NAME
setbuf, setbuffer, setlinebuf, setvbuf - 流缓冲操作
SYNOPSIS 总览
#include <stdio.h>
void setbuf(FILE *stream, char *buf);
void setbuffer(FILE *stream, char *buf, size_t size);
void setlinebuf(FILE *stream);
int setvbuf(FILE *stream, char *buf, int mode , size_t size);
DESCRIPTION 描述
有三种类型的缓冲策略，它们是无缓冲，块缓冲和行缓冲。当输出流无缓冲时，信息在写的同时出现于目标文件或终端上；当是块缓冲时，字符被暂存，然后一起写入；当是行缓冲时，字符被暂存，直到要输出一个新行符，或者从任何与终端设备连接的流中 (典型的是 stdin) 读取输入时才输出。函数 fflush(3) 可以用来强制提前输出。(参见 fclose(3)) 通常所有文件都是块缓冲的。当文件 I/O 操作在文件上发生时，将调用 malloc(3) ，获得一个缓冲。如果流指向一个终端 (通常 stdout 都是这样)，那么它是行缓冲的。标准错误流 stderr 默认总是无缓冲的。
函数 setvbuf 可以用在任何打开的流上，改变它的缓冲。参数 mode 必须是下列三个宏之一：
- _IONBF
- 无缓冲
- _IOLBF
- 行缓冲
- _IOFBF
- 完全缓冲
除非是无缓冲的文件，否则参数 buf 应当指向一个长度至少为 size 字节的缓冲；这个缓冲将取代当前的缓冲。如果参数 buf 是 NULL ，只有这个模式会受到影响；下次 read 或 write 操作还将分配一个新的缓冲。函数 setvbuf 只能在打开一个流，还未对它进行任何其他操作之前使用。
其他三个函数调用是函数 setvbuf 的别名，函数 setbuf 与使用下列语句完全等价：
- setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
函数 setbuffer 与此相同，但是缓冲的长度由用户决定，而不是由默认值 BUFSIZ 决定。函数 setlinebuf 与使用下列语句完全等价：
- setvbuf(stream, (char *)NULL, _IOLBF, 0);
RETURN VALUE 返回值
函数 setvbuf 成功执行时返回 0。它失败时可能返回任何值，但是当 It can return any value on failure, but returns nonzero when mode 不正确，或者不能实现请求时，必须返回非零值。它在失败时可能设置 errno 。其他函数没有返回值。
CONFORMING TO 标准参考
函数 setbuf 和 setvbuf 遵循 ANSI X3.159-1989 (``ANSI C'') 标准。
BUGS
函数 setbuffer 和 setlinebuf 无法移植到 4.2BSD 之前的 BSD 版本，在 Linux 中仅在 libc 4.5.21 之后的系统中可用。在 4.2BSD 和 4.3BSD 系统中， setbuf 总是使用非最优的缓冲大小，应当避免使用它。 在 stream 被关闭时，必须确保 buf 和它指向的空间仍然存在。这通常发生在程序终止时。 例如，下列调用是非法的：
#include <stdio.h>
int main()
{
char buf[BUFSIZ];
setbuf(stdin, buf);
printf("Hello, world!\n");
return 0;
}
SEE ALSO 参见
fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
#p#
NAME
setbuf, setbuffer, setlinebuf, setvbuf - stream buffering operations
SYNOPSIS
#include <stdio.h>
void setbuf(FILE *stream, char *buf);
void setbuffer(FILE *stream, char *buf, size_t size);
void setlinebuf(FILE *stream);
int setvbuf(FILE *stream, char *buf, int mode , size_t size);
DESCRIPTION
The three types of buffering available are unbuffered, block buffered, and line buffered. When an output stream is unbuffered, information appears on the destination file or terminal as soon as written; when it is block buffered many characters are saved up and written as a block; when it is line buffered characters are saved up until a newline is output or input is read from any stream attached to a terminal device (typically stdin). The function fflush(3) may be used to force the block out early. (See fclose(3).) Normally all files are block buffered. When the first I/O operation occurs on a file, malloc(3) is called, and a buffer is obtained. If a stream refers to a terminal (as stdout normally does) it is line buffered. The standard error stream stderr is always unbuffered by default.
The setvbuf function may be used on any open stream to change its buffer. The mode parameter must be one of the following three macros:
- _IONBF
- unbuffered
- _IOLBF
- line buffered
- _IOFBF
- fully buffered
Except for unbuffered files, the buf argument should point to a buffer at least size bytes long; this buffer will be used instead of the current buffer. If the argument buf is NULL, only the mode is affected; a new buffer will be allocated on the next read or write operation. The setvbuf function may only be used after opening a stream and before any other operations have been performed on it.
The other three calls are, in effect, simply aliases for calls to setvbuf. The setbuf function is exactly equivalent to the call
- setvbuf(stream, buf, buf ? _IOFBF : _IONBF, BUFSIZ);
The setbuffer function is the same, except that the size of the buffer is up to the caller, rather than being determined by the default BUFSIZ. The setlinebuf function is exactly equivalent to the call:
- setvbuf(stream, (char *)NULL, _IOLBF, 0);
RETURN VALUE
The function setvbuf returns 0 on success. It can return any value on failure, but returns nonzero when mode is invalid or the request cannot be honoured. It may set errno on failure. The other functions are void.
CONFORMING TO
The setbuf and setvbuf functions conform to ANSI X3.159-1989 (``ANSI C'').
BUGS
The setbuffer and setlinebuf functions are not portable to versions of BSD before 4.2BSD, and are available under Linux since libc 4.5.21. On 4.2BSD and 4.3BSD systems, setbuf always uses a suboptimal buffer size and should be avoided. You must make sure that both buf and the space it points to still exist by the time stream is closed, which also happens at program termination. For example, the following is illegal:
#include <stdio.h>
int main()
{
char buf[BUFSIZ];
setbuf(stdin, buf);
printf("Hello, world!\n");
return 0;
}
SEE ALSO
fclose(3), fflush(3), fopen(3), fread(3), malloc(3), printf(3), puts(3)
