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void f() {}
void g();

extern "C" {
    void ef() {}
    void eg();
}


void h() { g(); eg(); }

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g++ -c -std=c++11 -Wall -Wextra -pedantic -o main.o main.cpp
readelf -s main.o

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 8: 0000000000000000     7 FUNC    GLOBAL DEFAULT    1 _Z1fv
 9: 0000000000000007     7 FUNC    GLOBAL DEFAULT    1 ef
10: 000000000000000e    17 FUNC    GLOBAL DEFAULT    1 _Z1hv
11: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _GLOBAL_OFFSET_TABLE_
12: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND _Z1gv
13: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND eg

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$ c++filt _Z1fv
f()
$ c++filt _Z1hv
h()
$ c++filt _Z1gv
g()

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extern "C" {
    
    
    void f();
    void f(int i);

    
    
    template <class C> void f(C i) { }
}

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#include <cassert>

#include "c.h"

int main() {
    assert(f() == 1);
}

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#ifndef C_H
#define C_H



#ifdef __cplusplus
extern "C" {
#endif
int f();
#ifdef __cplusplus
}
#endif

#endif

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#include "c.h"

int f(void) { return 1; }

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g++ -c -o main.o -std=c++98 main.cpp
gcc -c -o c.o -std=c89 c.c
g++ -o main.out main.o c.o
./main.out

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main.cpp:6: undefined reference to `f()'

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#include <assert.h>

#include "cpp.h"

int main(void) {
    assert(f_int(1) == 2);
    assert(f_float(1.0) == 3);
    return 0;
}

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#ifndef CPP_H
#define CPP_H

#ifdef __cplusplus

int f(int i);
int f(float i);
extern "C" {
#endif
int f_int(int i);
int f_float(float i);
#ifdef __cplusplus
}
#endif

#endif

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#include "cpp.h"

int f(int i) {
    return i + 1;
}

int f(float i) {
    return i + 2;
}

int f_int(int i) {
    return f(i);
}

int f_float(float i) {
    return f(i);
}

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gcc -c -o main.o -std=c89 -Wextra main.c
g++ -c -o cpp.o -std=c++98 cpp.cpp
g++ -o main.out main.o cpp.o
./main.out

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main.c:6: undefined reference to `f_int'
main.c:7: undefined reference to `f_float'