    memset(&ser_addr, '\0', sizeof(ser_addr));
    ser_addr.sin_family =  AF_INET;
    ser_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    ser_addr.sin_port = htons(PORT);

    while(1){
        printf("==>");
        fflush(stdout);
        fgets(sendLine, LINES, stdin);
        
        if(strcmp(sendLine, "exit\n") == 0){
            break;
        }
                
        bytesent = sendto(sockfd, sendLine, strlen(sendLine), 0, (struct sockaddr*) &ser_addr, sizeof(ser_addr));
        if(bytesent == -1){
            perror("sendto error");
            exit(1);
        }

        if(strcmp(sendLine, "Terminate\n") != 0){
            printf("----VARIABLE VALUES BEFORE----\n");
            printf("recvLine size: %d\n", sizeof(recvLine));
            printf("echo_addr size: %d\n", sizeof(echo_addr));
            printf("len var address: %p\n", &len);
            printf("len value: %d\n", len);
            printf("byterec value: %zd\n", byterec);
            printf("\n");

            if ((byterec = recvfrom(sockfd, recvLine, sizeof(recvLine), 0, (struct sockaddr*) &echo_addr, &len)) < 0){
                perror("recvfrom error");
                exit(1);
            }

            printf("----VARIABLE VALUES AFTER----\n");
            printf("recvLine size: %d\n", sizeof(recvLine));
            printf("echo_addr size: %d\n", sizeof(echo_addr));
            printf("len var address: %p\n", &len);
            printf("len value: %d\n", len);
            printf("byterec value: %zd\n", byterec);
        }

        // recvLine[byterec] = '\0';
        printf("from server: %s\n", recvLine);
    }

    close(sockfd);
    printf("echo client normal end\n");

    return 0;
}

Additional Info: I am using cygwin for the compiler.

What I tried:

1. I tried to use char datatype for recvLine buffer variable. In this case, the value of byterec variable became 1.
2. I gave the input of length more than the size of buffer. The valuebyterec variable changed to 10. I assumed that it might be happening because of buffer overflow. Therefore, I tried step 3.
3. I gave the input of length less than the size of buffer. Still, the value of byterec variable is getting changed to 10.
4. I tried to change the buffer size, LINES constant, tried to match their values. Doesn't matter what I try? the value of byterec always changes to 10 after recvfrom() function.

After trying to debug for the whole day and countless googling, I am unable to figure out what's wrong with my code. Any help would be appreciated.

Thanks in advance.