I am trying to play a .mp3 file in python using winmm.dll (ctypes.windll.winmm). But when I try to get a length of some file in milliseconds, instead of actual length (05:23 = about 323000 ms) I get just 3. Time format got by status command is "m", and it doesn't change after set command. Here is some code that illustrates the problem:
from ctypes import windll, c_buffer
fp = 'song.mp3'
alias = 'test'
buf = c_buffer(255)
r = windll.winmm.mciSendStringW(f'open "{fp}" alias {alias}', buf, 254, 0)
print(r)
buf = c_buffer(255)
r = windll.winmm.mciSendStringW(f'status {alias} time format', buf, 254, 0)
print(r, buf.value)
buf = c_buffer(255)
r = windll.winmm.mciSendStringW(f'set {alias} time format milliseconds', buf, 254, 0)
print(r)
buf = c_buffer(255)
r = windll.winmm.mciSendStringW(f'status {alias} time format', buf, 254, 0)
print(r, buf.value)
buf = c_buffer(255)
r = windll.winmm.mciSendStringW(f'status {alias} length', buf, 254, 0)
print(r, buf.value)
And its output:
0
0 b'm'
0
0 b'm'
0 b'3'
Thanks in advance for your help!
As I specified in my comment this is a classic Undefined Behavior example. Check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for more details.
Also, you're mixing 8bit and 16bit strings.
Listing [Python 3.Docs]: ctypes - A foreign function library for Python.
code00.py:
#!/usr/bin/env python3
import sys
import ctypes as ct
from ctypes import wintypes as wt
def main(*argv):
winmm_dll = ct.WinDLL("Winmm.dll")
mciSendString = winmm_dll.mciSendStringW
mciSendString.argtypes = (wt.LPCWSTR, wt.LPWSTR, wt.UINT, wt.HANDLE)
mciSendString.restype = wt.DWORD
file_name = "./SampleAudio_0.4mb.mp3" # Downloaded from https://www.sample-videos.com/audio/mp3/crowd-cheering.mp3
alias_name = "test00"
status_commands = [
f"status {alias_name} time format",
f"status {alias_name} length",
]
commands = [f"open \"{file_name}\" alias {alias_name}"]
commands.extend(status_commands)
commands.append(f"set {alias_name} time format ms")
commands.extend(status_commands)
commands.append(f"close {alias_name}")
out_buf_len = 0xFF
out_buf = ct.create_unicode_buffer(out_buf_len)
for command in commands:
print(f"Sending {command}...")
res = mciSendString(command, out_buf, out_buf_len, None)
print(f" Result: {res}\n Output: {out_buf.value}")
if __name__ == "__main__":
print("Python {0:s} {1:d}bit on {2:s}\n".format(" ".join(item.strip() for item in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform))
main(*sys.argv[1:])
print("\nDone.")
Output:
[cfati@CFATI-5510-0:e:\Work\Dev\StackOverflow\q059343461]> "e:\Work\Dev\VEnvs\py_064_03.07.03_test0\Scripts\python.exe" code00.py Python 3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)] 64bit on win32 Sending open "./SampleAudio_0.4mb.mp3" alias test00... Result: 0 Output: 1 Sending status test00 time format... Result: 0 Output: milliseconds Sending status test00 length... Result: 0 Output: 27746 Sending set test00 time format ms... Result: 0 Output: Sending status test00 time format... Result: 0 Output: milliseconds Sending status test00 length... Result: 0 Output: 27746 Sending close test00... Result: 0 Output: Done.