Check is member in struct with C - c

Question

I want to access to member in struct but check if this member exist
#include <stdio.h>
int main()
{
struct MyStruct { int a;};
struct MyStruct temp ;
temp.a=3;
return 0;
}
Is there a way to check if member a is in struct m and if not access to b with #ifdef ?
something like #ifdef MyStruct.a temp.a=3; #else temp.b=3; #endif

Answer 1

It's not possible to do in C.
The usual way if you really want to detect a member of a struct, is to first compile a sample program that uses the struct, see if the compilation succeeds and depending on the result define or not a preprocessor macro to the compiler used to compile the rest of the project. Various build systems were created to ease up such tasks. A short example with cmake may look like the following:
cat >mystruct.h <<EOF
struct MyStruct {
int a; // a? maybe not? no one knows.
};
EOF
cat >CMakeLists.txt <<EOF
cmake_minimum_required(VERSION 3.11)
project(test_include C)
# will try to compile a testing file
try_compile(mystruct_has_a "${CMAKE_BINARY_DIR}/temp" ${CMAKE_SOURCE_DIR}/test_mystruct_has_a.c)
add_executable(main main.c)
if(mystruct_has_a)
target_compile_definitions(main PUBLIC MYSTRUCT_HAS_A)
endif()
EOF
cat >test_mystruct_has_a.c <<EOF
#include "mystruct.h"
int main() {
struct MyStruct temp;
temp.a = 3; // test - can we compile this?
}
EOF
cat >main.c <<EOF
#include <stdio.h>
#include "mystruct.h"
int main() {
struct MyStruct temp ;
#if MYSTRUCT_HAS_A
// it's ok to use `temp.a` here.
temp.a = 3;
printf("%d\n", temp.a);
#endif
}
EOF
Which can be compiled with from the command line:
cmake -S. -B_build && cmake --build _build --verbose
cmake will try to compile file test_mystruct_has_a.c. If the compilation is successful, then the macro MYSTRUCT_HAS_A will be added as a macro to compiler arguments. If not, the macro will not be added. Then main.c will compile with that macro or without depending on the previous result. Such a way is typically used in many, many various projects mostly to provide portability by detecting operating system specific stuff, from top of my head for example members of sigaction, siginfo_t or sched_param.