Max Bolingbroke Max Bolingbroke - 4 months ago 18
Linux Question

What is the encoding of argv?

It's not clear to me what encodings are used where in C's

argv
. In particular, I'm interested in the following scenario:


  • A user uses locale L1 to create a file whose name,
    N
    , contains non-ASCII characters

  • Later on, a user uses locale L2 to tab-complete the name of that file on the command line, which is fed into a program P as a command line argument



What sequence of bytes does P see on the command line?

I have observed that on Linux, creating a filename in the UTF-8 locale and then tab-completing it in (e.g.) the
zw_TW.big5
locale seems to cause my program P to be fed UTF-8 rather than
Big5
. However, on OS X the same series of actions results in my program P getting a
Big5
encoded filename.

Here is what I think is going on so far (long, and I'm probably wrong and need to be corrected):

Windows



File names are stored on disk in some Unicode format. So Windows takes the name
N
, converts from L1 (the current code page) to a Unicode version of
N
we will call
N1
, and stores
N1
on disk.

What I then assume happens is that when tab-completing later on, the name
N1
is converted to locale L2 (the new current code page) for display. With luck, this will yield the original name
N
-- but this won't be true if
N
contained characters unrepresentable in L2. We call the new name
N2
.

When the user actually presses enter to run P with that argument, the name
N2
is converted back into Unicode, yielding
N1
again. This
N1
is now available to the program in UCS2 format via
GetCommandLineW
/
wmain
/
tmain
, but users of
GetCommandLine
/
main
will see the name
N2
in the current locale (code page).

OS X



The disk-storage story is the same, as far as I know. OS X stores file names as Unicode.

With a Unicode terminal, I think what happens is that the terminal builds the command line in a Unicode buffer. So when you tab complete, it copies the file name as a Unicode file name to that buffer.

When you run the command, that Unicode buffer is converted to the current locale, L2, and fed to the program via
argv
, and the program can decode argv with the current locale into Unicode for display.

Linux



On Linux, everything is different and I'm extra-confused about what is going on. Linux stores file names as byte strings, not in Unicode. So if you create a file with name
N
in locale L1 that
N
as a byte string is what is stored on disk.

When I later run the terminal and try and tab-complete the name, I'm not sure what happens. It looks to me like the command line is constructed as a byte buffer, and the name of the file as a byte string is just concatenated onto that buffer. I assume that when you type a standard character it is encoded on the fly to bytes that are appended to that buffer.

When you run a program, I think that buffer is sent directly to
argv
. Now, what encoding does
argv
have? It looks like any characters you typed in the command line while in locale L2 will be in the L2 encoding, but the file name will be in the L1 encoding. So
argv
contains a mixture of two encodings!

Question



I'd really like it if someone could let me know what is going on here. All I have at the moment is half-guesses and speculation, and it doesn't really fit together. What I'd really like to be true is for
argv
to be encoded in the current code page (Windows) or the current locale (Linux / OS X) but that doesn't seem to be the case...

Extras



Here is a simple candidate program P that lets you observe encodings for yourself:

#include <stdio.h>

int main(int argc, char **argv)
{
if (argc < 2) {
printf("Not enough arguments\n");
return 1;
}

int len = 0;
for (char *c = argv[1]; *c; c++, len++) {
printf("%d ", (int)(*c));
}

printf("\nLength: %d\n", len);

return 0;
}


You can use
locale -a
to see available locales, and use
export LC_ALL=my_encoding
to change your locale.

Answer

Thanks everyone for your responses. I have learnt quite a lot about this issue and have discovered the following things that has resolved my question:

  1. As discussed, on Windows the argv is encoded using the current code page. However, you can retrieve the command line as UTF-16 using GetCommandLineW. Use of argv is not recommended for modern Windows apps with unicode support because code pages are deprecated.

  2. On Unixes, the argv has no fixed encoding:

    a) File names inserted by tab-completion/globbing will occur in argv verbatim as exactly the byte sequences by which they are named on disk. This is true even if those byte sequences make no sense in the current locale.

    b) Input entered directly by the user using their IME will occur in argv in the locale encoding. (Ubuntu seems to use LOCALE to decide how to encode IME input, whereas OS X uses the Terminal.app encoding Preference.)

This is annoying for languages such as Python, Haskell or Java, which want to treat command line arguments as strings. They need to decide how to decode argv into whatever encoding is used internally for a String (which is UTF-16 for those languages). However, if they just use the locale encoding to do this decoding, then valid filenames in the input may fail to decode, causing an exception.

The solution to this problem adopted by Python 3 is a surrogate-byte encoding scheme (http://www.python.org/dev/peps/pep-0383/) which represents any undecodable byte in argv as special Unicode code points. When that code point is decoded back to a byte stream, it just becomes the original byte again. This allows for roundtripping data from argv that is not valid in the current encoding (i.e. a filename named in something other than the current locale) through the native Python string type and back to bytes with no loss of information.

As you can see, the situation is pretty messy :-)