cmd/cgo: definition of GoString is incorrect for exported functions

[mirtchovski]

$ go version
go version devel +45468e9eae1f Wed May 22 18:51:15 2013 +0800 darwin/amd64

What steps will reproduce the problem?
Following Minux's suggested steps:

$ mkdir -p  /tmp/gopath/src/cgotest
$ cd /tmp/gopath/src/cgotest
$ cat > main.go <<EOF
package main

import (
        "fmt"
        "unsafe"
)

/*
        #include <stdlib.h>
        void my_printf(char* str);
*/
import "C"

//export hello_from_c
func hello_from_c(s string, i int) {
        fmt.Printf("From C: %s, %d\n", s, i)
}

func main() {
        cstr := C.CString("GoTest")
        defer C.free(unsafe.Pointer(cstr))
        C.my_printf(cstr)
}
EOF
$ cat > part.c <<EOF
#include <_cgo_export.h>
#include <stdio.h>
void my_printf(char *s){
        printf("From Go: %s\n", s);

        GoString d;
        d.p = "C Test";
        d.n = 7;

        // printf("C STRING: %s\n", d.p);
        // fflush(stdout);

        hello_from_c(d, 42);
}
EOF
$ GOPATH=/tmp/gopath go install cgotest
$ /tmp/gopath/bin/cgotest


What is the expected output?
From Go: GoTest
From C: C Test, 42

What do you see instead?
http://play.golang.org/p/QsokZKHkRr

Which operating system are you using?
OSX

[mirtchovski]

Comment 1:

from the go-nuts thread on the subject:
if the code is modified to http://play.golang.org/p/J6fUEF4E_9, then the function works
just fine, but if we replace strlen(t) on line 10 with the value it returns, 6, the
resulting program crashes again. putting a panic() inside hello_from_c in main.go gives
this interesting observation:
with strlen(t):
main.hello_from_c(0x40680b5, 0x6, 0x2a)
cgotest/_obj/main.cgo1.go:17 +0x119
with "6" instead:
[fp=0x41d1e30] main.hello_from_c(0x40680bd, 0x7fff00000006, 0x2a)
cgotest/_obj/main.cgo1.go:16 +0xdd

[mirtchovski]

Comment 2:

Ian Lance Taylor:
I see the bug.  _cgo_export.c looks like this:
void hello_from_c(GoString p0, GoInt p1)
{
        struct {
                GoString p0;
                GoInt p1;
        } __attribute__((packed)) a;
        a.p0 = p0;
        a.p1 = p1;
        crosscall2(_cgoexp_44bb5d457081_hello_from_c, &a, 24);
}
_cgo_export.h has this:
typedef struct { char *p; int n; } GoString;
This means that the generated hello_from_c code is going to convert
from GoInt to int when setting the string length of p0.  That will
break if it so happens that the upper 32 bits of the string length are
not already zero.

[ianlancetaylor]

Comment 3:

Labels changed: added priority-soon, go1.1.1, removed priority-triage.

Owner changed to @ianlancetaylor.

Status changed to Accepted.

[minux]

Comment 4:

the fix is easy (don't forget to also fix GoSlice), but i wonder
if we can write a good test case?

[ianlancetaylor]

Comment 5:

https://golang.org/cl/9643044

Status changed to Started.

[ianlancetaylor]

Comment 6:

This issue was closed by revision 8f7863d.

Status changed to Fixed.

[ianlancetaylor]

Comment 7:

Labels changed: removed go1.1.1.

[ianlancetaylor]

Comment 8:

Issue #5646 has been merged into this issue.