2011-10-10 0:24:49

最新推荐文章于 2025-09-13 12:47:04 发布

原创最新推荐文章于 2025-09-13 12:47:04 发布 · 635 阅读

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CC 4.0 BY-SA版权

文章标签：

#file #access #protocols #null #struct #url

本文详细解析了libavformat中的file协议实现，包括文件的打开、读取、写入、定位和关闭操作。

2011-10-10 0:24:49

libavformat/file.c文件的file协议:

static int file_open(URLContext *h, const char *filename, int flags)
{
   int access;
   int fd;

   av_strstart(filename, "file:", &filename);

   if (flags & URL_RDWR) {
   access = O_CREAT | O_TRUNC | O_RDWR;
   } else if (flags & URL_WRONLY) {
   access = O_CREAT | O_TRUNC | O_WRONLY;
   } else {
   access = O_RDONLY;
   }
#ifdef O_BINARY
   access |= O_BINARY;
#endif
   fd = open(filename, access, 0666);
   if (fd == -1)
   return AVERROR(errno);
   h->priv_data = (void *) (intptr_t) fd;
   return 0;
}

文件打开

static int file_read(URLContext *h, unsigned char *buf, int size)
{
   int fd = (intptr_t) h->priv_data;
   return read(fd, buf, size);
}
文件读取

static int file_write(URLContext *h, unsigned char *buf, int size)
{
   int fd = (intptr_t) h->priv_data;
   return write(fd, buf, size);
}
文件写

/* XXX: use llseek */
static int64_t file_seek(URLContext *h, int64_t pos, int whence)
{
   int fd = (intptr_t) h->priv_data;
   if (whence == AVSEEK_SIZE) {
   struct stat st;
   int ret = fstat(fd, &st);
   return ret < 0 ? AVERROR(errno) : st.st_size;
   }
   return lseek(fd, pos, whence);
}

static int file_close(URLContext *h)
{
   int fd = (intptr_t) h->priv_data;
   return close(fd);
}

static int file_get_handle(URLContext *h)
{
   return (intptr_t) h->priv_data;
}

URLProtocol file_protocol = {
   "file",
   file_open,
   file_read,
   file_write,
   file_seek,
   file_close,
   .url_get_file_handle = file_get_handle,
};

libavformat/allformats.c文件的av_register_all函数注册了file协议:
#define REGISTER_PROTOCOL(X,x) { \
   extern URLProtocol x##_protocol; \
   if(CONFIG_##X##_PROTOCOL) av_register_protocol(&x##_protocol); }

void av_register_all(void)
{
   /* 省略部分代码 */
   /* protocols */
   REGISTER_PROTOCOL (FILE, file);
   /* 省略部分代码 */
}

把注册协议函数也贴出来吧:

URLProtocol *first_protocol = NULL;

int av_register_protocol(URLProtocol *protocol)
{
   URLProtocol **p;
   p = &first_protocol;
   while (*p != NULL) p = &(*p)->next;
   *p = protocol;
   protocol->next = NULL;
   return 0;
}