创建替换stdin的文件描述符（在select（）中）

Question

Having in mind to communicate with a multi-client Unix domain socket without keyboard input, I want to create a file descriptor that replaces stdin.

With a multi-client socket, you have to add the stdin's file descriptor (which is 0, with FD_SET(0, &readfds) ) to readfds in order to send the keyboard input ( FD_ISSET(0, &readfds) ) to the server. Since I don't want to write everytime I launch my client, I want to replace this file descriptor by a custom one (by writing to this file from another program).

I checked out Create a file descriptor to create two programs:

One that writes to the file descriptor:

int main() {
    char buffer[] = "test";
    int fd = open("sample.dat", O_WRONLY | O_CREAT, 0666);
    dup2(fd, 5);

    if(write(5, buffer, 4) < 0)
        fprintf(stderr, "write %s\n", strerror(errno));

    close(5);    
    return 0;
}

And another one that reads from it:

int main() {
    char buffer[4];
    int fd = open("sample.dat", O_RDONLY);
    dup2(fd, 5);

    for(;;) {
        if(read(5, buffer, 4) < 0)
            fprintf(stderr, "read %s\n", strerror(errno));

        printf("%s\n", buffer);
        sleep(1);
    }

    return 0;
}

My question is : is it possible to make this file descriptor as stdin-like ? I mean actually my second program read "test" endlessly because (of course) sample.dat still contains "test" but I want to delete it once it has been read.

Answer 1

Yes. You can use pipes for this: https://linux.die.net/man/2/pipe

Or FIFOs: https://linux.die.net/man/3/mkfifo Using FIFOs will create a file in the filesystem which can be opened by the client and server process to communicate through.

Answer 2

If I understood correctly, you want to communicate to a program who can only read on the stdin. You have two ways to write in the stdin : the first way is to use your keyboard, and the second way is to use write() .

#include <unistd.h>
#include <string.h>

int         main(void)
{
    char    *s = "what you want to write";
    write(STDIN_FILENO, s, strlen(s));
    return (0);
}

This program will write on the stdin, in the same way you would write with your keyboard. Besides, you will see the string printed on your command-line interface.

Answer 3

The problem isn't that you're reading in "test" constantly, it's that you're not handling the call to read() properly. Whilst you're checking that it's not returning an error, you're not seeing how many bytes have been read in. read() doesn't null-terminate the string read in, so you should actually store the return value and use that to put the NUL in your buffer. Otherwise, since nothing in the buffer is changing, it'll appear as if you're re-reading the same text over and over.

    int err=read(fd, buffer, 4);
    if(err < 0)
        {
        fprintf(stderr, "read %s\n", strerror(errno));
        }
    else
        {
        buffer[err]='\0';
        }

If you make this change, you'll find that you only ever read in "test" once and after that you're not reading anything more in.

Note: You don't need the calls to dup2() - that's just for copying a file descriptor to known value. You can just use fd as is in your calls to read/write data.