extern "c" usage?!!

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  • Medvedev
    #1

    extern "c" usage?!!

    What's that preprocessor do
    #ifdef __cplusplus
    extern "C" {
    #endif
    ..
    ..
    ..
    #ifdef __cplusplus
    }
    #endif

    and how you say extern "C" , i mean how u extern constant!!
    Tags: None
    • Mike Wahler
      #2
      Re: extern "c&quot ; usage?!!


      "Medvedev" <3D.v.World@gma il.comwrote in message
      news:7fcf3a56-f5a7-457d-a87b-60429656b943@t5 4g2000hsg.googl egroups.com...
      What's that preprocessor do
      #ifdef __cplusplus
      extern "C" {
      #endif
      .
      .
      .
      #ifdef __cplusplus
      }
      #endif
      >
      and how you say extern "C" , i mean how u extern constant!!
      extern "C"
      {

      }

      ... causes any functions inside the braces to have
      "C linkage", so that they can be called from C
      functions. There's nothing about 'constant' here.

      The #ifdef will cause 'extern C' to be applied only
      if translating C++ code.

      -Mike


        Comment

        • Medvedev
          #3
          Re: extern &quot;c&quot ; usage?!!

          On Jul 6, 2:12 pm, "Mike Wahler" <mkwah...@mkwah ler.netwrote:
          "Medvedev" <3D.v.Wo...@gma il.comwrote in message
          >
          news:7fcf3a56-f5a7-457d-a87b-60429656b943@t5 4g2000hsg.googl egroups.com...
          >
          What's that preprocessor do
          #ifdef __cplusplus
          extern "C" {
          #endif
          .
          .
          .
          #ifdef __cplusplus
          }
          #endif
          >
          and how you say extern "C" , i mean how u extern constant!!
          >
          extern "C"
          {
          >
          }
          >
          .. causes any functions inside the braces to have
          "C linkage", so that they can be called from C
          functions. There's nothing about 'constant' here.
          sorry , but what differ C++ functions from C ones

            Comment

            • Jim Langston
              #4
              Re: extern &quot;c&quot ; usage?!!

              "Medvedev" <3D.v.World@gma il.comwrote in message
              news:ce4f3ac2-33fb-4cff-b631-3d0ec872d51b@e5 3g2000hsa.googl egroups.com...
              On Jul 6, 2:12 pm, "Mike Wahler" <mkwah...@mkwah ler.netwrote:
              >"Medvedev" <3D.v.Wo...@gma il.comwrote in message
              >>
              >news:7fcf3a5 6-f5a7-457d-a87b-60429656b943@t5 4g2000hsg.googl egroups.com...
              >>
              What's that preprocessor do
              #ifdef __cplusplus
              extern "C" {
              #endif
              .
              .
              .
              #ifdef __cplusplus
              }
              #endif
              >>
              and how you say extern "C" , i mean how u extern constant!!
              >>
              >extern "C"
              >{
              >>
              >}
              >>
              >.. causes any functions inside the braces to have
              >"C linkage", so that they can be called from C
              >functions. There's nothing about 'constant' here.
              >
              sorry , but what differ C++ functions from C ones
              C++ functions have overloading. C functions do not. This means there is
              only one definition for a C function, there can be more than one for a C++
              function. Compilers typcially handle this by "mangling". This will change
              the name of the C++ function in the object file so the linker can
              differentiate between calling parameters.

              stating:
              extern "C" {
              tells the compiler not to mangle the function names, there will only be one
              declaration for each function, so then when it is linked it has C linkage,
              the function name would be the same as if it was a C function, and C
              programs/objects can call the function as they can now link to them.


                Comment

                • Mike Wahler
                  #5
                  Re: extern &quot;c&quot ; usage?!!


                  "Medvedev" <3D.v.World@gma il.comwrote in message
                  news:ce4f3ac2-33fb-4cff-b631-3d0ec872d51b@e5 3g2000hsa.googl egroups.com...
                  On Jul 6, 2:12 pm, "Mike Wahler" <mkwah...@mkwah ler.netwrote:
                  >"Medvedev" <3D.v.Wo...@gma il.comwrote in message
                  >>
                  >news:7fcf3a5 6-f5a7-457d-a87b-60429656b943@t5 4g2000hsg.googl egroups.com...
                  >>
                  What's that preprocessor do
                  #ifdef __cplusplus
                  extern "C" {
                  #endif
                  .
                  .
                  .
                  #ifdef __cplusplus
                  }
                  #endif
                  >>
                  and how you say extern "C" , i mean how u extern constant!!
                  >>
                  >extern "C"
                  >{
                  >>
                  >}
                  >>
                  >.. causes any functions inside the braces to have
                  >"C linkage", so that they can be called from C
                  >functions. There's nothing about 'constant' here.
                  >
                  sorry , but what differ C++ functions from C ones
                  Think about overloading. You can have more than a
                  single function with the same name (the number and/or
                  types of arguments would differ.

                  int foo(void);
                  int foo(int);
                  int foo(int,int);

                  C++ needs to distinguish these functions from one another.
                  There's no specific method required by the standard, but
                  typically the compiler will 'mangle' the names (change and/or
                  add characters) so that they become unique.

                  "extern C" prevents this 'mangling', so that the function
                  can be linked with C (using its 'real' name).

                  Anyway, I should have already cited this link from the C++ FAQ:
                  How to mix C and C++, C++ FAQ
                  http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html


                  This explains it better than I.

                  -Mike



                    Comment

                    • James Kanze
                      #6
                      Re: extern &quot;c&quot ; usage?!!

                      On Jul 7, 12:18 am, Medvedev <3D.v.Wo...@gma il.comwrote:
                      On Jul 6, 2:12 pm, "Mike Wahler" <mkwah...@mkwah ler.netwrote:
                      "Medvedev" <3D.v.Wo...@gma il.comwrote in message
                      news:7fcf3a56-f5a7-457d-a87b-60429656b943@t5 4g2000hsg.googl egroups.com....
                      What's that preprocessor do
                      #ifdef __cplusplus
                      extern "C" {
                      #endif
                      .
                      .
                      .
                      #ifdef __cplusplus
                      }
                      #endif
                      and how you say extern "C" , i mean how u extern constant!!
                      extern "C"
                      {
                      }
                      .. causes any functions inside the braces to have
                      "C linkage", so that they can be called from C
                      functions. There's nothing about 'constant' here.
                      sorry , but what differ C++ functions from C ones
                      Whatever the implementation wants. There's no fundamental
                      reason for two different languages to use the same calling
                      conventions. One typical difference might be that in C++, the
                      call stack is cleaned up in the called function (since it
                      involves calling destructors, etc.), where as in C, it is
                      cleaned up in the callee (since historically, C didn't have
                      prototypes, and allowed calling a function with extra arguments,
                      which were ignored). Also, C++ has overloading, which means
                      that some sort of information concerning the type and number of
                      arguments must be maintained in the object file. And because C
                      allowed extra arguments, a C compiler will not want to do this.

                      --
                      James Kanze (GABI Software) email:james.kan ze@gmail.com
                      Conseils en informatique orientée objet/
                      Beratung in objektorientier ter Datenverarbeitu ng
                      9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

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