C5 ：标准IO库
在第三章中，所有IO函数都是围绕文件描述符展开，文件描述符用于后续IO操作。由于文件描述符相关的操作是不带缓冲的IO，需要操作者本人指定缓冲区分配、IO长度等，对设备环境要求一定的了解。
本章引入文件流概念，IO库将处理很多底层IO细节。
1 缓冲
提供缓冲的目的是尽量减少read和write的调用次数。标准IO提供了以下3种缓冲：
1.1 全缓冲
在填满标准IO缓冲区后再进行实际IO操作，标准IO函数通常使用malloc获取使用的缓冲区
1.2 行缓冲
在输入和输出中遇到换行符时，标准IO才执行IO操作。对于行缓冲有2个限制
- 标准IO库用来收集每一行的缓冲区长度固定，所以只要是填满了缓冲区，就会进行IO操作
- 只要是通过标准IO从一个不带缓冲的流或者一个行缓冲的流得到输入数据，那么就会flush行缓冲输出流
1.3 不带缓冲
标准库不对字符进行缓冲存储，标准错误流通常是不带缓冲的，这样就可以尽快将信息显示出来
可以使用setbuf函数打开或者关闭缓冲机制，为了带缓冲进行IO，参数buf必须指向一个长度为BUFSIZE的缓冲区，头文件stdio.h。成功返回-，出错返回-1。
/* If BUF is NULL, make STREAM unbuffered.
Else make it use buffer BUF, of size BUFSIZ. */
extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __THROW; /* Make STREAM use buffering mode MODE.
If BUF is not NULL, use N bytes of it for buffering;
else allocate an internal buffer N bytes long. */
extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf, int __modes, size_t __n) __THROW;
setvbuf的mode参数可以精确说明缓冲类型，mode参数实现为：
_IOFBF 全缓冲，_IOLBF 行缓冲，_IONBF 不带缓冲。
- 如果指定一个不带缓冲的流，则忽略buf和size参数。
- 如果指定全缓冲和行缓冲，则buf和size可选择指定一个缓冲区及其长度。
- 如果流是带缓冲的，而buf是NULL，则标准IO库自动为流分配合适长度的缓冲区，长度由BUFSIZE指定。
2 流
2.1 打开、关闭流
可以使用fopen等函数打开一个IO流，头文件stdio.h。成功返回文件指针，出错返回NULL。
/* Open a file and create a new stream for it.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern FILE *fopen (const char *__restrict __filename,
const char *__restrict __modes) __wur;
/* Open a file, replacing an existing stream with it.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern FILE *freopen (const char *__restrict __filename,
const char *__restrict __modes,
FILE *__restrict __stream) __wur;
fopen函数打开路径为__filename的文件
freopen函数在一个指定的流上打开文件。如果该流已经打开，则先关闭该流。如果该流已经定向，则使用freopen清除该定向。
mode参数指定对该IO流的读写方式，ISO C规定mode参数可以右15种取值。
使用fclose可以关闭一个打开的流，头文件stdio.h。成功返回0，失败返回-1。
/* Close STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int fclose (FILE *__stream);
/* Flush STREAM, or all streams if STREAM is NULL.
2.2 读、写流
2.2.1 每次一个字符的IO
下面函数用于一次读取一个字符，头文件stdio.h。成功返回下一个字符，达到文件尾或者出错，返回EOF（定义在libio.h）。
/* Read a character from STREAM.
These functions are possible cancellation points and therefore not
marked with __THROW. */
extern int fgetc (FILE *__stream);
extern int getc (FILE *__stream);
/* Read a character from stdin.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int getchar (void);
#ifndef EOF
# define EOF (-)
#endif
前两个函数的区别是，getc可以被实现为宏，而fgetc不能实现为宏？
getchar等价于geic (stdin)。
这三个函数达到文件尾或者出错，都返回EOF。为区别文件尾或者出错，需要调用以下函数进行判断，头文件stdio.h。成功返回非0，失败返回0。可以作为bool型判断。
/* Clear the error and EOF indicators for STREAM. */
extern void clearerr (FILE *__stream) __THROW;
/* Return the EOF indicator for STREAM. */
extern int feof (FILE *__stream) __THROW __wur;
/* Return the error indicator for STREAM. */
extern int ferror (FILE *__stream) __THROW __wur;
调用clearerr可以清除这两个标志。
也可以使用ubgetc将字符再写回流缓冲区，头文件stdio.h。成功返回c，失败返回EOF。
/* Push a character back onto the input buffer of STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int ungetc (int __c, FILE *__stream);
对应的写缓冲区函数为：
/* Write a character to STREAM.
These functions are possible cancellation points and therefore not
marked with __THROW.
These functions is a possible cancellation point and therefore not
marked with __THROW. */
extern int fputc (int __c, FILE *__stream);
extern int putc (int __c, FILE *__stream);
/* Write a character to stdout.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int putchar (int __c);
2.2.2 每次一行IO
下面两个函数每次读写输出一行，头文件stdio.h。成功返回buf，到达文件尾或者出错，返回NULL。
/* Get a newline-terminated string of finite length from STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
__wur;
/* Get a newline-terminated string from stdin, removing the newline.
