CODEWITHSUNDEEP
cnamed-pipesmkfifo
José Moreira
José Moreira

Reputation: 135

Named pipes how to identify client

Supose i have a server that creates captcha images on request.

Clients comunicate with the server via named pipes with a 6 chat word and an ID.

Server creates the image and send it to the client via named pipes too.

Client has the function create_captcha_files(const char* word) that comunicates with the server and get the result and saves in a word.png file.

Server has a already implemented function size_t captcha(const char* word, char * buffer) that writes on the buffer the corresponding image, returning the number of written bytes at the maximum of 16384bytes.

So the client is like this:

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>


int main()
{
    int fd, fdin, fdpng;
    char * myfifo = "captcha";
    char id = "001";
    char text[9];
    char buf[1024];
    char png[10];
    char palavra[6];


create_captcha_file(const char* palavra) {
    //write to fifo "captcha" the word + id
    mkfifo(myfifo, 0666);
    strcat(strcat(text,palavra),id);
    fd = open(myfifo, O_WRONLY);
    write(fd,text,9):
    close(fd);
    unlink(myfifo);

    //read anwser from server
    mkfifo(id,0666);
    fdin = open(id,O_RDONLY);
    strcat(strcat(png,palavra),".png");
    //create the .png file
    int fdpng = open(id,O_WRONLY | O_CREAT | O_APPEND,S_IRWXU);
        while((read(fdin,buf,1))) 
            {
             write(fdpng,buf,1);
             }
        close(fdpng);
        close(fdin);
     }
    unlink(id);


    return 0;
}

and the server:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>

int main()
{


    int fd;
    char texto[9];
    char palavra[6];
    char id[3];
    char * myfifo = "captcha";
    buffer[16384];

    //create and read from fifo
    mkfifo(myfifo, 0666);
    fdin = open(myfifo, O_RDONLY);
    read(fdin, texto, 9);
    close(fdin);
    //separate word from id
    for(i=0;i<=9;i++) {
        if(i<7) strcat(palavra,texto[i])
        else strcat(id,texto[i]
    }

size_t captcha(const *char palavra, char * buffer) {
    //create the captcha image and save to buffer
    buffer = create_captcha(palavra);
    return(size_of(buffer));

    }
   captcha(palavra, buffer);

    //write to pipe id the image
    mkfifo(id, 0666);
    fd = open(id, O_WRONLY);
    write(fd,buffer,size_of(buffer)):
    close(fd);
    unlink(fd);

}

Upvotes: 1

Views: 421

Answers (1)

Aleksandar Makragić
Aleksandar Makragić

Reputation: 1997

You can use select() to wait for something to happen on multiple file descriptors and then use FD_ISSET() to figure out which file descriptor sent message.

For example:

fd_set read_set;
FD_ZERO (&read_set);
FD_SET (filedes[0], &read_set); /* filedes[0] is read end of the pipe */
FD_SET (STDIN_FILENO, &read_set); /* some other file descriptor */

You would use FD_SET() for each client. And then, use select to wait for clients:

if (select (nfds, &read_set, NULL, NULL, NULL) == -1)
                /* Handle error */

Tricky thing is nfds here, it is maximum of file descriptors ids + 1.

Now, you wan't to check which client/file_descriptor sent message? Use FD_ISSET()!

if (FD_ISSET (filedes[0], &read_set))
               /* Do something */
if (FD_ISSET (STDIN_FILENO, &read_set)){
               /* Do something */

Note: You have to use FD_ISSET() for each file descriptor/client.

If you have any further questions, feel free to ask.

Upvotes: 1

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