无法使用g++用C++代码编译C库

Question

我正在尝试创建一个嵌入 FFMPEG 功能的 C++ 程序。为此，我采用了一个 FFMPEG 示例并按原样使用 g++ 重新编译它（只在需要的地方添加 extern "C" ）。编译成功，但未定义的符号链接失败。

cc -I/usr/include/x86_64-linux-gnu -Wall -g -fPIC  -c -o bin/decode_video.o decode_video.cpp

cc -Wl,--export-dynamic,--no-undefined -L /usr/lib/x86_64-linux-gnu/ -shared -lavdevice -lavutil -lavcodec  bin/decode_video.o -o decode_video.so

bin/decode_video.o: In function `decode(AVCodecContext*, AVFrame*, AVPacket*, char const*)':
/home/admin1/ffmpeg_examples/decode_video.cpp:60: undefined reference to `avcodec_send_packet'
/home/admin1/ffmpeg_examples/decode_video.cpp:67: undefined reference to `avcodec_receive_frame'
bin/decode_video.o: In function `main':
/home/admin1/ffmpeg_examples/decode_video.cpp:108: undefined reference to `av_packet_alloc'
/home/admin1/ffmpeg_examples/decode_video.cpp:116: undefined reference to `avcodec_find_decoder'
/home/admin1/ffmpeg_examples/decode_video.cpp:122: undefined reference to `av_parser_init'
/home/admin1/ffmpeg_examples/decode_video.cpp:128: undefined reference to `avcodec_alloc_context3'
/home/admin1/ffmpeg_examples/decode_video.cpp:139: undefined reference to `avcodec_open2'
/home/admin1/ffmpeg_examples/decode_video.cpp:150: undefined reference to `av_frame_alloc'
/home/admin1/ffmpeg_examples/decode_video.cpp:165: undefined reference to `av_parser_parse2'
/home/admin1/ffmpeg_examples/decode_video.cpp:184: undefined reference to `av_parser_close'
/home/admin1/ffmpeg_examples/decode_video.cpp:185: undefined reference to `avcodec_free_context'
/home/admin1/ffmpeg_examples/decode_video.cpp:186: undefined reference to `av_frame_free'
/home/admin1/ffmpeg_examples/decode_video.cpp:187: undefined reference to `av_packet_free'

检查 nm 我发现没有名称修改在起作用，所以 extern C 似乎运行良好：

nm bin/decode_video.o 

                 U avcodec_alloc_context3
                 U avcodec_find_decoder
                 U avcodec_free_context
                 U avcodec_open2
                 U avcodec_receive_frame
                 U avcodec_send_packet
                 U av_frame_alloc
                 U av_frame_free
                 U av_packet_alloc
                 U av_packet_free
                 U av_parser_close
                 U av_parser_init
                 U av_parser_parse2
                 U exit
                 U fclose
                 U feof
                 U fflush
                 U fopen
                 U fprintf
                 U fread
                 U fwrite
                 U _GLOBAL_OFFSET_TABLE_
000000000000026f T main
                 U memset
                 U printf
                 U snprintf
                 U __stack_chk_fail
                 U stderr
                 U stdout
00000000000000a3 t _ZL6decodeP14AVCodecContextP7AVFrameP8AVPacketPKc
0000000000000000 t _ZL8pgm_savePhiiiPc

检查库，对于 'missing' 个符号之一，我可以看到它已实现

nm /usr/lib/x86_64-linux-gnu/libavcodec.a | grep avcodec_send_packet

00000000000033e0 T avcodec_send_packet
                 U avcodec_send_packet

在 C 中编译时它工作得很好（当然没有 extern C）这让我想知道我错过了什么？

下面是 FFMPEG 示例代码供参考 (decode_video.cpp):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

extern "C"
{
#include <libavcodec/avcodec.h>
}

#define INBUF_SIZE 4096

static void pgm_save(unsigned char *buf, int wrap, int xsize, int ysize,
                     char *filename)
{
    FILE *f;
    int i;

    f = fopen(filename,"w");
    fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);
    for (i = 0; i < ysize; i++)
        fwrite(buf + i * wrap, 1, xsize, f);
    fclose(f);
}

static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,
                   const char *filename)
{
    char buf[1024];
    int ret;

    ret = avcodec_send_packet(dec_ctx, pkt);
    if (ret < 0) {
        fprintf(stderr, "Error sending a packet for decoding\n");
        exit(1);
    }

    while (ret >= 0) {
        ret = avcodec_receive_frame(dec_ctx, frame);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return;
        else if (ret < 0) {
            fprintf(stderr, "Error during decoding\n");
            exit(1);
        }

        printf("saving frame %3d\n", dec_ctx->frame_number);
        fflush(stdout);

