我有一个C ++ DLL可供C＃使用。我有一个函数，它接受一个字符串传递给它，我在C ++函数参数上设置为const char *，如下所示：
int __stdcall extract_all_frames(const char* szDestination, float scaleFactor)
此函数的主体直接从工作的FFmpeg示例函数复制，所以我几乎可以肯定问题不在那里。我觉得这个问题出现在我对它的修改中：
//Open file
char szFilename[32];
sprintf_s(szFilename, sizeof(szFilename), "frame%d.ppm\0", iFrame);
// JOIN szFILENAME AND szDESTINATION
std::string buffer(szDestination, sizeof(szDestination));
buffer.append("\\");
buffer.append(szDestination);
这应该是一个连接的路径和目录。然后我将buffer.c_str()传递给fopen_s()，const char *而不是std::string。每当从C＃调用此函数时，我都会遇到以下异常：
A first chance exception of type 'System.AccessViolationException' occurred in XRF FFmpeg Invoke Test.exe
Additional information: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
这是完整的代码：
#include "stdafx.h"
#pragma comment (lib, "avcodec.lib")
#pragma comment (lib, "avformat.lib")
#pragma comment (lib, "avutil.lib")
#pragma comment (lib, "swscale.lib")
extern "C"
{
#include <libavcodec\avcodec.h>
#include <libavformat\avformat.h>
#include <libavutil\avutil.h>
#include <libswscale\swscale.h>
}
#include <string>
#include "Xrf.FFmpeg.hpp"
void save_frame(AVFrame* pFrame, int iFrame, const char* szDestination)
{
//Open file
char szFilename[32];
sprintf_s(szFilename, sizeof(szFilename), "frame%d.ppm\0", iFrame);
// JOIN szFILENAME AND szDESTINATION
std::string buffer(szDestination, sizeof(szDestination));
buffer.append("\\");
buffer.append(szDestination);
FILE* pFile;
errno_t openError = fopen_s(&pFile, buffer.c_str(), "wb");
if (pFile == NULL)
{
return;
}
//Write header
int width = pFrame->width;
int height = pFrame->height;
fprintf(pFile, "P6\n%d %d\n255\n", width, height);
//Write pixel data
for (int y = 0; y < height; y++)
{
fwrite(pFrame->data[0] + y * pFrame->linesize[0], 1, width * 3, pFile);
}
// Close file
fclose(pFile);
}
int __stdcall extract_all_frames(const char* szPath, const char* szDestination, float scaleFactor)
{
// Check if scaleFactor is valid
if ((scaleFactor != 0.f) &&
(scaleFactor > 3.f))
{
fprintf(stderr, "Xrf: Scale factor '%f' out of bounds!\nMust be greater than 0, and less then or equal to 3.0.\n", scaleFactor);
return -1;
}
// Register all formats and codecs
av_register_all();
AVFormatContext* pFormatCtx;
if (avformat_open_input(&pFormatCtx, szPath, nullptr, nullptr) != 0)
{
fprintf(stderr, "libavformat: Couldn't open file '%s'!\n", szPath);
return -1;
}
// Retrieve stream information
if (avformat_find_stream_info(pFormatCtx, nullptr) < 0)
{
fprintf(stderr, "libavformat: Unable to find stream information!\n");
return -1;
}
// Dump information about file onto standard error
av_dump_format(pFormatCtx, 0, szPath, 0);
// Find the first video stream
size_t i;
int videoStream = -1;
for (i = 0; i < pFormatCtx->nb_streams; i++)
{
if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO)
{
videoStream = i;
break;
}
}
if (videoStream == -1)
{
fprintf(stderr, "libavformat: No video stream found!\n");
return -1;
}
// Get a pointer to the codec context for the video stream
AVCodecContext* pCodecCtx = pFormatCtx->streams[videoStream]->codec;
// Scale the frame
int scaleHeight = static_cast<int>(floor(pCodecCtx->height * scaleFactor));
int scaleWidth = static_cast<int>(floor(pCodecCtx->width * scaleFactor));
//Check if frame sizes are valid (not 0, because that's dumb)
if (scaleHeight == 0 || scaleWidth == 0)
