Error compiling a C program which calls a C++ function declared in header file and defined in a separate cpp file

Question

I'm trying to call a C++ function in a C program. I declared the function in a .h file as follows- data_fields.h:

#ifdef __cplusplus
extern "C"
{
#endif
    char GetCharFromHexAscii(char *);
#ifdef __cplusplus
}
#endif

And I call the GetCharFromHexAscii() in my .c file as follows- GUIForRenamer.c:

#include<stdio.h>
#include<gtk/gtk.h>
#include<unistd.h>
#include<string.h>
#include<ctype.h>
#include"data_fields.h"
char * AsciiDecode(char *Path)
{
    int k = 0;
    char *convertedPath, *asciiChar;
    convertedPath = (char *)malloc(strlen(Path) + 1 * sizeof(char));
    asciiChar = (char *)malloc(3 * sizeof(char));
    for(int i = 0; Path[i] != '\0'; i++)
        convertedPath[k++] = GetCharFromHexAscii(asciiChar);
    return convertedPath;
}

The definition of the C++ function in .cpp is as follows- LibFunc.cpp:

#include<iostream>
#include<cstdlib>
#include"data_fields.h"
char GetCharFromHexAscii(char *asciiChar)
{
    int hexVal = stoi(asciiChar, 0, 16);
    return (char)hexVal;
}

Here's the command I use to compile my C code-

gcc GUIForRenamer.c -lm `pkg-config --libs --cflags gtk+-3.0` -export-dynamic FileExtensionRenamer.c LibFunc.cpp

But the compilation fails with the following error-

LibFunc.cpp: In function ‘char GetCharFromHexAscii(char*)’:
LibFunc.cpp:7:15: error: ‘stoi’ was not declared in this scope
  int hexVal = stoi(asciiChar, 0, 16);
               ^~~~
LibFunc.cpp:7:15: note: suggested alternatives:
In file included from /usr/include/c++/7/string:52:0,
                 from /usr/include/c++/7/bits/locale_classes.h:40,
                 from /usr/include/c++/7/bits/ios_base.h:41,
                 from /usr/include/c++/7/ios:42,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from LibFunc.cpp:1:
/usr/include/c++/7/bits/basic_string.h:6477:3: note:   ‘std::__cxx11::stoi’
   stoi(const wstring& __str, size_t* __idx = 0, int __base = 10)
   ^~~~
/usr/include/c++/7/bits/basic_string.h:6477:3: note:   ‘std::__cxx11::stoi’

But, if I try to compile the same function by calling it in a C++ program, compilation happens successfully.

Could anyone help me figure out, what else could be done for this?

Based on the below suggestions, I updated the code in LibFunc.cpp as follows-

#include<iostream>
#include<cstdlib>
#include"data_fields.h"
#include<string>
using namespace std;
char GetCharFromHexAscii(char *asciiChar)
{
    int hexVal = stoi(asciiChar, 0, 16);
    return (char)hexVal;
}

