Question

I use select() to receive data from stdin.

The code is here:

#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
int main()
{
    fd_set rfds;
    struct timeval tv;
    int retval;
    char buf[100];

    FD_ZERO(&rfds);
    FD_SET(STDIN_FILENO, &rfds);

    tv.tv_sec = 5;
    tv.tv_usec = 0;

    retval = select(1,&rfds,0,0,&tv);

    if( retval == -1)
        perror("select reset\n");
    else if(retval == 0)
        printf("timeout\n");
    else
    {
        printf("data available\n");
        if(FD_ISSET(STDIN_FILENO, &rfds))
        {
           //int ret = recv(STDIN_FILENO, buf, sizeof(buf), 0); // ret get -1.
           int ret = read(STDIN_FILENO, buf, sizeof(buf));       // ret get correct data. 
           printf("buf: %s ret: %d\n", buf,ret);            
        }
    }    
    return 0;
}

In this code, recv() will always return -1, but read() can get correct data.

I find that read() is equivalent to recv() with a flags parameter of 0. Why then are the behaviors of recv() and read() not the same in my code?

Answer 1

Because recv() is for use on sockets, not generic file descriptors like stdin. Sockets can be treated like any descriptor, but descriptors don’t necessarily have a socket behind them (and in the case of stdin, they don’t).

In that particular piece of code, I suspect that if you check errno it will have a value of EINVAL or similar.