0. 序曲
寫這篇短文的起因是,前兩天想去天大的acm在線系統(tǒng)找?guī)椎李}做做。為什么呢?因?yàn)楸救颂齑螽厴I(yè),這個(gè)天大呢可是中國最早的大學(xué),原名北洋大學(xué)堂,這可絕對(duì)是貨真價(jià)實(shí)的第一所大學(xué)。給大家推薦推薦啊,學(xué)風(fēng)那是相當(dāng)?shù)暮谩?/p>
扯多了,還是回到本來的話題上。上了acm系統(tǒng)之后,就先看了1001。那道題的意思是輸入一些正整數(shù)(以EOF結(jié)束),把對(duì)應(yīng)的字符輸出。這個(gè)簡單,程序很快就出來了:
#include <stdio.h>
int main()
{
int c;
while(scanf("%d", &c) != EOF)
{
putchar(c);
}
return 0;
}
程序運(yùn)行,輸入103 102 105 107<enter>
輸出gfik。
當(dāng)時(shí)運(yùn)行完之后馬上想,為什么不是輸入一個(gè)數(shù)字馬上輸出一個(gè)字符呢,因?yàn)榭闯绦虼_實(shí)是這樣的邏輯,只要不是EOF,就會(huì)輸出。又一想,對(duì)了,是緩沖的問題。想起來APUE里邊說得stdin應(yīng)該是行緩沖的,另外,可以用setbuf,setvbuf設(shè)定流的緩沖。于是想將stdin設(shè)成無緩沖的。于是程序變成這樣:
#include <stdio.h>
int main()
{
int c;
setbuf(stdin, NULL);
while(scanf("%d", &c) != EOF)
{
putchar(c);
}
return 0;
}
可是編譯運(yùn)行,還是老樣子,沒有變化。想了想,沒想出是啥原因,于是開始google和APUE。終于算是明白了些,整理在這兒。
聲明：
本文很大部分內(nèi)容來自APUE－－UNIX環(huán)境高級(jí)編程。
1. 緩沖類型。
標(biāo)準(zhǔn)庫提供緩沖是為了減少對(duì)read和write的調(diào)用。提供的緩沖有三種類型(整理自APUE):
在這種情況下,實(shí)際的I/O操作只有在緩沖區(qū)被填滿了之后才會(huì)進(jìn)行。對(duì)駐留在磁盤上的文件的操作一般是有標(biāo)準(zhǔn)I/O庫提供全緩沖。緩沖區(qū)一般是在第一次對(duì)流進(jìn)行I/O操作時(shí),由標(biāo)準(zhǔn)I/O函數(shù)調(diào)用malloc函數(shù)分配得到的。
術(shù)語flush描述了標(biāo)準(zhǔn)I/O緩沖的寫操作。緩沖區(qū)可以由標(biāo)準(zhǔn)I/O函數(shù)自動(dòng)flush(例如緩沖區(qū)滿的時(shí)候);或者我們對(duì)流調(diào)用fflush函數(shù)。
在這種情況下,只有在輸入/輸出中遇到換行符的時(shí)候,才會(huì)執(zhí)行實(shí)際的I/O操作。這允許我們一次寫一個(gè)字符,但是只有在寫完一行之后才做I/O操作。一般的,涉及到終端的流--例如標(biāo)注輸入(stdin)和標(biāo)準(zhǔn)輸出(stdout)--是行緩沖的。
標(biāo)準(zhǔn)I/O庫不緩存字符。需要注意的是,標(biāo)準(zhǔn)庫不緩存并不意味著操作系統(tǒng)或者設(shè)備驅(qū)動(dòng)不緩存。
ISO C要求:
-
當(dāng)且僅當(dāng)不涉及交互設(shè)備時(shí),標(biāo)準(zhǔn)輸入和標(biāo)準(zhǔn)輸出是全緩存的。
-
標(biāo)準(zhǔn)錯(cuò)誤絕對(duì)不是全緩存的。
但是,這并沒有告訴我們當(dāng)標(biāo)準(zhǔn)輸入/輸出在涉及交互設(shè)備時(shí),它們是無緩存的還是行緩存的;也沒有告訴我們標(biāo)準(zhǔn)錯(cuò)誤應(yīng)該是行緩存的還是無緩存的。不過,大多數(shù)實(shí)現(xiàn)默認(rèn)的緩存類型是這樣的:
-
標(biāo)準(zhǔn)錯(cuò)誤總是無緩存的。
-
對(duì)于所有的其他流來說,如果它們涉及到交互設(shè)備,那么就是行緩存的;否則是全緩存的。
2. 改變默認(rèn)緩存類型
可以通過下面的函數(shù)改變緩存類型(摘自APUE):
void setbuf(FILE *restrict fp, char *restrict buf);
int setvbuf(FILE *restrict fp, char *restrict buf, int mode,
size_t size);
這些函數(shù)必須在流打開之后、但是未對(duì)流做任何操作之前被調(diào)用(因?yàn)槊總€(gè)函數(shù)都需要一個(gè)有效的文件指針作為第一個(gè)參數(shù))。
利用setbuf，可以打開或者關(guān)閉緩存。為了打開緩存，buf參數(shù)必須一個(gè)大小為BUFSIZ的緩存，BUFSIZ是定義在stdio。h中的常量。&lt;&lt;ISO/IEC
9899&gt;&gt;要求：BUFSIZ至少為256。如果要關(guān)閉緩存，可以將buf設(shè)成NULL。
利用setvbuf，我們可以設(shè)定緩存類型。這是通過mode參數(shù)指定的。
關(guān)于這兩個(gè)函數(shù)，可以看下表（摘自APUE）：
|
Function
|
mode
|
buf
|
Buffer and length
|
Type of
buffering
|
|
setbuf
|
|
non-null
|
user buf of length
BUFSIZ
|
fully buffered or line buffered
|
|
NULL
|
(no buffer)
|
unbuffered
|
|
setvbuf
|
_IOLBF
|
non-null
|
user buf of length size
|
fully buffered
|
|
NULL
|
system buffer of appropriate length
|
|
_IOFBF
|
non-null
|
user buf of length size
|
line buffered
|
|
NULL
|
system buffer of appropriate length
|
|
