为什么scanf函数（QUOT;％D＆QUOT，[...]）不会消耗的'\\ n'？而scanf函数（QUOT;％C＆QUOT;）呢？ [英] Why scanf("%d", [...]) does not consume '\n'? while scanf("%c") does?

问题描述

这里，我看到了在接受的答案这条语句：

  

大部分的转换说明的跳过前导空白包括换行，但％C 没有。​​

对于我来说，目前还不清楚这下不同行为的理由，我本来期望一个统一的一个（例如总是跳跃或从不）。

我来到这样的问题了一块C $ C $的C是这样的：

 的#includestdio.h中诠释主要（无效）{    焦炭CH;
    INT actualNum;    的printf（插入一个数字：）;
    scanf函数（％d个，＆安培; actualNum）;
    //的getchar（）;    的printf（插入字符：）;
    scanf函数（％C，＆安培; CH）;    返回0;
}
 

交换这两个 scanf函数取值可以解决问题，还有（评论）的getchar ，否则的'\\ n'第一插入将由第二 scanf函数与％C 。我对GCC都在Linux和Windows的测试，该行为是一样的：

  

海湾合作委员会（GCC）4.7.2 20120921（红帽4.7.2-2）结果
  版权所有（C）2012自由软件基金会，公司

  这是自由软件;参见复印条件的来源。有否
  保证;即使不是针对特定目的的适销。

我的问题是：为什么％d个和％C 表现不同w.r.t. 的'\\ n'在 scanf函数？

解决方案

从的马的嘴：

7.21.6.2 fscanf函数

...结果
5空白字符（S）组成的指令被读取输入到执行
第一个非空白字符（这仍然未读），或直到没有更多的字符可以
被读取。该指令永远不会失败。结果
...结果
7的指令，这是一个特定的转换阳离子科幻科幻德未列名一组匹配的输入序列，如
下面针对每个特定连接器描述。 A转换特定网络阳离子通过以下步骤执行：搜索结果
8输入空白字符（由 isspace为函数特定网络版）被跳过，除非
在特定网络阳离子包括 [， C 或 N 特定连接器。
^284）搜索结果
9的输入项目是从流中读取，除非特定网络阳离子包括 N 特定连接器。 的
输入项目被定义为不超过输入字符序列最长
任何特定网络版场宽度，并且，或者是，一个匹配的输入序列pre网络的X.
^285）
在第一个字符，如果有的话，之后的输入项保持未读。如果输入的长度
项是零，该指令失败的执行;这个条件是一个匹配失败，除非
最终OF-音响文件，编码错误，或从流读误差prevented输入，在这种
情况下，它是一个输入故障。

---

^{284）这些空白字符不是针对一个特定的编场宽度计算。结果
285）的fscanf推回至多一个输入字符到的输入流。因此，一些序列
可以接受的关于strtod，strtol将等等，都是不能接受的FSCANF。}

重点由我补充道。

空格不是有效的整数字符串的一部分，所以它是有道理的％d个转换说明跳过任何前导空格。不过，空白可能是对自己有效，因此，它是有道理的％C 转换说明符的不的跳过它。

根据以上第5条，如果你把一个空白的前％C 指令格式字符串，所有领先的空白将被忽略：

  scanf函数（％C，＆安培; CH）;
 

Here, I saw this statement in the accepted answer:

Most of the conversion specifiers skip leading whitespace including newlines but %c does not.

For me it is not clear the rationale under this different behaviors, I would have expected a uniform one (e.g. always skipping or never).

I came into this kind of problem with a piece of C code like this:

#include "stdio.h"

int main(void){

    char ch;
    int actualNum;

    printf("Insert a number: ");
    scanf("%d", &actualNum);
    // getchar();

    printf("Insert a character: ");
    scanf("%c", &ch);

    return 0;
}

Swapping the two scanfs solves the problem, as well as the (commented) getchar, otherwise the '\n' of the first insertion would be consumed by the second scanf with %c. I tested on gcc both on linux and windows, the behavior is the same:

gcc (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2)
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

So my question is: Why does %d and %c behave differently w.r.t. '\n' in scanf?

解决方案

From the horse's mouth:

7.21.6.2 The fscanf function

...
5 A directive composed of white-space character(s) is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read. The directive never fails.
...
7 A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

8 Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [, c, or n specifier. ²⁸⁴⁾

9 An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters which does not exceed any specified field width and which is, or is a prefix of, a matching input sequence. ²⁸⁵⁾ The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails; this condition is a matching failure unless end-of-file, an encoding error, or a read error prevented input from the stream, in which case it is an input failure.

---

^{284) These white-space characters are not counted against a specified field width.
285) fscanf pushes back at most one input character onto the input stream. Therefore, some sequences that are acceptable to strtod, strtol, etc., are unacceptable to fscanf.}

Emphasis added by me.

Whitespace is not part of a valid integer string, so it makes sense for the %d conversion specifier to skip any leading whitespace. However, whitespace may be valid on its own, so it makes sense for the %c conversion specifier to not skip it.

Per clause 5 above, if you put a blank space in the format string prior to the %c directive, all leading whitespace will be skipped:

scanf(" %c", &ch);

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