void *logctx= ap && ap->prealloced_context ? *ic_ptr : NULL; // 因為 ap == NULL, 所以 logctx 也 == NULL.
if (!fmt && (err = ff_probe_input_buffer(&pb, &fmt, filename, logctx, 0, logctx ? (*ic_ptr)->probesize : 0)) < 0) {
goto fail;
}
// fmt == NULL 時才執(zhí)行 ff_probe_input_buffer 函數(shù), 因為 fmt 就等于NULL, 成立.
ff_probe_input_buffer函數(shù)的原型:
int ff_probe_input_buffer(ByteIOContext **pb, // 字節(jié)IO上下文, 執(zhí)行url_fopen得到的
AVInputFormat **fmt, // 輸出參數(shù): 輸入的格式
const char *filename, // 文件名
void *logctx, // NULL
unsigned int offset, // 0
unsigned int max_probe_size) // 0
關(guān)鍵的代碼片斷:
/* 讀待探測的數(shù)據(jù) */
buf = av_realloc(buf, probe_size + AVPROBE_PADDING_SIZE);
if ((ret = get_buffer(*pb, buf + buf_offset, probe_size - buf_offset)) < 0) {
/* fail if error was not end of file, otherwise, lower score */
if (ret != AVERROR_EOF) {
av_free(buf);
return ret;
}
score = 0;
ret = 0; /* error was end of file, nothing read */
}
pd.buf_size += ret;
pd.buf = &buf[offset];
memset(pd.buf + pd.buf_size, 0, AVPROBE_PADDING_SIZE);
/* 猜測文件格式 */
*fmt = av_probe_input_format2(&pd, 1, &score);
get_buffer函數(shù), 有兩處比較關(guān)鍵:
int get_buffer(ByteIOContext *s, unsigned char *buf, int size);
{
/* 省略部分代碼 */ /* 讀包 */
if(s->read_packet)
len = s->read_packet(s->opaque, buf, size);
/* 省略部分代碼 */
/* 填充緩沖 */
fill_buffer(s);
/* 省略部分代碼 */
}
fill_buffer函數(shù), 有一處比較關(guān)鍵:
static void fill_buffer(ByteIOContext *s)
{ /* 省略部分代碼 */ /* 讀包 */
if(s->read_packet)
len = s->read_packet(s->opaque, dst, len);
/* 省略部分代碼 */
}
好了, 到第二次探測輸入格式的地方了:
*fmt = av_probe_input_format2(&pd, 1, &score);
進入av_probe_input_format2函數(shù):
AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)
{
AVInputFormat *fmt1, *fmt;
int score;
fmt = NULL;
for(fmt1 = first_iformat; fmt1 != NULL; fmt1 = fmt1->next) {
if (!is_opened == !(fmt1->flags & AVFMT_NOFILE))
continue;
/* 這次 is_opened == 1, fmt1->flags設置AVFMT_NOFILE標志才時成立 */
/* 由于 h264_demuxer.flags == AVFMT_GENERIC_INDEX, 所以上面不成立, 繼續(xù)執(zhí)行 */ score = 0;
if (fmt1->read_probe) {
score = fmt1->read_probe(pd); /* 調(diào)用h264_demuxer.h264_probe */
} else if (fmt1->extensions) {
if (av_match_ext(pd->filename, fmt1->extensions)) { /* 文件名和格式擴展名的匹配 */
/* h264_demuxer.extensions = "h26l,h264,264" */ score = 50;
}
}
if (score > *score_max) {
*score_max = score;
fmt = fmt1;
}else if (score == *score_max)
fmt = NULL;
}
return fmt;
}
av_match_ext函數(shù):
int av_match_ext(const char *filename, const char *extensions)
{
const char *ext, *p;
char ext1[32], *q;
if(!filename)
return 0;
ext = strrchr(filename, '.');
if (ext) {
ext++;
p = extensions;
for(;;) {
q = ext1;
while (*p != '' && *p != ',' && q-ext1<sizeof(ext1)-1)
*q++ = *p++;
*q = '';
if (!strcasecmp(ext1, ext))
return 1;
if (*p == '')
break;
p++;
}
}
return 0;
}
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