[Windows] tmpnam() in runPreprocessor generates root-drive paths, fails with permission denied for non-admin users

[divyansharma001]

Environment

• OS: Windows 11, non-admin user
• Build: dmd debug build, phobos make BUILD=debug

Steps to reproduce

1. Clone dmd and phobos
2. Build dmd: rdmd build.d BUILD=debug (from compiler/src/)
3. Build phobos: make -j4 BUILD=debug (from phobos/)

Error

src\core\stdc\errno.c: fatal error C1083: Cannot open compiler generated file: '\sla8.': Permission denied
Error: C preprocess command ...cl.exe failed for file src\core\stdc\errno.c, exit status 2

Root cause

runPreprocessor() in compiler/src/dmd/link.d:965 calls tmpnam(null) to generate
a temp file name for cl.exe's /Fi output flag. On Windows, MSVC CRT's tmpnam()
always returns paths in the format \sXXXXXX. (at the drive root — e.g. C:\sla8.),
regardless of the TMP/TEMP environment variables. Non-admin users cannot write
to the drive root, so cl.exe fails.

Fix

Replace tmpnam(null) with GetTempPathA + GetTempFileNameA, which correctly
use the user's temp directory:

import core.sys.windows.winbase : GetTempFileNameA, GetTempPathA;
char[260] tempDir = void;
char[260] tempFile = void;
GetTempPathA(260, tempDir.ptr);
GetTempFileNameA(tempDir.ptr, "dmd", 0, tempFile.ptr);
const(char)[] ifilename = tempFile[0 .. strlen(tempFile.ptr) + 1];

[Naveen447-a11y]

Hello divyansharma001

While reviewing the Windows linking flow in compiler/src/dmd/link.d, I noticed that runPreprocessor() (around line 965) currently uses tmpnam(null) to generate a temporary file name for cl.exe’s /Fi output flag.

On Windows, MSVC CRT’s tmpnam() returns file paths in the format \sXXXXXX at the drive root (e.g., C:\sabc.), regardless of the user's TMP or TEMP environment variables. Since non-admin users typically do not have write access to the drive root, this causes cl.exe to fail when attempting to write the preprocessed output.

To address this, I am considering replacing the use of tmpnam(null) with GetTempPathA and GetTempFileNameA, which correctly resolve the user's temporary directory. A possible approach would be:

import core.sys.windows.winbase : GetTempFileNameA, GetTempPathA;
char[260] tempDir = void;
char[260] tempFile = void;
GetTempPathA(260, tempDir.ptr);
GetTempFileNameA(tempDir.ptr, "dmd", 0, tempFile.ptr);
const(char)[] ifilename = tempFile[0 .. strlen(tempFile.ptr) + 1];

I would like to investigate this further and submit a patch if appropriate. Would it be alright if I take up this issue? Please let me know if there are any existing discussions or implementation considerations I should review beforehand.

Thank you!