On compiling this code, I got the following issues-

/tmp/ccr7aSCZ.o: In function `GetCharFromHexAscii':
LibFunc.cpp:(.text+0x24): undefined reference to `std::allocator<char>::allocator()'
LibFunc.cpp:(.text+0x3b): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&)'
LibFunc.cpp:(.text+0x60): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
LibFunc.cpp:(.text+0x6c): undefined reference to `std::allocator<char>::~allocator()'
LibFunc.cpp:(.text+0x8f): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string()'
LibFunc.cpp:(.text+0xa0): undefined reference to `std::allocator<char>::~allocator()'
/tmp/ccr7aSCZ.o: In function `__static_initialization_and_destruction_0(int, int)':
LibFunc.cpp:(.text+0xe0): undefined reference to `std::ios_base::Init::Init()'
LibFunc.cpp:(.text+0xf5): undefined reference to `std::ios_base::Init::~Init()'
/tmp/ccr7aSCZ.o: In function `std::__cxx11::stoi(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned long*, int)':
LibFunc.cpp:(.text._ZNSt7__cxx114stoiERKNS_12basic_stringIcSt11char_traitsIcESaIcEEEPmi[_ZNSt7__cxx114stoiERKNS_12basic_stringIcSt11char_traitsIcESaIcEEEPmi]+0x1b): undefined reference to `std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::c_str() const'
/tmp/ccr7aSCZ.o: In function `int __gnu_cxx::__stoa<long, int, char, int>(long (*)(char const*, char**, int), char const*, char const*, unsigned long*, int)':
LibFunc.cpp:(.text._ZN9__gnu_cxx6__stoaIlicJiEEET0_PFT_PKT1_PPS3_DpT2_EPKcS5_PmS9_[_ZN9__gnu_cxx6__stoaIlicJiEEET0_PFT_PKT1_PPS3_DpT2_EPKcS5_PmS9_]+0x62): undefined reference to `std::__throw_invalid_argument(char const*)'
LibFunc.cpp:(.text._ZN9__gnu_cxx6__stoaIlicJiEEET0_PFT_PKT1_PPS3_DpT2_EPKcS5_PmS9_[_ZN9__gnu_cxx6__stoaIlicJiEEET0_PFT_PKT1_PPS3_DpT2_EPKcS5_PmS9_]+0xa3): undefined reference to `std::__throw_out_of_range(char const*)'
/tmp/ccr7aSCZ.o:(.data.rel.local.DW.ref.__gxx_personality_v0[DW.ref.__gxx_personality_v0]+0x0): undefined reference to `__gxx_personality_v0'
collect2: error: ld returned 1 exit status

I thought this is the issue caused due to passing char * as parameter instead of passing string object to stoi function. So, I converted the char * to string object and passed it to the function. But, this didn't resolve the error and was still giving the same compilation issue.

Could anyone please help me understand why I am getting this error, and also suggest a way to resolve it?

You need to compile the C and C++ separately and then link them together. — NathanOliver
– NathanOliver, Commented Aug 18, 2020 at 15:15
stoi is declared in <string>, and lives in the std namespace. — molbdnilo
– molbdnilo, Commented Aug 18, 2020 at 15:18
malloc(strlen(Path) + 1 * sizeof(char)) only works because, by definition, sizeof (char) is 1 ... remember multiplication has priority over addition, so your expression is equivalent to strlen(Path) + (1 * sizeof(char)) == strlen(Path) + sizeof(char) == strlen(Path) + 1 ... if you really want to keep the redundant multiplication by 1, do malloc((strlen(Path) + 1) * sizeof(char)) ... to emphasize: malloc(strlen(Path) + 1 * sizeof(char)) is wrong. — pmg
– pmg, Commented Aug 19, 2020 at 14:41

eerorika · Accepted Answer · 2020-08-18 15:24:38Z

1

You call a function stoi. You haven't defined such function.

Like the error message also noticed, you may have instead intended to call std::stoi function from the C++ standard library. Notice that it is declared in the std namespace. Furthermore, you should include the header which declares it (<string>).

In addition, you may need to link with the C++ standard library. This would happen automatically if you used g++ frontend instead of gcc.

answered Aug 18, 2020 at 15:24

eerorika

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2 Comments

Priya.D Over a year ago

Thank u for the suggestion of using std, as that helped me move a step further. But, compiling with g++ doesn't help me resolve my issues, it rather gives me more compilation issues, since my other files are .c files and are properly compiled using gcc only.

eerorika Over a year ago

@Priya.D Compiler C++ files with g++ and C files with gcc.

Dragos Popescu · Accepted Answer · 2020-08-18 15:26:07Z

1

The function stoi is located in the <string> header (not string.h, you can check https://en.cppreference.com/w/cpp/string/basic_string/stol).

Since there is no using namespace std in your code, you should change your call to stoi to std::stoi

A final working code can look something like this

#include <string>
char GetCharFromHexAscii(char *asciiChar)
{
    int hexVal = std::stoi(asciiChar, 0, 16);
     return (char)hexVal;
}

answered Aug 18, 2020 at 15:26

Dragos Popescu

666 bronze badges

1 Comment

Priya.D Over a year ago

Thank u for the suggestion of using std, as this helped me move a step further, but, I got another set of errors. I've updated the above question with the next set of errors. Kindly check and let me know if u can help me with it.

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