(1) C and C++ (2) gets() and fgets()

Sam Varshavchik mrsam@courier-mta.com
Mon Dec 26 21:42:00 GMT 2011


Matthew D. Gutchess writes:

>     (1) C versus C++.  The ".h" is not used in C++ include files, correct?

Not really. There's nothing wrong with including a .h from a C++ include  
file. The only thing to watch out for is that the C include should be "C++  
aware", that is, if processed as C++ source, it conditionally wraps all  
declaration into extern "C" linkage. This is most certainly the case with  
your system headers.
 
> I assume that C and C++ object files can be linked into an executable, but  
> are C and C++ incompatible with each other?

C++ translation units can certainly be linked with, and C++ code, can call C  
code, if functions C functions are declared with "C" linkage:

extern int foo();

Compiled as C, this declares a C function foo(). Compiled as C++, the code  
would expect to call a C++ function foo(), which is not the same. For C++,  
if it's declared as

extern "C" int foo();

Then the C++ translation unit will use C linkage, and be able to call the C  
function foo(). Conversely, a C++ function can also be, carefully, declared  
with C linkage, allowing it to be called from. But just because it can, is  
obviously not the whole story. C, for example, doesn't know anything about  
C++ exceptions, and may not have any idea what to do with a thrown  
exception, if some C function is in the call stack.

If you carefully unwrap your system header files, stdio.h would be a good  
example, and trace all the preprocessor stuff in there, somewhere you'll  
find in there something along the lines

#if defined(__cplusplus)
extern "C" {
#endif

…

#if defined(__cplusplus)
};
#endif

This may not exactly match what you see on your platform, but you'll have  
something very similar. All the meat in stdio.h gets shoved in the middle.  
When processed as C, all the declarations are visible directly. When  
processed as C++, the predefined "__cplusplus" preprocessor symbol makes  
everything declared inside extern "C" linkage, making all C library  
functions from stdio.h callable from C++ translation units. The same goes  
for all other system header files.

>                                             Should one use C or C++ or are  
> the two dialects compatible?  What can one accomplish in C++ that cannot be  
> accomplished in C?

Well, exceptions for once. Templates, for another. There's no real  
equivalent to templates, in C. Then this whole object-oriented thing…

>     (2) Man pages state that gets() is "dangerous" because it can be used to  
> purposely cause buffer overflows and recommend using fgets().  gets()  
> accepts input from stdin, but fgets() accepts input from stream.

stdin is an ordinary stream.

> For example, how would the program below be modified to use fgets() instead  
> of gets()?:

> 	printf("Filename: ");
>
> 	gets(pathname);

fgets(pathname, sizeof(pathname), stdin);

Plus some error checking would be nice here, too.

But, this is really C, not C++. With C++, you would use std::cin, and  
std::getline().


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