CGO C.Bytes Not Work As Expected

[LawyZheng]

What version of Go are you using (go version)?

$ go version  1.17.3

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env centos Output

$ go env
GO111MODULE=""

GOARCH="amd64"

GOBIN=""

GOCACHE="/root/.cache/go-build"

GOENV="/root/.config/go/env"

GOEXE=""

GOEXPERIMENT=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOINSECURE=""

GOMODCACHE="/root/go/pkg/mod"

GONOPROXY=""

GONOSUMDB=""

GOOS="linux"

GOPATH="/root/go"

GOPRIVATE=""

GOPROXY="https://goproxy.cn,direct"

GOROOT="/home/xxxxx/go"

GOSUMDB="sum.golang.org"

GOTMPDIR=""

GOTOOLDIR="/home/xxxxxx/go/pkg/tool/linux_amd64"

GOVCS=""

GOVERSION="go1.17.3"

GCCGO="gccgo"

AR="ar"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD="/home/xxxxxx/cgo-test/go.mod"

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2463505218=/tmp/go-build -gno-record-gcc-switches"

go env windows Output

$ go env
set GO111MODULE=

set GOARCH=amd64

set GOBIN=

set GOCACHE=C:\Users\xxxxxx\AppData\Local\go-build

set GOENV=C:\Users\xxxxxx\AppData\Roaming\go\env

set GOEXE=.exe

set GOEXPERIMENT=

set GOFLAGS=

set GOHOSTARCH=amd64

set GOHOSTOS=windows

set GOINSECURE=

set GOMODCACHE=C:\Users\xxxxxx\go\pkg\mod

set GONOPROXY=

set GONOSUMDB=

set GOOS=windows

set GOPATH=C:\Users\xxxxxx\go

set GOPRIVATE=

set GOPROXY=https://goproxy.cn,direct

set GOROOT=C:\Program Files\Go

set GOSUMDB=sum.golang.org

set GOTMPDIR=

set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64

set GOVCS=

set GOVERSION=go1.17.3

set GCCGO=gccgo

set AR=ar

set CC=gcc

set CXX=g++

set CGO_ENABLED=1

set GOMOD=NUL

set CGO_CFLAGS=-g -O2

set CGO_CPPFLAGS=

set CGO_CXXFLAGS=-g -O2

set CGO_FFLAGS=-g -O2

set CGO_LDFLAGS=-g -O2

set PKG_CONFIG=pkg-config

set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\xxxxxx\AppData\Local\Temp\go-build2918900668=/tmp/go-build -gno-record-gcc-switches

What did you do?

I run a C code with CGO.
Use C.CBytes to Convert Go Bytes to C Bytes.

package main

/*
#include <stdio.h>

static void SayHello(const void* s) {
    puts(s);
}
*/
import "C"

func main() {
    b := []byte("go")
    C.SayHello(C.CBytes(b))
}

What did you expect to see?

Output: go

What did you see instead?

Output: goq∩
The extra char seems to be random. It will change every time I execute the binary.

Environment

I test it in different environments,
In CentOS and Windows, this error happens, while not reproducing in MacOS(M1) and Windows WSL(ubuntu).

What Should I Do

Is it a bug for CGO,
Or am I supposed to pass the length of []bytes to C function?

[ianlancetaylor]

The C function puts takes a pointer to a sequence of char values that is terminated by a NUL byte. The Go expression []byte("go") returns a slice containing two bytes, with no NUL byte. The C.CBytes function takes a byte slice, copies it into C memory as a sequence of char values, and returns a pointer to the sequence. It returns a sequence of exactly the bytes in the slice, and does not add a NUL byte. So you are taking a sequence of char values with no NUL and passing them to a C function that expects a sequence of char values that is terminated with NUL. So you get garbage in the C code.