MSVC casts pointers from any type to any type implicitly in .c files, not giving errors or warnings. Why?

Question

See this example:

FILE* f = NULL; // MSVC warns if I do not initialize them.
char* s = NULL;
s = f; // these two lines compiles in .c file but not in .cpp file
f = s; // this is really dangerous!

I found this bug (or, maybe it is a feature in MSVC?), because several hours ago, I wrote something like this:

fread(f, 1, size, str); // crash! exchange f and str!

which compiles without any warning, let alone errors, and crashes my program until I change the file extension to.cpp.

Why doesn't MSVC perform such a basic check in.c files?

Answer 1

MSVS does warn you ( https://godbolt.org/z/e5oq7fexM ):

<source>(16): warning C4133: '=': incompatible types - from 'FILE *' to 'char *'
<source>(17): warning C4133: '=': incompatible types - from 'char *' to 'FILE *'

Why doesn't MSVC perform such a basic checks.c files?

It does, you probably have disabled all the warnings.

these two lines compiles in.c file but not in.cpp file

Because C++ & C are different languages. C allows implicit pointer conversions, C++ does not. It is why you need to cast the result of malloc in C++ , but you do not in C

You can use -WX option to treat warnings as errors and your program will not compile anymore https://godbolt.org/z/GvssPbxEv

<source>(10): error C2220: the following warning is treated as an error
<source>(10): warning C4133: '=': incompatible types - from 'FILE *' to 'char *'
<source>(11): warning C4133: '=': incompatible types - from 'char *' to 'FILE *'

You can learn more about MSVC options here: https://learn.microsoft.com/en-us/cpp/build/reference/compiler-option-warning-level?view=msvc-170