{
fprintf(stderr, "Xrf: Scale factor caused a zero value in either width or height!\n");
return -1;
}
// Find the decoder for the video stream
AVCodec* pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
if (pCodec == NULL)
{
fprintf(stderr, "libavcodec: Unsupported codec!\n");
return -1; // Codec not found
}
// Open codec
AVDictionary* optionsDict = nullptr;
if (avcodec_open2(pCodecCtx, pCodec, &optionsDict) < 0)
{
fprintf(stderr, "libavcodec: Couldn't open codec '%s'!\n", pCodec->long_name);
return -1;
}
// Allocate video frame
AVFrame* pFrame = av_frame_alloc();
// Allocate an AVFrame structure
AVFrame* pFrameRGB = av_frame_alloc();
if (pFrameRGB == NULL)
{
fprintf(stderr, "libavformat: Unable to allocate a YUV->RGB resampling AVFrame!\n");
return -1;
}
// Determine required buffer size and allocate buffer
int numBytes = avpicture_get_size(PIX_FMT_RGB24, scaleWidth, scaleHeight);
uint8_t* buffer = static_cast <uint8_t *> (av_malloc(numBytes * sizeof(uint8_t)));
struct SwsContext* sws_ctx = sws_getContext(pCodecCtx->width,
pCodecCtx->height,
pCodecCtx->pix_fmt,
scaleWidth,
scaleHeight,
PIX_FMT_RGB24,
SWS_BILINEAR,
nullptr, nullptr, nullptr);
// Assign appropriate parts of buffer to image planes in pFrameRGB
// Note that pFrameRGB is an AVFrame, but AVFrame is a superset
// of AVPicture
avpicture_fill(reinterpret_cast <AVPicture *> (pFrameRGB),
buffer,
PIX_FMT_RGB24,
scaleWidth,
scaleHeight);
// Read frames and save first five frames to disk
AVPacket packet;
int frameFinished;
while (av_read_frame(pFormatCtx, &packet) >= 0)
{
// Is this a packet from the video stream?
if (packet.stream_index == videoStream)
{
// Decode video frame
avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
// Did we get a video frame?
if (frameFinished)
{
// Convert the image from its native format to RGB
sws_scale(sws_ctx,
static_cast <uint8_t const * const *> (pFrame->data),
pFrame->linesize,
0,
pCodecCtx->height,
pFrameRGB->data,
pFrameRGB->linesize);
// Save the frame to disk
if (++i <= 5)
{
save_frame(pFrameRGB, i, szDestination);
}
}
}
// Free the packet that was allocated by av_read_frame
av_free_packet(&packet);
}
av_free(buffer); // Free the RGB image
av_free(pFrameRGB);
av_free(pFrame); // Free the YUV frame
avcodec_close(pCodecCtx); // Close the codec
avformat_close_input(&pFormatCtx); // Close the video file
return 0;
}
我不知道错误是否在我的修改中（很可能是我对C ++非常新）或其他代码，因为异常仅抛出C＃中的调用行，而不是实际的C ++行导致问题。
答案 0 :(得分：1)
这是错误的：
std::string buffer(szDestination, sizeof(szDestination));
szDestination是一个指针，因此sizeof(szDestination)将返回指针大小，以字节为单位，而不是字符数。
如果szDestination是以空字符结尾的字符串，请使用strlen或类似函数来确定字符数。如果它不是以null结尾，那么你需要传递要复制的字节数作为参数。
更好的办法是调用DLL函数时：
int __stdcall extract_all_frames(const char* szPath, const char* szDestination, float scaleFactor)
接受这些指针，立即将它们分配给std::string。然后从那里删除char *或const char *的所有用法。你的帮助函数不需要处理“哑”字符指针。
示例：
int __stdcall extract_all_frames(const char* szPath, const char* szDestination, float scaleFactor)
{
std::string sPath = szPath;
std::string sDestination = sDestination;
// From here, use sPath and sDestination
//...
}
// redefinition of save_frame
//...
void save_frame(AVFrame* pFrame, int iFrame, const std::string& szDestination)
{
//Open file
std::string buffer = "frame" + to_string(iFrame) + ".ppm\0";
buffer.append("\\");
buffer.append(szDestination);
//...
}