程序员的自我修养: fread-C语言是怎样读取文件的

水深无声 2022-07-13 00:46 316阅读 0赞

为了效率的考虑,不至于频繁调用系统函数和访问IO设备,MSVC CRT的fread采用缓冲设计.

C语言提供的关于缓冲的函数有:

int flush(FILE\* stream); int setvbuf(FILE\* stream, char\* buf, int mode, size\_t size); /\* 缓冲模式mode有: 1. 无缓冲模式 \_IONBF 2. 行缓冲模式 \_IOLBF 每收到一个换行符(/n或/r/n), 就将缓冲flush掉 3. 全缓冲模式 \_IOFBF 仅当缓冲满时才进行flush \*/ void setbuf(FILE\* stream, char\* buf); 等价于 (void)setvbuf(stream, buf, \_IOBBF, BUFSIZ);

fread的调用过程大概是:

fread -> fread\_s(增加缓冲溢出保护, 加锁) -> \_fread\_nolock\_s(循环读取,缓冲) -> \_read(换行符转换) -> ReadFile(读取文件)

加注释的FILE结构如下:

struct \_iobuf \{ char \*\_ptr; int \_cnt; //剩余未读的字节数 char \*\_base; //文件的缓冲基址 int \_flag; //打开文件的属性 int \_file; //打开文件的编号 int \_charbuf; int \_bufsiz; //文件的缓冲的总的大小 char \*\_tmpfname; \}; typedef struct \_iobuf FILE;

核心函数\_fread\_nolock\_s(循环读取,缓冲)如下:

size\_t \_\_cdecl \_fread\_nolock\_s( void \*buffer, size\_t bufferSize, size\_t elementSize, size\_t num, FILE \*stream ) \{ char \*data; /\* point inside the destination buffer to where we need to copy the read chars \*/当前放进字节的尾部 size\_t dataSize; /\* space left in the destionation buffer (in bytes) //buffer中剩余字节数\*/ size\_t total; /\* total bytes to read //总共要读的字节数\*/ size\_t count; /\* num bytes left to read //剩下要读的字节数\*/ unsigned streambufsize; /\* size of stream buffer \*/ unsigned nbytes; /\* how much to read now \*/ unsigned nread; /\* how much we did read \*/ int c; /\* a temp char \*/ /\* initialize local vars \*/ data = buffer; dataSize = bufferSize; if (elementSize == 0 || num == 0) \{ return 0; \} /\* validation \*/ \_VALIDATE\_RETURN((buffer != NULL), EINVAL, 0); if (stream == NULL || num > (SIZE\_MAX / elementSize)) \{ if (bufferSize != SIZE\_MAX) \{ memset(buffer, \_BUFFER\_FILL\_PATTERN, bufferSize); \} \_VALIDATE\_RETURN((stream != NULL), EINVAL, 0); \_VALIDATE\_RETURN(num <= (SIZE\_MAX / elementSize), EINVAL, 0); \} count = total = elementSize \* num; if (anybuf(stream)) \{ /\* already has buffer, use its size \*/ streambufsize = stream->\_bufsiz; \} else \{ /\* assume will get \_INTERNAL\_BUFSIZ buffer \*/ streambufsize = \_INTERNAL\_BUFSIZ; \} /\* here is the main loop -- we go through here until we're done \*/ while (count != 0) \{ /\* if the buffer exists and has characters, copy them to user buffer \*/ if (anybuf(stream) && stream->\_cnt != 0) \{ if(stream->\_cnt < 0) \{ \_ASSERTE(("Inconsistent Stream Count. Flush between consecutive read and write", stream->\_cnt >= 0)); stream->\_flag |= \_IOERR; return (total - count) / elementSize; \} /\* how much do we want? \*/ nbytes = (count < (size\_t)stream->\_cnt) ? (unsigned)count : stream->\_cnt; if (nbytes > dataSize) \{ if (bufferSize != SIZE\_MAX) \{ memset(buffer, \_BUFFER\_FILL\_PATTERN, bufferSize); \} \_VALIDATE\_RETURN(("buffer too small", 0), ERANGE, 0) \} memcpy\_s(data, dataSize, stream->\_ptr, nbytes); /\* update stream and amt of data read \*/ count -= nbytes; stream->\_cnt -= nbytes; stream->\_ptr += nbytes; data += nbytes; dataSize -= nbytes; \} else if (count >= streambufsize) \{ /\* If we have more than streambufsize chars to read, get data by calling read with an integral number of bufsiz blocks. Note that if the stream is text mode, read will return less chars than we ordered. \*/ if (streambufsize) \{ /\* In 64bit apps size\_t is bigger than unsigned \* (which is 32bit even in 64 bit machines), so \* we need to split the read into INT\_MAX chunks \* since \_read() only support up to \_signed\_ int \* (even though the in parameter is unsigned). \*/ if (count > INT\_MAX) \{ /\* calc chars to read -- the largest multiple of streambufsize \* smaller then INT\_MAX \*/ nbytes = (unsigned)(INT\_MAX - INT\_MAX % streambufsize); \} else \{ /\* calc chars to read -- (count/streambufsize) \* streambufsize \*/ nbytes = (unsigned)(count - count % streambufsize); \} \} else \{ nbytes = (count > INT\_MAX)?(unsigned)INT\_MAX: (unsigned)count; \} if (nbytes > dataSize) \{ if (bufferSize != SIZE\_MAX) \{ memset(buffer, \_BUFFER\_FILL\_PATTERN, bufferSize); \} \_VALIDATE\_RETURN(("buffer too small", 0), ERANGE, 0) \} nread = \_read(\_fileno(stream), data, nbytes); if (nread == 0) \{ /\* end of file -- out of here \*/ stream->\_flag |= \_IOEOF; return (total - count) / elementSize; \} else if (nread == (unsigned)-1) \{ /\* error -- out of here \*/ stream->\_flag |= \_IOERR; return (total - count) / elementSize; \} /\* update count and data to reflect read \*/ count -= nread; data += nread; dataSize -= nread; \} else \{ /\* less than streambufsize chars to read, so call \_filbuf to fill buffer \*/ if ((c = \_filbuf(stream)) == EOF) \{ /\* error or eof, stream flags set by \_filbuf \*/ return (total - count) / elementSize; \} /\* \_filbuf returned a char -- store it \*/ if (dataSize == 0) \{ if (bufferSize != SIZE\_MAX) \{ memset(buffer, \_BUFFER\_FILL\_PATTERN, bufferSize); \} \_VALIDATE\_RETURN(("buffer too small", 0), ERANGE, 0) \} \*data++ = (char) c; --count; --dataSize; /\* update buffer size \*/ streambufsize = stream->\_bufsiz; \} \} /\* we finished successfully, so just return num \*/ return num; \}

