CODEWITHSUNDEEP
c++socketsunix
S1J0
S1J0

Reputation: 199

How to read/write UNIX sockets the right way?

I want to implement the functionality of this bash-command that read/write to a UNIX socket:

echo „READ <a command>“ | nc -U /tmp/socket

this command produces the following output:

Click::ControlSocket/1.3
200 Read handler 'debug_handler.ping' OK
DATA 0

My current c/c++ implementation looks something like this:

myprogram.hh

class Myprogram {
public:
    Myprogram(int argc, char** argv);
    ~Myprogram() { };   
private:
    foo();
    int sockfd;
    int len;
    sockaddr_un address;
};

myprogram.cc

Myprogram::foo() {
    sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
    address.sun_family = AF_UNIX;
    strcpy(address.sun_path, "/tmp/socket");
    len = sizeof(address);

    if(connect(sockfd, (sockaddr*)&address, len) == -1)
        return;

    std::string args = "READ <a command>";

    if (write(sockfd, args.c_str(), sizeof(args.c_str())) == -1){
        std::cout << "Error" << std::endl;
        return;
    }   

    int n;
    char ret_value[200];
    do {
        n = read(sockfd, ret_value, 200);
        std::cout << std::string(ret_value) << std::endl;
        std::cout << "n=" << std::to_string(n) << std::endl;
        *ret_value+= n;
    } while (n > 0);

    close(sockfd);
}

I only get this output:

Click::ControlSocket/1.3
n=26

the next read() call doesn‘t return and the program waits. I have tried a lot to read the socket, all with the same result, so it would be great if someone could give me a hint what I'm doing wrong.

Upvotes: 0

Views: 10385

Answers (2)

Remy Lebeau
Remy Lebeau

Reputation: 596287

Try something more like this:

Myprogram::foo() {
    sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
    if (sockfd == -1) {
        std::cout << "Error" << std::endl;
        return;
    }

    address.sun_family = AF_UNIX;
    strcpy(address.sun_path, "/tmp/socket");

    if (connect(sockfd, (sockaddr*)&address, sizeof(address)) == -1) {
        std::cout << "Error" << std::endl;
        close(sockfd);
        return;
    }

    std::string args = "READ <a command>\n";
    if (write(sockfd, args.c_str(), args.size())) == -1) {
        std::cout << "Error" << std::endl;
        close(sockfd);
        return;
    }   

    int n;
    char ret_value[200];
    do {
        n = read(sockfd, ret_value, sizeof(ret_value));
        if (n < 0) {
            std::cout << "Error" << std::endl;
            break;
        }

        if (n == 0) {
            break;
        }

        std::cout.write(ret_value, n) << std::endl;
        std::cout << "n=" << n << std::endl;
    } while (true);

    close(sockfd);
}

Of course, if you really want the output to appear just like the original than don't add your own characters to cout (except maybe in case of an error):

int n;
char ret_value[200];
do {
    n = read(sockfd, ret_value, sizeof(ret_value));
    if (n < 0) {
        //std::cout << "Error" << std::endl;
        break;
    }

    if (n == 0) {
        break;
    }

    std::cout.write(ret_value, n);
} while (true);

Upvotes: 3

kfsone
kfsone

Reputation: 24249

std::string args = "READ <a command>";

the server is waiting for a newline (echo adds one by default), so:

std::string args = "READ <a command>\n";

Also

char ret_value[200];
do {
    n = read(sockfd, ret_value, 200);

should be more like

const size_t buflen = 200;
char ret_value[buflen+1];
do {
    n = read(sockfd, ret_value, buflen);
    if (n < 0)
      // error
    else
      ret_value[n] = 0;

and this line

    *ret_value+= n;

seems unclear - you're incrementing the first byte of ret_value by n for what reason?

Upvotes: 3

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