Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. Mobile and Embedded
  4. template with C linkage [Error]
Forum Updated to NodeBB v4.3 + New Features

template with C linkage [Error]

Scheduled Pinned Locked Moved Solved Mobile and Embedded
14 Posts 5 Posters 15.1k Views 3 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • jsulmJ Online
    jsulmJ Online
    jsulm
    Lifetime Qt Champion
    wrote on last edited by jsulm
    #5

    Well, you should change it:

    ifdef __cplusplus
extern "C" {
#include "db.h"
} // Add this
#endif

    Else all your C++ stuff will be inside Extern "C"!

    And if you do it in C++ code you do not have to check whether it is C++ or not, just do:

    extern "C" {
#include "db.h"
}

    https://forum.qt.io/topic/113070/qt-code-of-conduct

    K 1 Reply Last reply
    0
    • jsulmJ jsulm

      Well, you should change it:

      ifdef __cplusplus
extern "C" {
#include "db.h"
} // Add this
#endif

      Else all your C++ stuff will be inside Extern "C"!

      And if you do it in C++ code you do not have to check whether it is C++ or not, just do:

      extern "C" {
#include "db.h"
}
      K Offline
      K Offline
      kumararajas
      wrote on last edited by kumararajas
      #6

      @jsulm That's correct. I was suppose to close it immediately after including C header.

      But I am still getting error when I do this

      extern "C" {
      #include "db.h"
      }

      Here is the error list

      /usr/include/c++/4.6/bits/cpp_type_traits.h:73:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:86:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:90:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:96:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:104:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:111:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:119:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:126:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:136:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:146:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:153:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:160:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:167:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:175:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:199:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:206:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:213:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:220:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:227:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:234:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:241:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:248:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:258:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:266:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:273:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:280:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:290:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:297:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:307:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:314:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:325:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:333:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:341:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:349:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:356:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:364:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:372:3: error: template with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:379:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:386:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:393:3: error: template specialization with C linkage
/usr/include/c++/4.6/bits/cpp_type_traits.h:403:3: error: template with C linkage

      --Kumar

      1 Reply Last reply
      0
      • jsulmJ Online
        jsulmJ Online
        jsulm
        Lifetime Qt Champion
        wrote on last edited by
        #7

        What is in db.h?
        Is it really pure C?
        And what does db.h include (cpp_type_traits.h?)?

        https://forum.qt.io/topic/113070/qt-code-of-conduct

        K 1 Reply Last reply
        0
        • M Offline
          M Offline
          mcosta
          wrote on last edited by
          #8

          Hi,

          as I said in my previous post you can use extern "C" only if your header file does not contain C++ code.
          It must be used to use C code inside C++

          Once your problem is solved don't forget to:

          • Mark the thread as SOLVED using the Topic Tool menu
          • Vote up the answer(s) that helped you to solve the issue

          You can embed images using (http://imgur.com/) or (http://postimage.org/)

          K 1 Reply Last reply
          0
          • jsulmJ jsulm

            What is in db.h?
            Is it really pure C?
            And what does db.h include (cpp_type_traits.h?)?

            K Offline
            K Offline
            kumararajas
            wrote on last edited by
            #9

            @jsulm To explain about my db.h,

            db.h is an interface, containing set of APIs. And it's a C code.
            db.h interfaces C++ code (odb.h)which written for accessing database.

            C++ code, gets accessed in C library.
            And in my C++ code and using that C library.

            (Sorry if I confused you)

            In short, db.h is a C header file and I am linking C library (db.a) in my project file.

            --Kumar

            1 Reply Last reply
            0
            • M mcosta

              Hi,

              as I said in my previous post you can use extern "C" only if your header file does not contain C++ code.
              It must be used to use C code inside C++

              K Offline
              K Offline
              kumararajas
              wrote on last edited by
              #10

              @mcosta Thanks!
              My db.h contains C code, basically a set of global functions acting as an interface.
              It does not contain any C++ code, but it invokes C++ code.

              Example -
              In db.h

              IfcFunc()
{
// C Code
   OtherFunc();
}

              In odb.h

              OtherFunc()
{
// C++ code
}

              I am linking db library and including the header file.

              --Kumar

              1 Reply Last reply
              0
              • K Offline
                K Offline
                kumararajas
                wrote on last edited by
                #11

                I have removed extern "C" { } now.
                Because, db.h has extern "C" in it.

                After removing that, I don't get template with C linkage error.

                But when I use an API in my code, which comes from the library, it says, undefined reference.

                I am linking as

                LIBS += -L$(PATH)/lib -ldb

                Am I missing something?

                --Kumar

                1 Reply Last reply
                0
                • B Offline
                  B Offline
                  bsomervi
                  wrote on last edited by bsomervi
                  #12

                  Seems to me that OtherFunc() has C++ linkage, you cannot mention it in a C compatible header file. If you need to access C++ from a C library that has a C compatible header then you must only access the C++ code from within the implementation files of the C library.

                  Do some research on how C++ deals with function overloading with name mangling and you should then understand how C++ linkage differs from C linkage and why symbols with C++ linkage cannot be in C compatible header files.

                  1 Reply Last reply
                  0
                  • K Offline
                    K Offline
                    kumararajas
                    wrote on last edited by
                    #13

                    I could get rid of issue. I did not want extern C

                    Reason is that, my code is accessing a library and C compatibility is taken care in the library.

                    Here is a bird eye view -
                    myLibrary->linking libdb

                    I do have a different problem now. That is,
                    I have an application that is linking myLibrary which is accessing libdb

                    MyApp->linkingmyLibrary->linking libdb

                    I am getting 'undefined reference to' when my Library calls an API from libDB.

                    I see it as a linkage order issue. Is there any thoughts on this?

                    Thank you,
                    Kumara

                    --Kumar

                    1 Reply Last reply
                    0
                    • K Offline
                      K Offline
                      kumararajas
                      wrote on last edited by
                      #14

                      Hello all,

                      Problem is solved.

                      Issue is that, I was accessing a library, and that link multiple libraries.

                      As an application point of view, I should be linking those libraries as well, which I was missing to do.
                      By linking those dependencies as well, I could build my application successfully.

                      Thanks,
                      Kumara

                      --Kumar

                      1 Reply Last reply
                      0

                      • Login
                      • Login or register to search.
                      • First post
                        Last post
                      0
                      • Categories
                      • Recent
                      • Tags
                      • Popular
                      • Users
                      • Groups
                      • Search
                      • Get Qt Extensions
                      • Unsolved