At least one some versions of libc, whether you include <stdlib.> before or after <ctype.h> changes the way the toupper and tolower functions are defined. See for example here: https://godbolt.org/z/4a7htx If you include the <ctype.h> header first, the bodies of extern inline functions are exposed in the header, and are inlined (note the loop contains no toupper call). If you swap the order, and include <stdlib.h> first, then __NO_CTYPE gets defined somewhere in the header chain, and when <ctype.h> is included the inline bodies are not available, and so inlining doesn't happen. See the block guarded by __NO_CTYPE in ctype.h at: https://sourceware.org/git/?p=glibc.git;a=blob;f=ctype/ctype.h;h=d17f727cf0dc2a0f6c62fa50aff799b175dcb426;hb=2a764c6ee848dfe92cb2921ed3b14085f15d9e79#l205 At least this causes a large performance variation (I measure the non-inlined version as 3 to 4 times slower), but if you pick through that conditionally defined block I bet you can find functional differences too. I would expect __NO_CTYPE to defined consistently regardless of header include order.
Are you compiling as C++? Some libstdc++ headers define __NO_CTYPE (because C++ prohibits headers from defining standard functions as macros), but I don't see anything in the glibc headers defining that macro.
Yes, I'm compiling as C++, and I should have made that clear from the start. In C, the flow is totally different and toupper/tolower are macros, not functions (as you pointed out). Although I'm compiling as C++, the only headers I'm including are <stdlib.h> and <ctype.h>. I'm not sure exactly where __NO_CTYPE is getting defined in this case, but in another case I looked at (this time including C++ headers like <algorithm>, the __NO_CTYPE definition came from <bits/os_defines.h>).
Here's an example that shows that <stdlib.h> defines __NO_CTYPE when compiled in C++: https://godbolt.org/z/y6Quxs In C it is not defined.