Sockets sendto() returns EINVAL after previous successful call

Question

I've modified this C datagram (UDP) socket client example:

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

#define SERVERPORT "4950"

volatile sig_atomic_t is_shutdown = 0;

void handler_signal(int signal)
{
    if (signal == SIGINT)
    {
        is_shutdown = 1;
    }
}

int main(int argc, char *argv[])
{
    int socket_fd;
    struct addrinfo hints, *server_info, *temp;
    int result;
    int number_bytes;

    if (argc != 2)
    {
        fprintf(stderr, "usage: executable [hostname]\n");
        return 1;
    }

    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_DGRAM;

    result = getaddrinfo(argv[1], SERVERPORT, &hints, &server_info);

    if (result != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(result));
        return 1;
    }

    // loop through all the results and make a socket
    for(temp = server_info; temp != NULL; temp = temp->ai_next)
    {
        socket_fd = socket(temp->ai_family, temp->ai_socktype, temp->ai_protocol);

        if (socket_fd == -1)
        {
            fprintf(stderr, "socket error: %s\n", strerror(errno));
            continue;
        }

        break;
    }

    freeaddrinfo(server_info);

    if (temp == NULL)
    {
        fprintf(stderr, "failed to create socket\n");
        return 2;
    }

    //int hostname_length = strlen(argv[2]);
    unsigned long counter = 0;
    const size_t buffer_length = 50;
    char buffer[buffer_length];
    memset(&buffer, 0, buffer_length);

    while (!is_shutdown)
    {
        snprintf(buffer, buffer_length, "%lu", counter++);

        number_bytes = sendto(socket_fd, buffer, buffer_length, 0, temp->ai_addr, temp->ai_addrlen);

        if (number_bytes == -1)
        {
            fprintf(stderr, "sendto error: %s\n", strerror(errno));
            break;
        }

        printf("sent %d bytes to %s.\n", number_bytes, argv[1]);
    }

    result = close(socket_fd);

    if (result == -1)
    {
        fprintf(stderr, "close error: %s\n", strerror(errno));
    }

    printf("exiting application\n");

    return 0;
}

It fails on sendto function with Invalid argument (errno 22) on the second iteration of the loop. It works on the first.

However, the code works (client keeps sending messages in an infinite loop and server successfully receives them) if I change it to the following:

    //...

    //create a copy to pass it later
    socklen_t length = temp->ai_addrlen;
    struct sockaddr *address = temp->ai_addr;

    while (!is_shutdown)
    {
        snprintf(buffer, buffer_length, "%lu", counter++);

        number_bytes = sendto(socket_fd, buffer, buffer_length, 0, address, length);

   //...

What have I done here? What am I missing? I am not sure what is going on here.

`if (number_bytes == -1)`:: check errno before breaking out of the loop. Could be EAGAIN , EINTR, or even resource-related for UDP. — wildplasser, Feb 11 '18 at 19:17
@wildplasser Thanks, I will modify it. But right now It only becomes EINVAL. — CorellianAle, Feb 11 '18 at 19:20
You answered your own question: `number_bytes = sendto(socket_fd, buffer, buffer_length, 0, &temp->ai_addr, temp->ai_addrlen);` :the address needs to be passed by pointer. — wildplasser, Feb 11 '18 at 19:43
@wildplasser then why does it work on a first call and only fails on the second? `temp->ai_addr` is already a pointer. — CorellianAle, Feb 11 '18 at 20:07
`temp` is a pointer to `struct addrinfo` which value is assigned from `server_info`. But when you `freeaddrinfo(server_info);` the memory is released and `temp` points to nowhere. It works the first time because no memory reuse has happened yet. But after memory reuse it become invalid. Try to make a copy of data before to free the structure. — Frankie_C, Feb 11 '18 at 20:28
Thank you, I totally missed that `temp`'s fields point to the `server_info`'s fields. — CorellianAle, Feb 11 '18 at 20:41

score -1 · Accepted Answer · answered Feb 11 '18 at 20:44

As Frankie_C pointed in his [comment]:

temp is a pointer to struct addrinfo which value is assigned from server_info. But when you freeaddrinfo(server_info) the memory is released and temp points to nowhere. It works the first time because no memory reuse has happened yet. But after memory reuse it become invalid. Try to make a copy of data before to free the structure.

Sockets sendto() returns EINVAL after previous successful call

1 Answers1