error: cast between incompatible function types from 'FARPROC' on MinGW

[kenhys@…]

Problem:

​https://groonga.org bundles nginx to provide http server, it builds bundled nginx 1.17.3 with MinGW.

​https://github.com/groonga/groonga/commit/091fba777241c793b3b3eff582fb64fdb9a74a57/checks

During executing recent CI, it reports error.

Actual:

2019-10-07T01:05:15.7349316Z cc -c -pipe  -O -W -Wall -Wpointer-arith -Wno-unused-parameter -Werror -g -I/d/a/groonga/groonga/include -I/d/a/groonga/groonga/include   -DNGX_HTTP_GROONGA_LOG_PATH=\""/mingw64/var/log/groonga/httpd/groonga.log"\" -DNGX_HTTP_GROONGA_QUERY_LOG_PATH=\""/mingw64/var/log/groonga/httpd/groonga-query.log"\" -I src/core -I src/event -I src/event/modules -I src/os/win32 -I objs \
2019-10-07T01:05:15.7361486Z    -o objs/src/os/win32/ngx_win32_init.o \
2019-10-07T01:05:15.7376066Z    src/os/win32/ngx_win32_init.c
error: cast between incompatible function types from 'FARPROC'

This error is similar to #1546, but occurred place is different.

2019-10-07T01:05:16.3986206Z src/os/win32/ngx_win32_init.c: In function 'ngx_os_init':
2019-10-07T01:05:16.4540987Z src/os/win32/ngx_win32_init.c:243:15: error: cast between incompatible function types from 'FARPROC' {aka 'long long int (*)()'} to 'int (*)(struct pollfd *, ULONG,  INT)' {aka 'int (*)(struct pollfd *, long unsigned int,  int)'} [-Werror=cast-function-type]
2019-10-07T01:05:16.4695725Z      WSAPoll = (ngx_wsapoll_pt) GetProcAddress(hmod, "WSAPoll");
2019-10-07T01:05:16.4715602Z                ^
2019-10-07T01:05:16.4717443Z cc1.exe: all warnings being treated as errors

Expected:

build succeeds without error.

Addtional Information:

2019-10-07T00:48:02.4678733Z  + MINGW64_NT-10.0 2.11.1(0.329/5/3) x86_64
2019-10-07T00:48:02.5551913Z  + using GNU C compiler
2019-10-07T00:48:02.6050959Z  + gcc version: 8.2.0 (Rev1, Built by MSYS2 project) 
2019-10-07T00:48:02.7918999Z checking for gcc -pipe switch ... found
2019-10-07T00:48:02.8123895Z checking for MINGW64_NT-10.0 specific features
2019-10-07T00:48:03.1517890Z configuring additional modules

[kenhys@…]

CI build log (compressed)

[Maxim Dounin]

Thanks for the report. It looks like gcc fails to handle normal GetProcAddress() usage on Windows, and issues a clearly wrong warning on casts between function types it thinks are not compatible. The following patch should fix this:

# HG changeset patch
# User Maxim Dounin <mdounin@mdounin.ru>
# Date 1571170186 -10800
#      Tue Oct 15 23:09:46 2019 +0300
# Node ID 7a2e380f0eb74c84d9e8f58719d15cb348b4e1dc
# Parent  be5858ead6d61339441fb1ef8eb684cef0458b74
Win32: silenced -Wcast-function-type GCC warning (ticket #1865).

With MinGW-w64, building 64-bit nginx binary with GCC 8 and above
results in warning due to cast of GetProcAddress() result to ngx_wsapoll_pt,
which GCC thinks is incorrect.  Added intermediate cast to "void *" to
silence the warning.

diff -r be5858ead6d6 -r 7a2e380f0eb7 src/os/win32/ngx_win32_init.c
--- a/src/os/win32/ngx_win32_init.c	Fri Oct 11 16:15:32 2019 +0300
+++ b/src/os/win32/ngx_win32_init.c	Tue Oct 15 23:09:46 2019 +0300
@@ -240,7 +240,7 @@ ngx_os_init(ngx_log_t *log)
         goto nopoll;
     }
 
-    WSAPoll = (ngx_wsapoll_pt) GetProcAddress(hmod, "WSAPoll");
+    WSAPoll = (ngx_wsapoll_pt) (void *) GetProcAddress(hmod, "WSAPoll");
     if (WSAPoll == NULL) {
         ngx_log_error(NGX_LOG_NOTICE, log, ngx_errno,
                       "GetProcAddress(\"WSAPoll\") failed");

[Maxim Dounin <mdounin@…>]

In 7586:4f6e315eef15/nginx:

Win32: silenced -Wcast-function-type GCC warning (ticket #1865).

With MinGW-w64, building 64-bit nginx binary with GCC 8 and above
results in warning due to cast of GetProcAddress() result to ngx_wsapoll_pt,
which GCC thinks is incorrect. Added intermediate cast to "void *" to
silence the warning.

[Maxim Dounin]

Fix committed.