Calling C++ standard header (cstdint) from C file - c++

Question

I have an external library written in C++ such as
external.h
#ifndef OUTPUT_FROM_CPP_H
#define OUTPUT_FROM_CPP_H
#include <cstdint>
extern "C" uint8_t myCppFunction(uint8_t n);
#endif
external.cpp
#include "external.h"
uint8_t myCppFunction(uint8_t n)
{
return n;
}
Currently I have no choice but use this C++ library in my current C project. But my compiler is telling me
No such file or director #include <cstdint>
when used in my C project
main.c
#include "external.h"
int main()
{
int a = myCppFunction(2000);
return a;
}
I understand that this is because cstdint is a C++ standard library that I'm trying to use through my C file.
My questions are:
Is there a way I can manage to use this C++ library in my C project without modifying my libary ?
If no, what whould I have to do on the library side to make it possible ?

Answer 1

The c prefix in cstdint is because it's really a header file incorporated from C. The name in C is stdint.h.
You need to conditionally include the correct header by detecting the __cplusplus macro. You also need this macro to use the extern "C" part, as that's C++ specific:
#ifndef OUTPUT_FROM_CPP_H
#define OUTPUT_FROM_CPP_H
#ifdef __cplusplus
// Building with a C++ compiler
# include <cstdint>
extern "C" {
#else
// Building with a C compiler
# include <stdint.h>
#endif
uint8_t myCppFunction(uint8_t n);
#ifdef __cplusplus
} // Match extern "C"
#endif
#endif

Answer 2

You have to modify the library.
Replace <cstdint> with <stdint.h>. Normally the former is recommended, but only the latter exists in C.
You should also be getting errors on extern "C". That's solved by putting following right below the includes:
#ifdef __cplusplus
extern "C" {
#endif
With a matching section at the end of file:
#ifdef __cplusplus
}
#endif
Then extern "C" can be removed from individual functions.

Answer 3

Is there a way I can manage to use this C++ library in my C project without modifying my libary ?
Create a separate header that is portable with C and use that header when compiling with C compiler:
// external_portable_with_c.h
// rewrite by hand or generate from original external.h
// could be just sed 's/cstdint/stdint.h/; s/extern "C"//'
#include <stdint.h>
uint8_t myCppFunction(uint8_t n);
// c_source_file.c
#include "external_portable_with_c.h"
void func() {
myCppFunction(1);
}
If no, what whould I have to do on the library side to make it possible ?
Is answered by other answers. Protect the C++ parts with #ifdef __cplusplus.
Note that (some? all?) compilers require the main function to be compiled with C++ compiler for C++ and C to work together properly. https://isocpp.org/wiki/faq/mixing-c-and-cpp#overview-mixing-langs

Answer 4

If you don't want to modify the library header, create a new file, for example includes_for_cpp/cstdint with content
#include <stdint.h>
Add the directory includes_for_cpp to the include path of your C project. After that, #include <cstdint> should find your file.