unix socket send() succeeds, but recv() fails

Question

I am writing a simple client server program using unix domain sockets, but am having issues with the recv() call in my client program.

The program executes as follows:

• Server sets up socket and waits for a connection
• Client connects and sends a string
• Server receives string, and sends string back to client (like an echo)
• Client recv() call fails, returning "resource temporarily unavailable"
• Client exits
• Server waits for another connection

I have also tried using a poll() call in my client to wait for the response from the server. In this case however, the recv() call simply receives a 0, implying the connection has been closed serverside, which it has not.

I have exhausted google on this error, but no fixes I came accross seem applicable to my code.

I have included my client (with poll() code commented out) and server code below.

I'm probably missing something obvious... but any insight would be greatly appreciated!

Server code:

    /*
 *  testServer.c
 *
 */

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <termios.h>
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>
#include <stdbool.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/un.h>
#include <linux/spi/spidev.h>
#include <linux/sockios.h>
#include <errno.h>

#define SOCK_PATH "/var/run/ts.serv" 

void handleSockIO(int *sockDesc);

int main ()
{
    int sock;
    struct sockaddr_un sock_addr;
    int len, p;
    struct pollfd poll_fd[1];

    printf("[TS] testServer Started.\r\n");

    if ((sock = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) 
    {
        perror("[TS]wr_sock creation");
    }
    else 
    {
        printf("[TS] Created socket descriptor.\r\n");
    }


    sock_addr.sun_family = AF_UNIX;
    strcpy(sock_addr.sun_path, SOCK_PATH);
    unlink(sock_addr.sun_path);

    len = strlen(sock_addr.sun_path) + sizeof(sock_addr.sun_family);

    if (bind(sock, (struct sockaddr *)&sock_addr, len) == -1) 
    {
        perror("[TS]sock bind failed\r\n");
    }
    else 
    {
        printf("[TS] Bound socket to sock_addr.\r\n");
    }

    if (listen(sock, 5) == -1) 
    {
        perror("[TS] sock listen fail");
    }
    else 
    {
        printf("[TS] Socket now listening.\r\n");
    }

    poll_fd[0].fd = sock;
    poll_fd[0].events = POLLIN;

    printf("[TS] Waiting for a connection...\r\n");

    while (1) 
    {
        p = poll(poll_fd, 1, 1); //Wait for 1 ms for data

        if (p == -1) 
        {
            perror("[TS] Poll");
        } 
        else if (p == 0) 
        {
            //printf("Timeout occurred!\n"); 
        } 
        else 
        {
            if (poll_fd[0].revents & POLLIN)//Data available to read without blocking
            { 
                printf("[TS] Data available on sock..\r\n");
                handleSockIO(&sock);

                printf("[TS] Waiting for another connection...\r\n");
            }
        }
    }//While(1)

    return 0;
}

void handleSockIO(int *sockDesc)
{
    int ioSock, n;
    socklen_t t;
    struct sockaddr_un remote_addr;
    char str[15];

    memset(str, ' ', sizeof(str));

    t = sizeof(remote_addr);

    if ((ioSock = accept(*sockDesc, (struct sockaddr *)&remote_addr, &t)) == -1) 
    {
        perror("accept failed\r\n");
    }
    else 
    {
        printf("[TS] Receiving...\r\n");

        n = recv(ioSock, str, sizeof(str), 0);

        if (n < 0) 
            printf("[TS] recvfrom failed: %s\r\n", strerror(errno));
        else if(n == 0)
        {
            printf("[TS] Received %d on ioSock...\r\n", n);
        }
        else if(n > 0)
        {
            printf("[TS] Received: %s, which is %d long.\r\n", str, strlen(str));

            printf("[TS] Echoing response...\r\n");

            if (send(ioSock, str, n, 0) == -1) //Echo str back
            {
                printf("[TS] send failed: %s\r\n", strerror(errno));
            }
            else 
            {
                printf("[TS] Send successful\r\n");
            }

