I did this

I was building 8.16.0 release on AIX in 32 bit mode and the build failed (64 bit build is good):

  CC       libcurl_la-altsvc.lo
altsvc.c:230:8: error: call to undeclared library function 'fopen' with type 'FILE *(const char *, const char *)'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
  fp = fopen(file, FOPEN_READTEXT);
       ^
altsvc.c:230:8: note: include the header <stdio.h> or explicitly provide a declaration for 'fopen'

The problem is very similar to the issue raised yesterday: #18500

So it looks like for 32 bit builds, the _LARGE_FILES is defined in system headers. And then checking /usr/include/stdio.h we find this:

#ifdef _LARGE_FILES
#define fseeko fseeko64
#define ftello ftello64
#define fgetpos fgetpos64
#define fsetpos fsetpos64
#define fopen fopen64
#define freopen freopen64
#endif /* _LARGE_FILES */

So when doing 32 bit build, fopen becomes a #define.

And now we get back to same problem as yesterday. stdio.h is included from curl_setup.h. At the end of curl_setup.h we include curl_mem_undef.h. And here we do #undef fopen.

I expected the following

I expect a clean build.

curl/libcurl version

curl 8.16.0 tag

operating system

AIX 7.2.5
Only when building in 32 bit mode