其中,

int \_\_cdecl \_filwbuf ( FILE \*str ) \#endif /\* \_UNICODE \*/ \{ REG1 FILE \*stream=NULL; /\* In safecrt, we assume we always have a buffer \*/ \_VALIDATE\_RETURN(str != NULL, EINVAL, \_TEOF); /\* Init pointer to \_iob2 entry. \*/ stream = str; if (!inuse(stream) || stream->\_flag & \_IOSTRG) return(\_TEOF); if (stream->\_flag & \_IOWRT) \{ stream->\_flag |= \_IOERR; return(\_TEOF); \} stream->\_flag |= \_IOREAD; /\* Get a buffer, if necessary. \*/ if (!anybuf(stream)) \{ \#ifndef \_SAFECRT\_IMPL \_getbuf(stream); \#else /\* \_SAFECRT\_IMPL \*/ /\* In safecrt, we assume we always have a buffer \*/ \_VALIDATE\_RETURN(FALSE, EINVAL, \_TEOF); \#endif /\* \_SAFECRT\_IMPL \*/ \} else \{ stream->\_ptr = stream->\_base; \} stream->\_cnt = \_read(\_fileno(stream), stream->\_base, stream->\_bufsiz); \#ifndef \_UNICODE if ((stream->\_cnt == 0) || (stream->\_cnt == -1)) \{ \#else /\* \_UNICODE \*/ if ((stream->\_cnt == 0) || (stream->\_cnt == 1) || stream->\_cnt == -1) \{ \#endif /\* \_UNICODE \*/ stream->\_flag |= stream->\_cnt ? \_IOERR : \_IOEOF; stream->\_cnt = 0; return(\_TEOF); \} if ( !(stream->\_flag & (\_IOWRT|\_IORW)) && ((\_osfile\_safe(\_fileno(stream)) & (FTEXT|FEOFLAG)) == (FTEXT|FEOFLAG)) ) stream->\_flag |= \_IOCTRLZ; /\* Check for small \_bufsiz (\_SMALL\_BUFSIZ). If it is small and if it is our buffer, then this must be the first \_filbuf after an fseek on a read-access-only stream. Restore \_bufsiz to its larger value (\_INTERNAL\_BUFSIZ) so that the next \_filbuf call, if one is made, will fill the whole buffer. \*/ if ( (stream->\_bufsiz == \_SMALL\_BUFSIZ) && (stream->\_flag & \_IOMYBUF) && !(stream->\_flag & \_IOSETVBUF) ) \{ stream->\_bufsiz = \_INTERNAL\_BUFSIZ; \} \#ifndef \_UNICODE stream->\_cnt--; return(0xff & \*stream->\_ptr++); \#else /\* \_UNICODE \*/ stream->\_cnt -= sizeof(wchar\_t); return (0xffff & \*((wchar\_t \*)(stream->\_ptr))++); \#endif /\* \_UNICODE \*/ \}

代码中分了三种情况:

1) 缓冲区不为空

此时, 把缓冲区中的数据复制到传入的字符数组中.

2) 缓冲区为空, 需要读取的数据大于缓冲的尺寸

此时, 直接调用函数\_fread把文件中的内容写到传入的字符数组中.

3) 缓冲区为空, 需要读取的数据不大于缓冲的尺寸

此时, 调用函数\_fread读满缓冲区, 并再写缓冲区的一个字符到传入的字符数组中.

若未读满传入的字符数组, 循环执行上述1~3过程, 直到读满或读到文件末尾(EOF).