tee.py

import os
import sys
import ctypes

ld = ctypes.CDLL(None, use_errno=True)

SPLICE_F_NONBLOCK = getattr(os, "SPLICE_F_NONBLOCK", 0x02)


def tee(fd_in, fd_out, length, flags=SPLICE_F_NONBLOCK):
    result = ld.tee(
        ctypes.c_int(fd_in),
        ctypes.c_int(fd_out),
        ctypes.c_size_t(length),
        ctypes.c_uint(flags),
    )

    if result == -1:
        errno = ctypes.get_errno()
        raise OSError(errno, os.strerror(errno))

    return result


def main():
    stdin = sys.stdin.fileno()
    stdout = sys.stdout.fileno()

    with open("tee.out", "wb") as f:
        while True:
            length = tee(stdin, stdout, 1 << 31)
            if not length:
                break
            # In Python 3.10, could use os.splice. See tee(2).
            buf = sys.stdin.buffer.read(length)
            f.write(buf)


if __name__ == "__main__":
    main()