我正在尝试创建一个嵌入FFMPEG功能的C ++程序。为此，我举了一个FFMPEG示例，并使用g ++对其进行了重新编译（仅在需要时添加extern“ C”）。编译成功，但未定义符号的链接失败。
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，我发现实际上没有名称混乱，因此外部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“缺失”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
用C编译时，它可以完美地工作（当然没有外部C），这让我想知道我缺少什么？
在下面找到FFMPEG示例代码以供参考（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;
}
感谢@WhozCraig帮助我弄清楚了。链接时我以为我保留了完整的库符号时使用了错误的标志，但这是错误的标志。因此，链接器只通过了一次，对象顺序错误（因为我自然认为这没关系）要保留完整的库符号，请将--whole-archive传递给链接器：
-Wl,--whole-archive