        /* the picture is allocated by the decoder. no need to
           free it */
        snprintf(buf, sizeof(buf), "%s-%d", filename, dec_ctx->frame_number);
        pgm_save(frame->data[0], frame->linesize[0],
                 frame->width, frame->height, buf);
    }
}

int main(int argc, char **argv)
{
    const char *filename, *outfilename;
    const AVCodec *codec;
    AVCodecParserContext *parser;
    AVCodecContext *c= NULL;
    FILE *f;
    AVFrame *frame;
    uint8_t inbuf[INBUF_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
    uint8_t *data;
    size_t   data_size;
    int ret;
    AVPacket *pkt;

    if (argc <= 2) {
        fprintf(stderr, "Usage: %s <input file> <output file>\n"
                "And check your input file is encoded by mpeg1video please.\n", argv[0]);
        exit(0);
    }
    filename    = argv[1];
    outfilename = argv[2];

    pkt = av_packet_alloc();
    if (!pkt)
        exit(1);

    /* set end of buffer to 0 (this ensures that no overreading happens for damaged MPEG streams) */
    memset(inbuf + INBUF_SIZE, 0, AV_INPUT_BUFFER_PADDING_SIZE);

    /* find the MPEG-1 video decoder */
    codec = avcodec_find_decoder(AV_CODEC_ID_MPEG1VIDEO);
    if (!codec) {
        fprintf(stderr, "Codec not found\n");
        exit(1);
    }

    parser = av_parser_init(codec->id);
    if (!parser) {
        fprintf(stderr, "parser not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);
    if (!c) {
        fprintf(stderr, "Could not allocate video codec context\n");
        exit(1);
    }

    /* For some codecs, such as msmpeg4 and mpeg4, width and height
       MUST be initialized there because this information is not
       available in the bitstream. */

    /* open it */
    if (avcodec_open2(c, codec, NULL) < 0) {
        fprintf(stderr, "Could not open codec\n");
        exit(1);
    }

    f = fopen(filename, "rb");
    if (!f) {
        fprintf(stderr, "Could not open %s\n", filename);
        exit(1);
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate video frame\n");
        exit(1);
    }

    while (!feof(f)) {
        /* read raw data from the input file */
        data_size = fread(inbuf, 1, INBUF_SIZE, f);
        if (!data_size)
            break;

        /* use the parser to split the data into frames */
        data = inbuf;
        while (data_size > 0) {
            ret = av_parser_parse2(parser, c, &pkt->data, &pkt->size,
                                   data, data_size, AV_NOPTS_VALUE, AV_NOPTS_VALUE, 0);
            if (ret < 0) {
                fprintf(stderr, "Error while parsing\n");
                exit(1);
            }
            data      += ret;
            data_size -= ret;

            if (pkt->size)
                decode(c, frame, pkt, outfilename);
        }
    }

    /* flush the decoder */
    decode(c, frame, NULL, outfilename);

    fclose(f);

    av_parser_close(parser);
    avcodec_free_context(&c);
    av_frame_free(&frame);
    av_packet_free(&pkt);

    return 0;
}

Answer 1

感谢@WhozCraig 帮我弄明白了。我在链接时使用了错误的标志，以为我保留了完整的库符号，但这是错误的标志。所以链接器做了一次，对象顺序是错误的（因为我自然认为这无关紧要）要保留完整的库符号，请将 --whole-archive 传递给链接器：

-Wl,--whole-archive

无法使用g++用C++代码编译C库

Can't compile C library with C++ code using g++

c

c++

ffmpeg

g++

cc