msvcrt Module¶

The msvcrt module exposes a handful of routines from the Microsoft Visual C++ runtime: unbuffered console I/O, file locking, and conversion between C file descriptors and Win32 handles.

It is Windows-only. On other platforms importing it raises ModuleNotFoundError.

Complexity Reference¶

Console I/O¶

Operation	Time	Space	Notes
`getch()`	O(1) + blocks	O(1)	Reads one keypress, no echo; blocks until a key is pressed
`getwch()`	O(1) + blocks	O(1)	Wide-character variant
`getche()`	O(1) + blocks	O(1)	Like `getch()` but echoes
`getwche()`	O(1) + blocks	O(1)	Wide-character variant
`kbhit()`	O(1)	O(1)	Non-blocking check for a pending keypress
`putch(c)`	O(1) + syscall	O(1)	Write one byte to the console
`putwch(c)`	O(1) + syscall	O(1)	Wide-character variant
`ungetch(c)`	O(1)	O(1)	Push one character back; one pending char only
`ungetwch(c)`	O(1)	O(1)	Wide-character variant

Files and Descriptors¶

Operation	Time	Space	Notes
`locking(fd, mode, nbytes)`	O(1) + syscall	O(1)	Lock a byte range; may block or retry
`setmode(fd, flags)`	O(1) + syscall	O(1)	Switch between text and binary mode
`open_osfhandle(handle, flags)`	O(1) + syscall	O(1)	Win32 handle to C descriptor
`get_osfhandle(fd)`	O(1) + syscall	O(1)	C descriptor to Win32 handle
`heapmin()`	O(n)	O(1)	Releases unused heap blocks to the OS

getch() and friends are O(1) in work but block indefinitely, so treat them as I/O waits rather than computation.

Reading Keypresses Without Enter¶

import msvcrt

# Poll without blocking - O(1) per check
while True:
    if msvcrt.kbhit():
        key = msvcrt.getch()      # O(1), returns immediately after kbhit()
        if key == b"q":
            break
        print(key)

Locking a File Region¶

import msvcrt
import os

fd = os.open("data.bin", os.O_RDWR)
try:
    # Lock 1024 bytes from the current position - O(1) + syscall
    msvcrt.locking(fd, msvcrt.LK_LOCK, 1024)
    try:
        os.write(fd, b"exclusive")
    finally:
        os.lseek(fd, 0, os.SEEK_SET)
        msvcrt.locking(fd, msvcrt.LK_UNLCK, 1024)
finally:
    os.close(fd)

Windows only

For portable console input use input(), or a cross-platform library. For portable file locking, see fcntl.flock() on Unix and select the implementation at runtime.

kbhit() before getch()

getch() blocks. Guarding it with kbhit() turns an indefinite wait into an O(1) poll, which is what you want inside an event loop.

Version Notes¶

All Python 3 versions: available on Windows builds only
All versions: per-call complexity is unchanged; costs are dominated by the underlying C runtime and console