What does the throw keyword mean in C

January 6, 2023

I tried googling it but all results were about C++’s throw std::exception().

I was reading through libraries (was curious how stuff like printf, malloc and FILE were implemented) and came across the definition for the malloc function:

extern void *malloc (size_t __size) __THROW __attribute_malloc__
     __attribute_alloc_size__ ((1)) __wur;

When using the IDE (Visual Studio Code) to trace back to definitions for each thing, __THROW led to this:

# if !defined __cplusplus && __GNUC_PREREQ (3, 3)
   // stuff that doesn't happen
# else
#  if defined __cplusplus && __GNUC_PREREQ (2,8)
#   define __THROW  throw ()
#   define __THROWNL    throw ()
#   define __NTH(fct)   __LEAF_ATTR fct throw ()
#   define __NTHNL(fct) fct throw ()
// continuation to the if-else macro

This confused me, as, as far as i know, c doesn’t have exceptions and instead uses int error codes. Even more, why are there parentheses as in a function call?

What does it mean and what does it do in the presented case?

>Solution :

There is no throw keyword in C. That’s a C++ thing.

As to why you find it in your code, the clue is right here in the preprocessor macros:

# if !defined __cplusplus && __GNUC_PREREQ (3, 3)
   // stuff that doesn't happen
# else
#  if defined __cplusplus && __GNUC_PREREQ (2,8)
#   define __THROW  throw ()
#   define __THROWNL    throw ()
#   define __NTH(fct)   __LEAF_ATTR fct throw ()
#   define __NTHNL(fct) fct throw ()
// continuation to the if-else macro

This code is written so it will compile with either a C or C++ compiler. The C++ bit that references throw and other C++isms will only be compiled if a C++ compiler is used, as such a compiler will define the __cplusplus macro.

When you compile it with a C compiler, only the bit marked // stuff that doesn't happen will be used, not the else block.