为什么不能使用“ scanf_s”一次读取一个字符和一个数字？

Question

This code crashes:

scanf_s("%c %d",&ch,&x);//Run error

But this code works:

scanf_s("%c",&ch);
scanf_s("%d",&x);//Run succeed

I want to know why the first code fragment is wrong.

'run error' means The compiler has a warning and something is wrong when I run the program input

Answer 1

As you are using scanf_s (in contrast to scanf ), you need to provide an additional length parameter for %c (cf, for example, this document from microsoft regarding scanf_s ):

Unlike scanf and wscanf, scanf_s and wscanf_s require you to specify buffer sizes for some parameters. Specify the sizes for all c, C, s, S, or string control set [] parameters.

So you will have to write

scanf_s ("%c %d", &ch, 1, &x);

indicating that the first %c shall read at most one character.

Note that scanf_s("%c",&ch); should issue the same warning/error as it lacks the mandatory length parameter for %c , too.

Command scanf_s("%d",&x); , in contrast, is OK as %d does not require (must not get provided) an additional length parameter.

Answer 2

I assume you are running MSVC, which gives an error if you use scanf instead of scanf_s . I also assume you want to use the former and not the latter. To disable the error, go to Project menu -> [Project name] Properties -> C/C++ -> Preprocessor. To the "Preprocessor Definitions" field, add the macro _CRT_SECURE_NO_WARNINGS . Then click "Apply."

Now you should be able to use scanf as you would on any other compiler/IDE.

Answer 3

As an expansion of the answer by Stephan Lechner , consider what happens when executing scanf_s("%c %d",&ch,&x) .

Technically, it's undefined behavior because you didn't specify the correct parameters to scanf_s .

However, practically, the code will try to interpret junk values in the memory as missing parameters. That is, the code is equivalent to something like this:

scanf_s("%c %d", &ch, 0xf0000000, 0xdeadbeef);

Here 0xf0000000 represents the address of the variable x . The code of scanf_s will interpret it as buffer size (a very large buffer), but this has no effect because %c tells it to read just one byte. Further, 0xdeadbeef represents junk, and the code interprets it as an address to write your integer result. Writing to random addresses is a big no-no, so it crashes.

When your code reads the char and the int separately, it never encounters writing to a random address, so everything works well, and if you ignore compiler warnings, you never know that your code is buggy.