_IONBF
|
(ignored)
|
(no buffer)
|
unbuffered
|
需要注意的是：如果在函數(shù)內(nèi)為流分配了自動(dòng)變量作為緩存，那么在退出之前需要將流關(guān)閉。因此最好讓系統(tǒng)自己分配緩存，這些緩存在流關(guān)閉的時(shí)候會(huì)自動(dòng)被釋放。
3.如果清理輸入緩存
關(guān)于這點(diǎn)可以參看comp.lang.c
FAQ的Question12.26b:
Q: If fflush won't work, what can I
use to flush input?
A: It depends on what you're trying to do. If you're
trying to get rid of an unread newline or other unexpected input after calling
scanf (see questions 12.18a-12.19), you really need to rewrite or replace the
call to scanf (see question 12.20). Alternatively, you can consume the rest of a
partially-read line with a simple code fragment like
while((c =
getchar()) != '\n' && c != EOF)
/* discard */ ;
(You may also
be able to use the curses flushinp function.)
There is no standard way to
discard unread characters from a stdio input stream. Some vendors do implement
fflush so that fflush(stdin) discards unread characters, although portable
programs cannot depend on this. (Some versions of the stdio library implement
fpurge or fabort calls which do the same thing, but these aren't standard,
either.) Note, too, that flushing stdio input buffers is not necessarily
sufficient: unread characters can also
accumulate in other, OS-level input buffers. If you're trying to actively
discard input (perhaps in anticipation of issuing an unexpected prompt to
confirm a destructive action, for which an accidentally-typed ``y'' could be
disastrous), you'll have to use a system-specific technique to detect the
presence of typed-ahead input; see questions 19.1 and 19.2. Keep in mind that
users can become frustrated if you discard input that happened to be
typed too quickly.
References: ISO Sec. 7.9.5.2
H&S Sec.
15.2
4. 幾點(diǎn)需要注意的地方
- 對(duì)輸入流進(jìn)行fflush操作是無定義的。
- 無緩存并不意味著一個(gè)個(gè)的那樣處理輸入，而是說當(dāng)操作系統(tǒng)返回它們時(shí)，對(duì)于標(biāo)準(zhǔn)庫函數(shù)來說它們是立即可用的。因?yàn)檫€可能有操作系統(tǒng)級(jí)甚至是硬件級(jí)的緩存，這些并不是setbuf可以控制的。
- 另外可以參考這里（我就是最先從這里開始看的）。還有這里。我從后面那個(gè)鏈接摘錄一些重要的下來：
setbuf() has to do with the
delivery of bytes between the
C library FILE* management layer and the OS I/O
layer.
Calls to fread(), fgets(),
fgetc(), and getchar() work within
whatever FILE* buffered data is available,
and when that data
is exhausted, the calls request that the FILE* buffer be
refilled
by the system I/O layer.
When full buffering is turned
on, that refill operation results in the
FILE* layer requesting that the
operating system hand it a full
buffer's worth of data; when buffering is
turned off, that
refill operation results in the FILE* layer requesting that
the
operating system return a single character.
...setting an input stream to
be unbuffered
does NOT tell the operating
system to tell the device driver
to go into any kind of "raw"
single-character mode. There are
system-specific calls such as ioctl() and
tcsetterm() that
control what the device driver will
do.