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List:       openjdk-serviceability-dev
Subject:    Re: RFR: JDK-8200178 Remove mapfiles for JDK native libraries
From:       Martin Buchholz <martinrb () google ! com>
Date:       2018-03-29 6:16:18
Message-ID: CA+kOe08bkip2eGy8rn8viwtQ3eDP8GQYqRUBnsN02WWV6MqwcA () mail ! gmail ! com
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On Wed, Mar 28, 2018 at 3:14 PM, Magnus Ihse Bursie <
magnus.ihse.bursie@oracle.com> wrote:

> On 2018-03-28 23:53, Martin Buchholz wrote:
>
> I can't find any documentation for what JNIEXPORT and friends actually do.
> People including myself have been cargo-culting JNIEXPORT and JNICALL for
> decades.
> Why aren't they in the JNI spec?
>
> That surprises me. I'm quite certain that javah (or rather, java -h
> nowadays) generate header files with JNIEXPORT and JNICALL.
>
> As you can see in the jni.h and jni_md.h files, JNIEXPORT equals
> __attribute__((visibility("default"))) for compilers that support it (gcc
> and friends), and __declspec(dllexport) for Windows. This means, that the
> symbol should be exported. (And it's ignored if you use mapfiles aka linker
> scripts.)
>
> As for JNICALL, it's empty on most compilers, but evaluates to __stdcall
> on Windows. This defines the calling convention to use. This is required
> for JNI calls from Java. (Ask the JVM team why.) While it's not technically
> required for calling from one dll to another, it's good practice to use it
> all time to be consistent. In any way, it doesn't hurt us.
>

Sure, I can see how JNIEXPORT and JNICALL are implemented, but what do they
*mean?*

For example, one might expect from the JNI prefix that these macros are
exclusively for use by JNI linking, i.e. unsupported except in the output
of javac -h.  But of course in practice they are used with arbitrary
symbols to communicate between components of user native code, not just to
communicate with the JVM.  Is that a bug?

[Attachment #3 (text/html)]

<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Mar \
28, 2018 at 3:14 PM, Magnus Ihse Bursie <span dir="ltr">&lt;<a \
href="mailto:magnus.ihse.bursie@oracle.com" \
target="_blank">magnus.ihse.bursie@oracle.com</a>&gt;</span> wrote:<br><blockquote \
class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc \
solid;padding-left:1ex">  
    
  
  <div text="#000000" bgcolor="#FFFFFF"><span class="">
    On 2018-03-28 23:53, Martin Buchholz wrote:<br>
    <blockquote type="cite">
      <div dir="ltr">
        <div class="gmail_extra">I can&#39;t find any documentation for what
          JNIEXPORT and friends actually do.</div>
        <div class="gmail_extra">People including myself have been
          cargo-culting  <span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">JNIEXPORT
  and JNICALL for decades.</span></div>
        <div class="gmail_extra"><span \
style="color:rgb(34,34,34);font-family:arial,sans-serif;font-size:small;font-style:nor \
mal;font-variant-ligatures:normal;font-variant-caps:normal;font-weight:400;letter-spac \
ing:normal;text-align:start;text-indent:0px;text-transform:none;white-space:normal;wor \
d-spacing:0px;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">Why
  aren&#39;t they in the JNI spec?</span></div>
      </div>
    </blockquote></span>
    That surprises me. I&#39;m quite certain that javah (or rather, java -h
    nowadays) generate header files with JNIEXPORT and JNICALL.<br>
    <br>
    As you can see in the jni.h and jni_md.h files, JNIEXPORT equals
    __attribute__((visibility(&quot;<wbr>default&quot;))) for compilers that support \
it  (gcc and friends), and __declspec(dllexport) for Windows. This
    means, that the symbol should be exported. (And it&#39;s ignored if you
    use mapfiles aka linker scripts.)<br>
    <br>
    As for JNICALL, it&#39;s empty on most compilers, but evaluates to
    __stdcall on Windows. This defines the calling convention to use.
    This is required for JNI calls from Java. (Ask the JVM team why.)
    While it&#39;s not technically required for calling from one dll to
    another, it&#39;s good practice to use it all time to be consistent. In
    any way, it doesn&#39;t hurt us.<span \
class=""><br></span></div></blockquote><div><br></div><div>Sure, I can see how \
JNIEXPORT and JNICALL are implemented, but what do they <b><i><font \
size="6">mean?</font></i></b></div><div><br></div><div>For example, one might expect \
from the JNI prefix that these macros are exclusively for use by JNI linking, i.e. \
unsupported except in the output of javac -h.   But of course in practice they are \
used with arbitrary symbols to communicate between components of user native code, \
not just to communicate with the JVM.   Is that a bug?</div><div><br></div><div>  \
</div></div></div></div>



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