This function is impossible to use safely. It has been officially
removed from ISO C11 and ISO C++14, and we have also removed it
from the _GNU_SOURCE feature list. It remains available when
explicitly using an old ISO C, Unix, or POSIX standard.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern char *gets (char *__s) __wur __attribute_deprecated__;
/* Write a string to STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int fputs (const char *__restrict __s, FILE *__restrict __stream);
/* Write a string, followed by a newline, to stdout.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int puts (const char *__s);
注意以下两点：
- 应当屏蔽使用puts或者gets。这两个函数都是从标准输入或输出读写，但由于无法指定缓冲区长度，将会导致缓冲区溢出错误
- 程序退出处，显示调用exit(0)，将会冲洗任何未写的数据，然后关闭所有打开的流
- 读写一行都是以NULL为终止，但是缓冲区的NULL不会写入文件
3 二进制IO
上一节的流IO无法解决以下问题：当读取的数据是结构化数据（非字符串，可能读取到NULL，0），流IO将会截断。
为解决该问题，需要引入二进制IO函数fread和fwrite，头文件stdio.h。函数返回读或写的对象数。
/* Read chunks of generic data from STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern size_t fread (void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __stream) __wur;
/* Write chunks of generic data to STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern size_t fwrite (const void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __s);
4 定位流（访问偏移）
可使用ftell获取当前文件访问偏移量、fseek对文件访问进行偏移、rewind将流设置到起始位置。
/* Seek to a certain position on STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int fseek (FILE *__stream, long int __off, int __whence);
/* Return the current position of STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern long int ftell (FILE *__stream) __wur;
/* Rewind to the beginning of STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern void rewind (FILE *__stream);
fseek的第三个参数whence与fseek的参数相同：
SEEK_SET指定从文件起始位置开始，SEEK_CUR指定从当前位置开始，SEEK_END指定从文件末尾
5 格式化IO
5.1 格式化输出
格式化输出由5个printf函数处理，头文件stdio.h。成功返回字符数，失败返回负值。
/* Write formatted output to stdout.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int printf (const char *__restrict __format, ...);
/* Write formatted output to STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int fprintf (FILE *__restrict __stream,
const char *__restrict __format, ...);
extern int dprintf (int __fd, const char *__restrict __fmt, ...)
__attribute__ ((__format__ (__printf__, , )));
/* Write formatted output to S. */
extern int sprintf (char *__restrict __s,
const char *__restrict __format, ...) __THROWNL;
/* Maximum chars of output to write in MAXLEN. */
extern int snprintf (char *__restrict __s, size_t __maxlen,
const char *__restrict __format, ...)
__THROWNL __attribute__ ((__format__ (__printf__, , )));
printf将格式化数据写到标准输出
fprintf将格式化数据写到指定的流
dprintf写到指定的文件描述符
sprintf写到数组S
snprintf是sprintf的安全版本，指定了缓冲区的长度，防止缓冲区溢出
5.2 格式化输入
格式化输入由3个scanf函数处理，头文件stdio.h。成功返回输入的参数，失败或者到达文件末尾，返回EOF。
/* Read formatted input from STREAM.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int fscanf (FILE *__restrict __stream,
const char *__restrict __format, ...) __wur;
/* Read formatted input from stdin.
This function is a possible cancellation point and therefore not
marked with __THROW. */
extern int scanf (const char *__restrict __format, ...) __wur;
/* Read formatted input from S. */
extern int sscanf (const char *__restrict __s,
const char *__restrict __format, ...) __THROW;
6 临时文件
可以使用tmpnam或者tmpfile创建临时二进制文件。头文件stdio.h。
/* Generate a temporary filename. */
extern char *tmpnam (char *__s) __THROW __wur; /* Create a temporary file and open it read/write. This function is a possible cancellation point and therefore not
marked with __THROW. */
extern FILE *tmpfile (void) __wur;
tmpnam生成一个和现有文件名不同的有效路径字符串，最多调用TMP_MAX次，定义在stdio.h。返回指向唯一路径名的指针。如果输入为NULL，则路径名存放在静态区；如果不是NULL，则应该指向长度为L_tmpnam个字符的数组。
tmpfile创建一个临时二进制文件，在关闭该文件或者程序结束时将自动删除该文件。可以使用tmpfile创建一个临时文件，然后立即unlink。
7 内存流
/* Create a new stream that refers to a memory buffer. */
extern FILE *fmemopen (void *__s, size_t __len, const char *__modes)
__THROW __wur; /* Open a stream that writes into a malloc'd buffer that is expanded as
necessary. *BUFLOC and *SIZELOC are updated with the buffer's location
and the number of characters written on fflush or fclose. */
extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __THROW __wur;