select() and pselect() allow a program to monitor multiple file descriptors,
waiting until one or more of the file descriptors become "ready" for some
class of I/O operation

#include <sys/types.h>
#include <sys/select.h>

#include <unistd.h>

#include <stdbool.h>

static inline int max(int lhs, int rhs) {
    if(lhs > rhs)
        return lhs;
    else
        return rhs;
}

void copy(int from, int to) {
    char buffer[10];
    int readp = 0;
    int writep = 0;
    bool eof = false;
    for(;;) {
        fd_set readfds, writefds;
        FD_ZERO(&readfds);
        FD_ZERO(&writefds);

        int ravail, wavail;
        if(readp < writep) {
            ravail = writep - readp - 1;
            wavail = sizeof buffer - writep;
        }
        else {
            ravail = sizeof buffer - readp;
            wavail = readp - writep;
        }

        if(!eof && ravail)
            FD_SET(from, &readfds);
        if(wavail)
            FD_SET(to, &writefds);
        else if(eof)
            break;
        int rc = select(max(from,to)+1, &readfds, &writefds, NULL, NULL);
        if(rc == -1)
            break;
        if(FD_ISSET(from, &readfds))
        {
            ssize_t nread = read(from, &buffer[readp], ravail);
            if(nread < 1)
                eof = true;
            readp = readp + nread;
        }
        if(FD_ISSET(to, &writefds))
        {
            ssize_t nwritten = write(to, &buffer[writep], wavail);
            if(nwritten < 1)
                break;
            writep = writep + nwritten;
        }
        if(readp == sizeof buffer && writep != 0)
            readp = 0;
        if(writep == sizeof buffer)
            writep = 0;
    }
}