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。
1 /* If BUF is NULL, make STREAM unbuffered. 2 Else make it use buffer BUF, of size BUFSIZ. */ 3 extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __THROW; 4 5 /* Make STREAM use buffering mode MODE. 6 If BUF is not NULL, use N bytes of it for buffering; 7 else allocate an internal buffer N bytes long. */ 8 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。
1 /* Open a file and create a new stream for it. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern FILE *fopen (const char *__restrict __filename, 6 const char *__restrict __modes) __wur; 7 /* Open a file, replacing an existing stream with it. 8 9 This function is a possible cancellation point and therefore not 10 marked with __THROW. */ 11 extern FILE *freopen (const char *__restrict __filename, 12 const char *__restrict __modes, 13 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。
1 /* Close STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern int fclose (FILE *__stream); 6 /* 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）。
1 /* Read a character from STREAM. 2 3 These functions are possible cancellation points and therefore not 4 marked with __THROW. */ 5 extern int fgetc (FILE *__stream); 6 extern int getc (FILE *__stream); 7 8 /* Read a character from stdin. 9 10 This function is a possible cancellation point and therefore not 11 marked with __THROW. */ 12 extern int getchar (void); 13 14 #ifndef EOF 15 # define EOF (-1) 16 #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。
1 /* Clear the error and EOF indicators for STREAM. */ 2 extern void clearerr (FILE *__stream) __THROW; 3 /* Return the EOF indicator for STREAM. */ 4 extern int feof (FILE *__stream) __THROW __wur; 5 /* Return the error indicator for STREAM. */ 6 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。
1 /* Push a character back onto the input buffer of STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern int ungetc (int __c, FILE *__stream);
对应的写缓冲区函数为：
1 /* Write a character to STREAM. 2 3 These functions are possible cancellation points and therefore not 4 marked with __THROW. 5 6 These functions is a possible cancellation point and therefore not 7 marked with __THROW. */ 8 extern int fputc (int __c, FILE *__stream); 9 extern int putc (int __c, FILE *__stream); 10 11 /* Write a character to stdout. 12 13 This function is a possible cancellation point and therefore not 14 marked with __THROW. */ 15 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。
1 /* Get a newline-terminated string of finite length from STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream) 6 __wur; 7 8 /* Get a newline-terminated string from stdin, removing the newline. 9 10 This function is impossible to use safely. It has been officially 11 removed from ISO C11 and ISO C++14, and we have also removed it 12 from the _GNU_SOURCE feature list. It remains available when 13 explicitly using an old ISO C, Unix, or POSIX standard. 14 15 This function is a possible cancellation point and therefore not 16 marked with __THROW. */ 17 extern char *gets (char *__s) __wur __attribute_deprecated__; 18 19 20 /* Write a string to STREAM. 21 22 This function is a possible cancellation point and therefore not 23 marked with __THROW. */ 24 extern int fputs (const char *__restrict __s, FILE *__restrict __stream); 25 26 /* Write a string, followed by a newline, to stdout. 27 28 This function is a possible cancellation point and therefore not 29 marked with __THROW. */ 30 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。
1 /* Read chunks of generic data from STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern size_t fread (void *__restrict __ptr, size_t __size, 6 size_t __n, FILE *__restrict __stream) __wur; 7 /* Write chunks of generic data to STREAM. 8 9 This function is a possible cancellation point and therefore not 10 marked with __THROW. */ 11 extern size_t fwrite (const void *__restrict __ptr, size_t __size, 12 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。
1 /* Seek to a certain position on STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern int fseek (FILE *__stream, long int __off, int __whence); 6 /* Return the current position of STREAM. 7 8 This function is a possible cancellation point and therefore not 9 marked with __THROW. */ 10 extern long int ftell (FILE *__stream) __wur; 11 /* Rewind to the beginning of STREAM. 12 13 This function is a possible cancellation point and therefore not 14 marked with __THROW. */ 15 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。
1 /* Write formatted output to stdout. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern int printf (const char *__restrict __format, ...); 6 7 /* Write formatted output to STREAM. 8 9 This function is a possible cancellation point and therefore not 10 marked with __THROW. */ 11 extern int fprintf (FILE *__restrict __stream, 12 const char *__restrict __format, ...); 13 14 extern int dprintf (int __fd, const char *__restrict __fmt, ...) 15 __attribute__ ((__format__ (__printf__, 2, 3))); 16 17 /* Write formatted output to S. */ 18 extern int sprintf (char *__restrict __s, 19 const char *__restrict __format, ...) __THROWNL; 20 21 /* Maximum chars of output to write in MAXLEN. */ 22 extern int snprintf (char *__restrict __s, size_t __maxlen, 23 const char *__restrict __format, ...) 24 __THROWNL __attribute__ ((__format__ (__printf__, 3, 4)));
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。
1 /* Read formatted input from STREAM. 2 3 This function is a possible cancellation point and therefore not 4 marked with __THROW. */ 5 extern int fscanf (FILE *__restrict __stream, 6 const char *__restrict __format, ...) __wur; 7 /* Read formatted input from stdin. 8 9 This function is a possible cancellation point and therefore not 10 marked with __THROW. */ 11 extern int scanf (const char *__restrict __format, ...) __wur; 12 /* Read formatted input from S. */ 13 extern int sscanf (const char *__restrict __s, 14 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。
1 /* Generate a temporary filename. */ 2 extern char *tmpnam (char *__s) __THROW __wur; 3 4 /* Create a temporary file and open it read/write. 5 6 This function is a possible cancellation point and therefore not 7 marked with __THROW. */ 8 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
1 /* Create a new stream that refers to a memory buffer. */ 2 extern FILE *fmemopen (void *__s, size_t __len, const char *__modes) 3 __THROW __wur; 4 5 /* Open a stream that writes into a malloc'd buffer that is expanded as 6 necessary. *BUFLOC and *SIZELOC are updated with the buffer's location 7 and the number of characters written on fflush or fclose. */ 8 extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __THROW __wur;