可变长度结构 [英] Variable length structures

问题描述

OMX提供了一个结构，定义如下

OMX provides a struct with following definition

/* Parameter specifying the content URI to use. */
typedef struct OMX_PARAM_CONTENTURITYPE
{
OMX_U32 nSize;
/**< size of the structure in bytes */
OMX_VERSIONTYPE nVersion; /**< OMX specification version information */
OMX_U8 contentURI[1];     /**< The URI name*/
}OMX_PARAM_CONTENTURITYPE;
OMX_IndexParamContentURI,
/**< The URI that identifies the target content. Data type is OMX_PARAM_CONTENTURITYPE. */

我有一个恒定的字符数组来设置。

I have got a constant char array to set.

char* filename = "/test.bmp";

据我的理解，我需要以某种方式的存储器复制文件名设置为struct.contentURI，然后相应地更新struct.size。我该怎么办呢？

As far as I understood I need somehow to set memcopy filename to struct.contentURI and then to update the struct.size accordingly. How can I do it?

最好的问候

推荐答案

首先，你需要分配足够的内存来包含固定大小部分和文件名：

First you need to allocate enough memory to contain the fixed-size parts and the filename:

size_t uri_size = strlen(filename) + 1;
size_t param_size = sizeof(OMX_PARAM_CONTENTURITYPE) + uri_size - 1;
OMX_PARAM_CONTENTURITYPE * param = malloc(param_size);

加1，以包括终止字符，并减去1，因为结构已经包含一个字节的阵列。

adding 1 to include the termination character, and subtracting 1 because the structure already contains an array of one byte.

在C ++中，你需要一个演员，你应该使用智能指针或异常安全的载体：

In C++, you'll need a cast, and you should use a smart pointer or a vector for exception safety:

std::vector<char> memory(param_size);
OMX_PARAM_CONTENTURITYPE * param = 
    reinterpret_cast<OMX_PARAM_CONTENTURITYPE *>(&memory[0]);

然后就可以在字段中填入：

Then you can fill in the fields:

param->nSize = param_size;
param->nVersion = whatever;
memcpy(param->contentURI, filename, uri_size);

和不要忘了免费（参数）一旦你完成了它。

and don't forget to free(param) once you've finished with it.

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