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Force resolution of fstat64 symbol with JNA #110807

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Merged
merged 1 commit into from
Jul 12, 2024
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Force resolution of fstat64 symbol with JNA #110807

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11 changes: 10 additions & 1 deletion libs/native/jna/src/main/java/org/elasticsearch/nativeaccess/jna/JnaPosixCLibrary.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@
import com.sun.jna.Library;
import com.sun.jna.Memory;
import com.sun.jna.Native;
import com.sun.jna.NativeLibrary;
import com.sun.jna.NativeLong;
import com.sun.jna.Pointer;
import com.sun.jna.Structure;
Expand Down Expand Up @@ -143,9 +144,17 @@ private interface FXStatFunction extends Library {
this.functions = Native.load("c", NativeFunctions.class);
FStat64Function fstat64;
try {
// JNA lazily finds symbols, so even though we try to bind two different functions below, if fstat64
// isn't found, we won't know until runtime when calling the function. To force resolution of the
// symbol we get a function object directly from the native library. We don't use it, we just want to
// see if it will throw UnsatisfiedLinkError
NativeLibrary.getInstance("c").getFunction("fstat64");
fstat64 = Native.load("c", FStat64Function.class);
} catch (UnsatisfiedLinkError e) {
// TODO: explain
// fstat has a long history in linux from the 32-bit architecture days. On some modern linux systems,
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Does this make the class name JnaPosixCLibrary a bit of a misnomer?

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Maybe, but "fstat" is part of posix, it's just that the implementation is a bit wonky due to historical reasons (and thus why we need to sometimes peek into the implementation side).

// fstat64 doesn't exist as a symbol in glibc. Instead, the compiler replaces fstat64 calls with
// the internal __fxstat method. Here we fall back to __fxstat, and staticall bind the special
// "version" argument so that the call site looks the same as that of fstat64
var fxstat = Native.load("c", FXStatFunction.class);
int version = System.getProperty("os.arch").equals("aarch64") ? 0 : 1;
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Hmm this seems worth documenting.

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That's what I was doing here with the comment. Do you see a better place to document it?

fstat64 = (fd, stat) -> fxstat.__fxstat(version, fd, stat);
Expand Down