            //============Wait to close IO descriptor=================
            int r;
            char temp[1]; //Arbitrary buffer to satisfy recv()
            do 
            {
                printf("[TS] Waiting for client to close connection...\r\n");
                r = recv(ioSock, temp, sizeof(temp), 0);

                if (r == 0)
                {
                    printf("[TS] Client closed connection, closing ioSock...\r\n");
                    close(ioSock);
                }
            } while (r != 0);

            //========================================================
        }//if(n>0) else...
    }       
}

Client code:

 /*
 *  testClient.c
 *
 */

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <termios.h>
#include <fcntl.h>
#include <poll.h>
#include <signal.h>
#include <unistd.h>
#include <string.h>
#include <stdbool.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <sys/un.h>
#include <linux/sockios.h>
#include <errno.h>

#define CHAR_BUF_SIZE 15
#define SEND_STRING "Hello!"
#define SOCK_PATH "/var/run/ts.serv" 

int main ()
{
    char str[CHAR_BUF_SIZE] = {0};
    int c, len, n, p;
    int s; // s will hold a socket descriptor returned by socket()
    struct sockaddr_un serv_addr;
    struct pollfd poll_fd[1];

    printf("[TC] testClient Started.\r\n");

    //===============SOCKET SETUP===============================

    if ((s = socket(AF_UNIX, SOCK_STREAM, 0)) == -1) 
    {
        printf("[TC] Socket failed: %s\r\n", strerror(errno));
    }

    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sun_family = AF_UNIX;
    strcpy(serv_addr.sun_path, SOCK_PATH);

    len = strlen(serv_addr.sun_path) + sizeof(serv_addr.sun_family);

    //==========================================================

    //  printf("[TC]Trying to connect to TS socket...\r\n");

    //===============RESPONSE POLL SETUP========================

    poll_fd[0].fd = s;
    poll_fd[0].events = POLLIN;

    //==========================================================

    printf("[TC] Connecting to SOCK_PATH...\r\n");

    c = connect(s, (struct sockaddr *)&serv_addr, len);

    if (c == -1)
    {
        printf("[TC] Connection failed: %s\r\n", strerror(errno));
    }
    else 
    {
        printf("[TC] Connected. Sending string....\r\n");

        if (send(s, SEND_STRING, strlen(SEND_STRING), 0) == -1) 
        {
            printf("[TC] send() failed: %s\r\n", strerror(errno));
        }
        else 
        { 
            printf("[TC] Send on SOCK_PATH successful.\r\n");
            //Sending complete------------------------------------------------
            //Wait for response...

            printf("[TC] Waiting for server response...\r\n");

//          p = poll(poll_fd, 1, -1); //Wait for a response
//          
//          if (p == -1) 
//          {
//              perror("[TC] Poll");
//          } 
//          else 
//          {
//              if(poll_fd[0].revents & POLLIN)
//              {
                    n = recv(s, str, sizeof(str), 0);

                    if (n < 0) 
                    {
                        printf("[TC] Receive on SOCK_PATH failed: %s\r\n", 
                               strerror(errno));
                    }
                    else if(n == 0)
                    {
                        printf("[TC] %d Received on SOCK_PATH.\r\n", n);
                    }
                    else if(n > 0)
                    {
                        printf("[TC] Received %d from SOCK_PATH: %s\r\n", 
                               n, str);
                    }
//              }
//          }
        }//if(send())
    }//if(connect())

    printf("[TC] Transction complete, closing connection and exiting.\r\n");

    close(s);

    return 0;
}

Answer 1

len = sizeof(serv_addr) instead of len = strlen(serv_addr.sun_path) + sizeof(serv_addr.sun_family) should solve you problem. Also do not ignore compiler warnings, say n = recv(s, str, strlen(str), 0) with n declared as int and ssize_t returned by recv. It will help you to avoid a future errors.