sendfile doesn't work for empty stdin

That seems to be the intended behavior of the sendfile call. Let me cite Linus Torvalds:

sendfile() on purpose only works on things that use the page cache. EINVAL is basically sendfile's way of saying "I would fall back on doing a read+write, so you might as well do it yourself in user space because it might actually be more efficient that way".
(https://yarchive.net/comp/linux/sendfile.html)

So, sendfile will work with stdin only if the kernel knows that stdin is actually a file, not pipe, block device, or anything else.

I wrote a simple program to check it myself:

#include <stdio.h> #include <sys/sendfile.h> int main() { sendfile(fileno(stdout), fileno(stdin), NULL, 8); perror("sendfile failed"); } 

Running it with various redirections clearly shows that the sendfile call is successful only if the input is directly bound to a regular file, empty or not:

$ ./sendfile-test sendfile failed: Invalid argument $ ./sendfile-test < empty.txt sendfile failed: Success $ ./sendfile-test < non-empty.txt (...) sendfile failed: Success $ ./sendfile-test < /dev/null sendfile failed: Invalid argument $ cat empty.txt | ./sendfile-test sendfile failed: Invalid argument $ cat non-empty.txt | ./sendfile-test sendfile failed: Invalid argument $ cat /dev/null | ./sendfile-test sendfile failed: Invalid argument