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cmd/cgo: update documentation on implementation details

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Change-Id: Iec771d5bbdf510b6c5ec17a614da90e7974a6348
Reviewed-on: https://go-review.googlesource.com/59870
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Hiroshi Ioka 2017-08-29 17:28:15 +09:00 committed by Ian Lance Taylor
parent b793372125
commit ec04f107ad
1 changed files with 65 additions and 32 deletions
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97
src/cmd/cgo/doc.go
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@ -406,21 +406,24 @@ about simple #defines for constants and the like. These are recorded
for later use.
Next, cgo needs to identify the kinds for each identifier. For the
identifiers C.foo and C.bar, cgo generates this C program:
identifiers C.foo, cgo generates this C program:
<preamble>
#line 1 "not-declared"
void __cgo_f_xxx_1(void) { __typeof__(foo) *__cgo_undefined__; }
void __cgo_f_1_1(void) { __typeof__(foo) *__cgo_undefined__1; }
#line 1 "not-type"
void __cgo_f_xxx_2(void) { foo *__cgo_undefined__; }
#line 1 "not-const"
void __cgo_f_xxx_3(void) { enum { __cgo_undefined__ = (foo)*1 }; }
#line 2 "not-declared"
void __cgo_f_xxx_1(void) { __typeof__(bar) *__cgo_undefined__; }
#line 2 "not-type"
void __cgo_f_xxx_2(void) { bar *__cgo_undefined__; }
#line 2 "not-const"
void __cgo_f_xxx_3(void) { enum { __cgo_undefined__ = (bar)*1 }; }
void __cgo_f_1_2(void) { foo *__cgo_undefined__2; }
#line 1 "not-int-const"
void __cgo_f_1_3(void) { enum { __cgo_undefined__3 = (foo)*1 }; }
#line 1 "not-num-const"
void __cgo_f_1_4(void) { static const double __cgo_undefined__4 = (foo); }
#line 1 "not-str-lit"
void __cgo_f_1_5(void) { static const char __cgo_undefined__5[] = (foo); }
#line 1 "not-signed-int-const"
#if 0 < -(foo)
#line 1 "not-signed-int-const"
#error found unsigned int
#endif
This program will not compile, but cgo can use the presence or absence
of an error message on a given line to deduce the information it
@ -430,45 +433,72 @@ errors that might stop parsing early.
An error on not-declared:1 indicates that foo is undeclared.
An error on not-type:1 indicates that foo is not a type (if declared at all, it is an identifier).
An error on not-const:1 indicates that foo is not an integer constant.
An error on not-int-const:1 indicates that foo is not an integer constant.
An error on not-num-const:1 indicates that foo is not a number constant.
An error on not-str-lit:1 indicates that foo is not a string literal.
An error on not-signed-int-const:1 indicates that foo is not a signed integer constant.
The line number specifies the name involved. In the example, 1 is foo and 2 is bar.
The line number specifies the name involved. In the example, 1 is foo.
Next, cgo must learn the details of each type, variable, function, or
constant. It can do this by reading object files. If cgo has decided
that t1 is a type, v2 and v3 are variables or functions, and c4, c5,
and c6 are constants, it generates:
that t1 is a type, v2 and v3 are variables or functions, and i4, i5
are integer constants, u6 is an unsigned integer constant, and f7 and f8
are float constants, and s9 and s10 are string constants, it generates:
<preamble>
__typeof__(t1) *__cgo__1;
__typeof__(v2) *__cgo__2;
__typeof__(v3) *__cgo__3;
__typeof__(c4) *__cgo__4;
enum { __cgo_enum__4 = c4 };
__typeof__(c5) *__cgo__5;
enum { __cgo_enum__5 = c5 };
__typeof__(c6) *__cgo__6;
enum { __cgo_enum__6 = c6 };
__typeof__(i4) *__cgo__4;
enum { __cgo_enum__4 = i4 };
__typeof__(i5) *__cgo__5;
enum { __cgo_enum__5 = i5 };
__typeof__(u6) *__cgo__6;
enum { __cgo_enum__6 = u6 };
__typeof__(f7) *__cgo__7;
__typeof__(f8) *__cgo__8;
__typeof__(s9) *__cgo__9;
__typeof__(s10) *__cgo__10;
long long __cgo_debug_data[] = {
long long __cgodebug_ints[] = {
0, // t1
0, // v2
0, // v3
c4,
c5,
c6,
i4,
i5,
u6,
0, // f7
0, // f8
0, // s9
0, // s10
1
};
double __cgodebug_floats[] = {
0, // t1
0, // v2
0, // v3
0, // i4
0, // i5
0, // u6
f7,
f8,
0, // s9
0, // s10
1
};
const char __cgodebug_str__9[] = s9;
const unsigned long long __cgodebug_strlen__9 = sizeof(s9)-1;
const char __cgodebug_str__10[] = s10;
const unsigned long long __cgodebug_strlen__10 = sizeof(s10)-1;
and again invokes the system C compiler, to produce an object file
containing debug information. Cgo parses the DWARF debug information
for __cgo__N to learn the type of each identifier. (The types also
distinguish functions from global variables.) If using a standard gcc,
cgo can parse the DWARF debug information for the __cgo_enum__N to
learn the identifier's value. The LLVM-based gcc on OS X emits
incomplete DWARF information for enums; in that case cgo reads the
constant values from the __cgo_debug_data from the object file's data
segment.
distinguish functions from global variables.) Cgo reads the constant
values from the __cgodebug_* from the object file's data segment.
At this point cgo knows the meaning of each C.xxx well enough to start
the translation process.
@ -553,9 +583,12 @@ _cgo_main.c:
int main() { return 0; }
void crosscall2(void(*fn)(void*, int, uintptr_t), void *a, int c, uintptr_t ctxt) { }
uintptr_t _cgo_wait_runtime_init_done() { }
uintptr_t _cgo_wait_runtime_init_done() { return 0; }
void _cgo_release_context(uintptr_t ctxt) { }
char* _cgo_topofstack(void) { return (char*)0; }
void _cgo_allocate(void *a, int c) { }
void _cgo_panic(void *a, int c) { }
void _cgo_reginit(void) { }
The extra functions here are stubs to satisfy the references in the C
code generated for gcc. The build process links this stub, along with