RFC: silence benign warning with pragma

[danielgustafsson]

Disclaimer: I don't have a system to reproduce the warning, and test the fix, on so this is perhaps more of a discussion piece than a patch. Anyways, here goes.

Commit 5bfaa86 introduced a new compiler warning on Windows cross compilation with GCC. The coding is however a documented pattern for acquiring a handle to write to, so rather than changing working code this wraps the offending cast in a GCC diagnostic pragma to silence the warning. See below for an example of the warning from the autobuild logs:

/src/tool_cb_wrt.c:175:9: warning: cast from function call of type 'intptr_t {aka long long int}' to non-matching type 'void *' [-Wbad-function-cast]
(HANDLE) _get_osfhandle(fileno(outs->stream)),
^

An example failing build can be found here: https://curl.haxx.se/dev/log.cgi?id=20181111190642-17820#prob1

[MarcelRaad]

I think you just have to store the result of the function call in an intermediate variable to suppress the warning. That would maybe be less verbose? At least that's what I did in other places.

[vszakats]

According to this, this workaround will silence it without using pragmas:

--- a/src/tool_cb_wrt.c
+++ b/src/tool_cb_wrt.c
@@ -161,6 +161,7 @@ size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
     DWORD in_len = (DWORD)(sz * nmemb);
     wchar_t* wc_buf;
     DWORD wc_len;
+    intptr_t fhnd = _get_osfhandle(fileno(outs->stream));
 
     /* calculate buffer size for wide characters */
     wc_len = MultiByteToWideChar(CP_UTF8, 0, buffer, in_len,  NULL, 0);
@@ -172,7 +173,7 @@ size_t tool_write_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
     wc_len = MultiByteToWideChar(CP_UTF8, 0, buffer, in_len, wc_buf, wc_len);
 
     if(!WriteConsoleW(
-        (HANDLE) _get_osfhandle(fileno(outs->stream)),
+        (HANDLE) fhnd,
         wc_buf,
         wc_len,
         &wc_len,

I can confirm the above silences the warning, using this command:

$ x86_64-w64-mingw32-gcc -c tool_cb_wrt.c -I../include -I../lib -Wbad-function-cast

(One issue with #pragma GCC diagnostic is that it requires at least gcc 4.6.)

[danielgustafsson]

Perfect, that's a much better solution! I've pushed an updated commit which will be squashed if accepted. In passing I also noticed that the possibility of an error from MultiByteToWideChar() wasn't considered so added a check for that as well while in the codepath.

[MarcelRaad]

Thanks!