Linux C/C++ socket send in multi-thread code

Indeed as the previous answer says: "The safest mechanism is for only a single thread to ever be submitting data to a socket."

But if you want to have multiple threads call read/write, you must guarantee thread safety yourself. I've written a wrapper around send and recv below that does this for you.

Send() and Recv() are thread safe in the sense that they wont cause a crash, but there is no guarantee that that data wont be "intermixed" with data sent from other threads. Since we most likely don't want that, we must block until all the thread's data is confirmed sent or received, or errored out. Such a long blocking call can be problematic, so make sure you're doing this on threads that can handle long blocking operations.

For linux:

#include <sys/types.h>
#include <sys/socket.h>
#include <map>
#include <mutex>



/* blocks until the full amount of bytes requested are read
 * thread safe
 * throws exception on error */
void recv_bytes(int sock, char* buf, int len, int flags){

    static std::map<int, std::mutex> mtx;

    std::lock_guard<std::mutex> lock(mtx[sock]);

    int bytes_received = 0;

    while (bytes_received != len){

        int bytes = recv(sock, buf + bytes_received, len - bytes_received, flags);

        //error check
        if (bytes == -1){
            throw std::runtime_error("Network Exception");
        }

        bytes_received += bytes;

    } 
}



/* blocks until the full amount of bytes requested are sent
 * thread safe
 * throws exception on error */
void send_bytes(int sock, char* buf, int len, int flags){

    static std::map<int, std::mutex> mtx;

    std::lock_guard<std::mutex> lock(mtx[sock]);

    int bytes_sent = 0; 

    while (bytes_sent != len){

        int bytes_s0 = send(sock, buf, len, flags);

        if (bytes_sent == -1) {
            throw std::runtime_error("Network Exception");
        }

        bytes_sent += bytes_s0;

    }
}