Question

I’ve read all the information I found regarding this matter and unfortunately none of it helps. I’ve written this small function as a non-blocking version of fgetc:

char nonblocking_fgetc(FILE *stream){
    static struct pollfd pfd;
    pfd.fd = fileno(stream);
    pfd.events=POLLIN;
    poll(&pfd, 1, 1);
    return ((pfd.revents&POLLIN)?fgetc(stream):-1);
}

This function should return a character if it is present in stream, or a -1 otherwise. It works as expected for the first call. However, any subsequent call returns -1.

The function above is cleaned up version. My version has many tests which check for polling errors. Nether POLLERR is set or a negative value is returned.

example:
main contains this small while loop:

while(1){
    c=nonblocking_fgetc(stdin);
    if (c!=-1) {fputc(c, stdout);fflush(stdout);}
}    

running echo "Test" | ./a.out would yield only T and then loops without any further output.

I would appreciate it if anyone can shed some light on this.

Answer 1

The <stdio.h> file streams are buffered. See setbuf for more. You could use directly the read syscall.