go - cgo调用共享库: cannot find lib or function?

Question

我正在使用 Go 编程语言第 13 章的示例代码，如下所示:

$ cat bzip2.c
#include <bzlib.h>

int bz2compress(bz_stream *s, int action,
                char *in, unsigned *inlen, char *out, unsigned *outlen) {
    s->next_in = in;
    s->avail_in = *inlen;
    s->next_out = out;
    s->avail_out = *outlen;
    int r = BZ2_bzCompress(s, action);
    *inlen -= s->avail_in;
    *outlen -= s->avail_out;
    s->next_in = s->next_out = NULL;
    return r;
}

$ cat usebzip2.go
// Package bzip provides a writer that uses bzip2 compression (bzip.org).
package main
import "C"

import (
    "io"
    "log"
    "os"
    "testing"
    "unsafe"
)

type writer struct {
    w      io.Writer // underlying output stream
    stream *C.bz_stream
    outbuf [64 * 1024]byte
}

// Close flushes the compressed data and closes the stream.
// It does not close the underlying io.Writer.
func (w *writer) Close() error {
    if w.stream == nil {
        panic("closed")
    }
    defer func() {
        C.BZ2_bzCompressEnd(w.stream)
        C.bz2free(w.stream)
        w.stream = nil
    }()
    for {
        inlen, outlen := C.uint(0), C.uint(cap(w.outbuf))
        r := C.bz2compress(w.stream, C.BZ_FINISH, nil, &inlen,
            (*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
        if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
            return err
        }
        if r == C.BZ_STREAM_END {
            return nil
        }
    }
}

// NewWriter returns a writer for bzip2-compressed streams.
func NewWriter(out io.Writer) io.WriteCloser {
    const blockSize = 9
    const verbosity = 0
    const workFactor = 30
    w := &writer{w: out, stream: C.bz2alloc()}
    C.BZ2_bzCompressInit(w.stream, blockSize, verbosity, workFactor)
    return w
}

func main() {
    w := NewWriter(os.Stdout)
    if _, err := io.Copy(w, os.Stdin); err != nil {
        log.Fatalf("bzipper: %v\n", err)
    }
    if err := w.Close(); err != nil {
        log.Fatalf("bzipper: close: %v\n", err)
    }
}

首先我编译 .c 文件:

gcc -I/usr/include -L/usr/lib -lbz2 --shared bzip2.c -fPIC -o libbzip2.so

linux环境LD_LIBRARY_PATH包含“.”，然后go build失败:

go build usebzip2.go
# command-line-arguments
/tmp/go-build677611698/b001/_x002.o: In function `_cgo_22d5d7fabfe4_Cfunc_bz2compress':
/tmp/go-build/cgo-gcc-prolog:118: undefined reference to `bz2compress'
collect2: error: ld returned 1 exit status

那么如何解决呢？我正在使用 ubuntu 18.04 LTS。非常感谢。

Answer 1

不要运行:

go build usebzip2.go

反而:

go build

(并且您不需要直接在 gcc 上调用 bzip2.c )。当你使用这个过程时，你会得到更多(但不同的)错误，因为你没有在之前输入正确的指令:

import "C"

线。您需要一条评论(或一系列评论)告诉 cgo 您打算提供的功能，或内联提供这些功能，并指导链接阶段使用 -lbz2 .特别是，您需要:

#include <bzlib.h>

提供bz2alloc功能

提供bz2free功能

为您的bz2compress 提供声明功能

设置LDFLAGS包括-lbz2

实际 bz2alloc和 bz2free简短而简单，因此可以直接包含在此 header block 中:

package main

/*
#cgo LDFLAGS: -lbz2     
#include <bzlib.h>
#include <stdlib.h>
bz_stream *bz2alloc() { return calloc(1, sizeof(bz_stream)); }
int bz2compress(bz_stream *s, int action,
    char *in, unsigned *intlen, char *out, unsigned *outlen);
void bz2free(bz_stream* s) { free(s); }
*/      
import "C"

如果插入并运行 go build您现在将看到一个不同且更有用的错误:

./usebzip2.go:60:2: cannot use w (type *writer) as type io.WriteCloser in return argument:
        *writer does not implement io.WriteCloser (missing Write method)

这当然是因为 type writer未实现 Write .

(练习 13.3 的完整版本——不是我的——在 https://github.com/torbiak/gopl/tree/master/ex13.3。请注意，他们已经扩充了他们的版本以使用锁定，从而可以安全地同时从多个 goroutine 调用 write 函数。)