C/C++

Problem:

While trying to compile your C/C++ program, you see an error message like

../src/main.c:48:9: error: implicit declaration of function 'StartBenchmark' [-Werror=implicit-function-declaration]
         StartBenchmark();

Solution:

implicit declaration of function means that you are trying to use a function that has not been declared. In our example above, StartBenchmark is the function that is implicitly declared.

This is how you call a function:

StartBenchmark();

This is how you declare a function:

void StartBenchmark();

The following bullet points list the most common reasons and how to fix them:

1. Missing #include: Check if the header file that contains the declaration of the function is #included in each file where you call the function (especially the file that is listed in the error message), before the first call of the function (typically at the top of the file). Header files can be included via other headers,
2. Function name typo: Often the function name of the declaration does not exactly match the function name that is being called. For example, startBenchmark() is declared while StartBenchmark() is being called. I recommend to fix this by copy-&-pasting the function name from the declaration to wherever you call it.
3. Bad include guard: The include guard that is auto-generated by IDEs often looks like this:

   #ifndef _EXAMPLE_FILE_NAME_H
   #define _EXAMPLE_FILE_NAME_H
   // ...
   #endif

   Note that the include guard definition _EXAMPLE_FILE_NAME_H is not specific to the header filename that we are using (for example Benchmark.h). Just the first of all header file names wil

4. Change the order of the #include statements: While this might seem like a bad hack, it often works just fine. Just move the #include statements of the header file containing the declaration to the top. For example, before the move:

   #include "Benchmark.h"
   #include "other_header.h"

   after the move:

   #include "Benchmark.h"
   #include "other_header.h"

Problem:

When trying to compile your C++ application, you see lots of undefined reference to messages like

AutoBenchmark.cpp:(.text+0x6a): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x81): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x8a): undefined reference to `std::chrono::_V2::system_clock::now()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0xcd): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0xe6): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: /tmp/cc356AdP.o: in function `AutoBenchmark::~AutoBenchmark()':
AutoBenchmark.cpp:(.text+0x17e): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x18e): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: /tmp/cc356AdP.o: in function `AutoBenchmark::Record(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
AutoBenchmark.cpp:(.text+0x1db): undefined reference to `std::chrono::_V2::system_clock::now()'
/usr/bin/ld: /tmp/cc356AdP.o: in function `AutoBenchmark::Record(char const*)':
AutoBenchmark.cpp:(.text+0x252): undefined reference to `std::allocator<char>::allocator()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x288): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x294): undefined reference to `std::allocator<char>::~allocator()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x2b9): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x2ce): undefined reference to `std::allocator<char>::~allocator()'
/usr/bin/ld: /tmp/cc356AdP.o: in function `AutoBenchmark::Print()':
AutoBenchmark.cpp:(.text+0x316): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::length() const'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x332): undefined reference to `std::cout'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x337): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x344): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x572): undefined reference to `std::cout'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x577): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x59c): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x5ab): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x5c2): undefined reference to `std::ostream::operator<<(double)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x5d1): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x607): undefined reference to `std::cout'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x60c): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x631): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x640): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x657): undefined reference to `std::ostream::operator<<(double)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x666): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x69c): undefined reference to `std::cout'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x6a1): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x6c6): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x6d5): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x6ec): undefined reference to `std::ostream::operator<<(double)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x6fb): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x731): undefined reference to `std::cout'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x736): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x75b): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <char, std::char_traits<char>, std::allocator<char> >(std::basic_ostream<char, std::char_traits<char> >&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x76a): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x781): undefined reference to `std::ostream::operator<<(double)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x790): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
/usr/bin/ld: AutoBenchmark.cpp:(.text+0x7c6): undefined reference to `std::cout'

Solution:

You need to link the stdc++ library by adding -lstdc++ to your compiler/linker flags.

For example, instead of

gcc -o myprogram *.c

use

gcc -o myprogram *.c -lstdc++

Problem:

When trying to compile an application on Linux, you see a compiler error message like

myheader.h:23:12: error: expected constructor, destructor, or type conversion before ‘(’ token
   23 |  __declspec(dllexport) int myfunc(

Solution:

__declspec(dllexport) is a Windows-specific feature and not available on Linux. In order to fix it in a compatible way, add this code either in a header that is included in every file containing __declspec(dllexport) or add it in each file where the error occurs:

#ifdef __linux__
#define __declspec(v)
#endif

This will basically ignore any __declspec() call on the preprocessor level.

Problem:

When trying to compile your application using gcc, you see an error message like

/usr/bin/ld: /tmp/ccxPIowU.o: in function `run_mymath':
mathstuff.c:(.text+0x15d): undefined reference to `sqrt'

Solution:

You need to link the math library using the -lm flag (-lxxx means: “link the xxx library”, i.e. -lm means “link the m” library)

For example, instead of

gcc -o myprogram *.c

use

gcc -o myprogram *.c -lm

Problem:

If you’re trying to compile Windows code on Linux, you will often see messages like

lmmin.c:261:10: warning: implicit declaration of function ‘_finite’; did you mean ‘finite’? [-Wimplicit-function-declaration]
  116 |     if(!_finite(myvalue)){

or

/usr/bin/ld: mymath.c:(.text+0x1057): undefined reference to `_finite'

and your code won’t compile.

Solution:

_finite is a function that is only available on Windows. In order to use it on Linux using GCC or G++, one option is to use isfinite() from math.h:

#include <math.h>

#define _finite(v) (isfinite((v)))

In case that function is not available (like on some Microcontroller platforms), you can use __builtin_isfinite(). Note that glibc defines isfinite() as an alias for __builtin_isfinite(). Use it like this:

#define _finite(v) (__builtin_isfinite(v))

In case you want to have code that is compatible with both platforms (Windows and Linux), use

#ifdef __linux__
#define _finite(v) (__builtin_isfinite(v))
#endif

or

#include <math.h>

#ifdef __linux__
#define _finite(v) (isfinite((v)))
#endif