C's printf and fprintf(stdout,) are not printing

问题

This is a bit of an odd one. My code wasn't outputting what I thought it should. I added some print statements at various stages to see where it was going wrong. Still nothing. So I added a printf statement at the start of main. That's where I got really confused.

So I presumed something funny was happening with the file descriptors. I changed the printf to a fprintf. Still nothing. Printing to stderr with fprintf does work! Why is this happening?

Removing all of the body from main except the initial print statement and the return does print.

Code

int main(void) {
    fprintf(stdout, "STARTED!");
    //Create an Internet domain socket
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    //If this fails exit and print the error
    if (sockfd == -1) {
        printf("Error %d, cannot create socket", errno);
        return 1;
    }
    printf("SOCKET CREATED!");

    //Creates a socket address
    struct sockaddr_in  addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(8080);
    addr.sin_addr.s_addr = INADDR_ANY;

    //Attempts to bind to the socket address, again prints to error if this fails.
    if (bind(sockfd, (struct sockaddr *) &addr, sizeof(addr)) == -1)
    {
        printf("Error %d, cannot bind", errno);
        return 1;
    }

    //Starts Listening for a client
    if (listen(sockfd, 1) == -1)
    {
        printf("Error %d, cannot listen", errno);
        return 1;
    }

    //If all is successful, server is operational
    while(1)
    {
        //Creates a file descripter for the connection
        int connfd;
        //And a socket address for the client
        struct sockaddr_in cliaddr;
        socklen_t cliaddrlen = sizeof(cliaddr);
        //If a connection attempt is made accepts it.
        connfd = accept(sockfd, (struct sockaddr *) &cliaddr, &cliaddrlen);
        if (connfd == -1) {
            //If the connection fails print an error
            printf("Error %d, cannot accept connection", errno);
            continue;
        }

        //Otherwise process the request
        else {
            printf("CONNECTED!");
            char end;
            end = 1;
            while (end)
            {
                process_request(connfd);
                end = 0;
            }
        }
        close(connfd);

    }
    close(sockfd);
    return 0;
}

回答1:

Output is often buffered by the system. You can call fflush, but sometimes, depending on how the caching works, simply ending the output with a newline is sufficient. So try changing

fprintf(stdout, "STARTED!");

fprintf(stdout, "STARTED!\n");

And, if that doesn't help, to

fprintf(stdout, "STARTED!\n");
fflush(stdout)

(And stderr often isn't cached, as you want to see errors immediately.)

Finally, you will see output when the program finishes (as things are flushed then), which probably explains the rest of the behaviour.

来源：https://stackoverflow.com/questions/14784367/cs-printf-and-fprintfstdout-are-not-printing

标签

file-descriptor

printf