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doc: add doc about C types that we map to uintptr instead of ptr

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Update #22906
Update #21897

Change-Id: I73709b2fdac6981d4bc2f7dab0767f2dd7be3be5
Reviewed-on: https://go-review.googlesource.com/82917
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Keith Randall 2017-12-08 08:38:15 -08:00
parent 36aa2b036d
commit 25363de226
2 changed files with 46 additions and 4 deletions
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27
doc/go1.10.html
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@ -262,8 +262,6 @@ Go structs and Go arrays are not supported in the type signatures of cgo-exporte
<p>
TODO: CL 70890 "permit passing string values directly between Go and C."
<br>
TODO: CL 66332 "special case C ptr types to use uintptr."
</p>
<p>
@ -279,6 +277,31 @@ Later <code>go</code> <code>build</code> commands refer to the <code>CC</code> e
variable or else the built-in default.
</p>
<p>
Cgo now translates some C types that would normally map to a pointer
type in Go, to a <code>uintptr</code> instead. These types include
the <code>CFTypeRef</code> hierarchy in Darwin's CoreFoundation
framework and the <code>jobject</code> hierarchy in Java's JNI
interface.
</p>
<p>
These types must be <code>uintptr</code> on the Go side because they
would otherwise confuse the Go garbage collector; they are sometimes
not really pointers but data structures encoded in a pointer type.
</p>
<p>
Because of this change, values of the affected types need to be
zero-initialized with the constant <code>0</code> instead of the
constant <code>nil</code>. Go 1.10 provides <code>gofix</code>
modules to help with that rewrite:
<pre>
go tool fix -r cftype <pkg>
go tool fix -r jni <pkg>
</pre>
</p>
<p>
For more details, see the <a href="/cmd/cgo/">cgo documentation</a>.
</p>

23
src/cmd/cgo/doc.go
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@ -403,16 +403,35 @@ the CF*Ref types from the CoreFoundation library on Darwin, including:
CFXMLParserRef
CFXMLTreeRef
Also the object types from Java's JNI interface:
jobject
jclass
jthrowable
jstring
jarray
jbooleanArray
jbyteArray
jcharArray
jshortArray
jintArray
jlongArray
jfloatArray
jdoubleArray
jobjectArray
jweak
These types are uintptr on the Go side because they would otherwise
confuse the Go garbage collector; they are sometimes not really
pointers but data structures encoded in a pointer type. All operations
on these types must happen in C. The proper constant to initialize an
empty such reference is 0, not nil.
This special case was introduced in Go 1.10. For auto-updating code
from Go 1.9 and earlier, use the cftype rewrite in the Go fix tool:
These special cases were introduced in Go 1.10. For auto-updating code
from Go 1.9 and earlier, use the cftype or jni rewrites in the Go fix tool:
go tool fix -r cftype <pkg>
go tool fix -r jni <pkg>
It will replace nil with 0 in the appropriate places.