Sendto returning random values

I call sendto in my udp socket client side like this:

if((num_w = sendto(cli_udp_sock, buffer, strlen(buffer), 0, (struct sockaddr *) &servaddr, sizeof(servaddr) )) < 0)

but there seems to be no nothing received at the server side and in very strange fashion, num_w, when printed out, gives values like:

-197379208 -1440076936 2054978424

And perror() displays “success”

Client code up till that point:

#include <sys/socket.h>       /*  socket definitions        */
#include <sys/types.h>        /*  socket types              */
#include <arpa/inet.h>        /*  inet (3) funtions         */
#include <unistd.h>           /*  misc. UNIX functions      */
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <netdb.h>
/*  Global constants  */

#define MAX_LINE           (1000)
#define LISTENQ (10)

/*  Function declarations  */
int ParseCmdLine(int argc, char *argv[], char **szPort, char **szAddress, char ** serv_udp_port);



/*  main()  */

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

    //
short int cli_port;                  /*  port number               */
short int serv_port;
struct    sockaddr_in servaddr;  /*  socket address structure  */
socklen_t serv_addrlen = sizeof(servaddr);
struct    sockaddr_in cliaddr;
char      buffer[MAX_LINE];      /*  character buffer          */
char     *szAddress;             /*  Holds remote IP address   */
char     *szPort;                /*  Holds remote port         */
char     *serv_udp_port;
char     *endptr;                /*  for strtol()              */
int cli_udp_sock;
/*  Get command line arguments  */

ParseCmdLine(argc, argv, &szPort, &szAddress, &serv_udp_port);


/*  Set the remote port */ 

cli_port = strtol(szPort, &endptr, 0);
serv_port = strtol(serv_udp_port, &endptr, 0);

if ( *endptr ) 
{
    printf("ECHOCLNT: Invalid port supplied.n");
    exit(EXIT_FAILURE);
}



/*  Set all bytes in socket address structure to zero, and fill in the relevant data members   */

memset(&cliaddr, 0, sizeof(cliaddr));
cliaddr.sin_family      = AF_INET;
cliaddr.sin_port        = htons(cli_port);
cliaddr.sin_addr.s_addr = htonl(INADDR_ANY);

memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family      = AF_INET;
servaddr.sin_port        = htons(serv_port);

/*  Set the remote IP address  */

if ( inet_aton(argv[2], &servaddr.sin_addr) <= 0 ) 
{
    printf("ECHOCLNT: Invalid remote IP address.n");
    exit(EXIT_FAILURE);
}

int list_s;
if ( (list_s = socket(AF_INET, SOCK_STREAM, 0)) < 0 ) 
{
    fprintf(stderr, "ECHOCLNT: Error creating listening socket.n");
    exit(EXIT_FAILURE);
}
if (listen(list_s, LISTENQ) < 0)
{
    printf("Failed to called listen.n");
}
if( ((cli_udp_sock = socket(AF_INET, SOCK_DGRAM, 0) )< 0))
{
    printf("UDP socket was not successfully created. Exiting...n");
    exit(0);
}

//if(bind(cli_udp_sock, (struct sockaddr *) &cliaddr, sizeof(cliaddr)) > 0); 

char command[100];
    /*  Get command from user  */
printf("==============================================n");

do
{     
    if(strcmp(command, "sn") == 0)
    {
        printf("______________________________________________n");
            printf("'s' entered, nEnter the string to requestn"); //prompt for string to enter
            strcpy(buffer, "CAPn");                //set first set of characteres to CAPn
            char bucket[100];                   //space for entered string
            fgets(bucket, sizeof bucket, stdin);
        printf("The string entered is: %s", bucket);
            strcat(buffer, bucket);

            /*  Send string to echo server, and retrieve response  */
            int num_w;
        if((num_w = sendto(cli_udp_sock, buffer, strlen(buffer), 0, (struct sockaddr *) &servaddr, sizeof(servaddr) )) < 0)
        {
            perror("error: ");
            printf("Problem writing data to server, errno: %d", errno);
            exit(0);
        }
        else
        {
            perror("error 2: ");
            printf("num_w: %dn");
        }

Thanks so much in advance! (Oh and I’m on Kali linux, virtual machine, Windows 10)

Answer

You aren’t printing num_w. You’ve left it out of the printf() statement. You’re just printing whatever random junk is on the stack.

And perror() displays “success”

So num_w was positive. Calling perror() in that case is pointless.

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Answer