从C＃使用C ++ DLL时出现AccessViolationException [英] AccessViolationException when using C++ DLL from C#
问题描述
我有一个C ++ DLL供C＃使用。我有一个传递给它的字符串的函数，我在C ++函数参数上将它们设置为 const char * 像这样：
I have a C++ DLL for use from C#. I have a function which takes a string passed to it, and I have those set on the C++ function parameters as const char * like so:
int __stdcall extract_all_frames(const char* szDestination, float scaleFactor)
此函数的主体直接从工作的FFmpeg示例函数复制而来，因此我几乎可以确定问题不存在。我觉得问题出在我对它所做的修改中：
The main body of this function is copied directly from a working FFmpeg example function so I'm almost certain the problem isn't there. I feel like the problem is in this modification I made to it:
//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);
应该是串联的路径和目录。然后，我将 buffer.c_str（）传递到 fopen_s（）中，这需要 const char * 不是 std :: string 。每当从C＃调用此函数时，都会出现以下异常：
Which is supposed to be a concatenated path and directory. I then pass buffer.c_str() into fopen_s(), which takes const char * not std::string. Whenever calling this function from C#, I get the following exception:
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 ++或其他代码非常陌生，因为异常仅会引发C＃的调用行，而不是引发问题的实际C ++行。
I don't know if the error is in my modification (most likely, I'm extremely new to C++), or the other code, as the exception only throws on the invocation line in C#, not the actual C++ line causing the problem.
推荐答案
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。
szDestination is a pointer, thus sizeof(szDestination) will return the pointer size, in bytes, not the number of characters.
如果 szDestination 是一个以null结尾的字符串，请使用 strlen 或类似的函数来确定字符数。如果它不是以null结尾的，那么您需要传递要复制的字节数作为参数。
If szDestination is a null terminated string, use strlen or similar function to determine the number of characters. If it isn't null terminated, then you need to pass the number of bytes to copy as a parameter.
更好的方法是调用DLL函数时：
The better thing to do is when your DLL function is called:
int __stdcall extract_all_frames(const char* szPath, const char* szDestination, float scaleFactor)
获取这些指针，并立即将它们分配给 std :: string 。然后从此处删除char *或const char *的所有用法。不需要您的辅助函数来处理哑字符指针。
take those pointers and immediately assign them to std::string. Then drop all usage of char* or const char* from there. There is no need for your helper functions to deal with "dumb" character pointers.
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);
//...
}
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