Redirect stdin and stdout in child in c

问题

I need to create a client-server app (using sockets), the basic idea is:

client sends string
server receives string and sends it as stdin of another app
server reads stdout of the app
server sends answer to client.

The "other app" is a closed source calc (a calculator which reads from stdin the sort of 4 5 + and prints 9).

I'm trying to create a double pipe on the server, fork, and use this pipes to redirect stdin and stdout of the calc:

if(!fork())
{

    close(STDOUT_FILENO);
    close(STDIN_FILENO);

    dup2(outfd[0], STDIN_FILENO);
    dup2(infd[1], STDOUT_FILENO);

    close(outfd[0]); /* Not required for the child */
    close(outfd[1]);
    close(infd[0]);
    close(infd[1]);

    system("./calc/calc ");
    exit(0);
}

and from the server write on one pipe and read the other

    close(outfd[0]); /* These are being used by the child */
    close(infd[1]);

    char c;
    do
    {
        c=getchar();
        write(outfd[1],&c,1);
    }while(c!='\n');
    input[read(infd[0],input,100)] = 0; /* Read from child’s stdout */

    printf("%s",input);

    close(outfd[1]);
    close(infd[0]);

I know this is very incomplete, but I think it should at least print the output of the first line in calc.

(Full code)

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main()
{

int outfd[2];
int infd[2];

pipe(outfd); /* Where the parent is going to write to */
pipe(infd); /* From where parent is going to read */

if(!fork())
{

    close(STDOUT_FILENO);
    close(STDIN_FILENO);

    dup2(outfd[0], STDIN_FILENO);
    dup2(infd[1], STDOUT_FILENO);

    close(outfd[0]); /* Not required for the child */
    close(outfd[1]);
    close(infd[0]);
    close(infd[1]);

    //system("/usr/bin/bc -q");
    system("./calc/calc ");
    exit(0);
}
else
{

    char input[100];

    close(outfd[0]); /* These are being used by the child */
    close(infd[1]);

    //write(outfd[1],"2^32\n",5); /* Write to child’s stdin */
    //write(outfd[1],"2 4 +\n",6);
    char c;
    do
    {
        c=getchar();
        write(outfd[1],&c,1);
    }while(c!='\n');
    input[read(infd[0],input,100)] = 0; /* Read from child’s stdout */

    printf("%s",input);

    close(outfd[1]);
    close(infd[0]);

}
return 0;

It's very basic, but it's just for testing. When I execute it, it does nothing.

If I ps -e | grep calc during the execution, I can see it is running, but whatever I type doesn't seems to do anything.

回答1:

Try moving:

close(outfd[1]);

between the do-while loop and input[read(infd[0],input,100)] = 0;.

This will send EOF on calc's stdin. It will exit, and this will flush its output buffer, so there will be something to read.

Your code is deadlocked: you're waiting for output before you close the pipe, but it's waiting for you to close the pipe before it sends output.

来源：https://stackoverflow.com/questions/27736042/redirect-stdin-and-stdout-in-child-in-c

标签

Linux

pipe