[Windows] fprintf miss matching with GCC

[K1ndWha1e]

Describe the bug

After running any program with gcc, I received this error.

Reproduction Steps

fn main() {
        print("HERE")
}

v -cc gcc run .\main.v

Expected Behavior

Prints "HERE"

Current Behavior

================== C compilation error (from gcc): ==============
cc: C:\\Users\\nikit\\AppData\\Local\\Temp\\v_0\\TE2216~1.C: In function 'builtin___memory_panic':
cc: C:\\Users\\nikit\\AppData\\Local\\Temp\\v_0\\TE2216~1.C:1952:17: error: implicit declaration of function 'fprintf' [-Werror=implicit-function-declaration]
cc:  1952 |                 fprintf(stderr, "%p", ((voidptr)(size)));
cc:       |                 ^~~~~~~
cc: C:\\Users\\nikit\\AppData\\Local\\Temp\\v_0\\TE2216~1.C:1257:1: note: include '<stdio.h>' or provide a declaration of 'fprintf'
cc:  1256 | #include <float.h>
cc:   +++ |+#include <stdio.h>
cc:  1257 | #else
cc: C:\\Users\\nikit\\AppData\\Local\\Temp\\v_0\\TE2216~1.C:1952:17: warning: incompatible implicit declaration of built-in function 'fprintf' [-Wbuiltin-declaration-mismatch]
cc:  1952 |                 fprintf(stderr, "%p", ((voidptr)(size)));
cc:       |                 ^~~~~~~
cc: C:\\Users\\nikit\\AppData\\Local\\Temp\\v_0\\TE2216~1.C:1952:17: note: include '<stdio.h>' or provide a declaration of 'fprintf'
...
cc: cc1.exe: some warnings being treated as errors
(note: the original output was 36 lines long; it was truncated to its first 12 lines + the last line)
=================================================================
Try passing `-g` when compiling, to see a .v file:line information, that correlates more with the C error.
(Alternatively, pass `-show-c-output`, to print the full C error message).
builder error: 
==================
C function `C.fprintf` was declared in V, but the C compiler did not see a matching C declaration for `fprintf`.
A declaration like `fn C.fprintf()` only tells V about an external C symbol; it does not define that symbol or include its header.
Include the C header that declares `fprintf` and add any needed `#flag -I`, `#flag -l`, or C source file.
If there is no header but the symbol is provided by a linked library/object, mark the V declaration with `@[c_extern]`.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.5.1 a3c8c55

Environment details (OS name and version, etc.)

V full version	V 0.5.1 9b26b5d.a3c8c55
OS	windows, Њ ©Єа®б®дв Windows 10 Pro 19045 64-а §ап¤ п
Processor	12 cpus, 64bit, little endian, Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
Memory	45.98GB/63.92GB
V executable	C:\Users\nikit\Downloads\v\v.exe
V last modified time	2026-06-20 16:54:08
V home dir	OK, value: C:\Users\nikit\Downloads\v
VMODULES	OK, value: C:\Users\nikit.vmodules
VTMP	OK, value: C:\Users\nikit\AppData\Local\Temp\v_0
Current working dir	OK, value: C:\Users\nikit\CExploringLab\PROJECTS\tiny-gpu-watcher
Git version	git version 2.47.1.windows.2
V git status	a3c8c55
.git/config present	true
cc version	cc (Rev6, Built by MSYS2 project) 13.1.0
gcc version	gcc (Rev6, Built by MSYS2 project) 13.1.0
clang version	clang version 18.1.1
msvc version	ЋЇвЁ¬Ё§ЁагойЁ© Є®¬ЇЁ«пв®а Microsoft (R) C/C++ ўҐабЁЁ 19.43.34808 ¤«п x64
tcc version	tcc version 0.9.27 (x86_64 Windows)
tcc git status	thirdparty-windows-amd64 b425ac82-dirty
emcc version	N/A
glibc version	N/A

Note

You can use the 👍 reaction to increase the issue's priority for developers.

Please note that only the 👍 reaction to the issue itself counts as a vote.
Other reactions and those to comments will not be taken into account.

[GGRei]

The same failure mode can be reproduced on hosted Windows GCC with the -no-relaxed-gcc14 flag. ( GCC MinGW 15.2.0 )

The issue points to the generated C headers for the Windows MinGW/GCC path. In vlib/v/gen/c/cheaders.v, that path provides FILE and stderr manually, but it does not expose the system stdio declarations. Later, the manual formatted-stdio prototype block also skips GCC+MinGW, to avoid mingw-w64 static/extern conflicts.

That leaves generated builtin code able to call fprintf(stderr, ...) without a visible C declaration.

So the fix should stay in the MinGW/GCC header handling. Make the system fprintf / vfprintf declarations visible for GCC+MinGW, while still avoiding manual formatted-stdio prototypes there.

[medvednikov]

Please confirm it's been fixed @K1ndWha1e

[K1ndWha1e]

Please confirm it's been fixed @K1ndWha1e

All works fine! Thank you!