How the printf function works in c when format specifier is not correct?

Question

In an Interview, I was asked the output of the following code snippet:

printf("%d", "computer");

I am a c# developer and earlier I had learned C too, but when this question was asked, I had no clue. When I run the same question in windows 10 computer (64-bit), it is giving putput as

3571712

Please give reasons why this is happening.

See also: [this question](http://stackoverflow.com/questions/31380628/how-printf-works-in-case-of-type-mismatch-with-type-specifier) and [this question](http://stackoverflow.com/questions/33648596/why-are-arguments-which-do-not-match-the-conversion-specifier-in-printf-undefine). — Paul R, Mar 14 '17 at 07:34
Either it was intentionally a tricky question to see if you knew that the code doesn't make sense, or it was unintentionally a complete crap question, in which case you didn't want that job anyway. The correct answer is _not_ "it will print the address of the string literal", because it uses the wrong format specifier for that. — Lundin, Mar 14 '17 at 07:49

score 4 · Answer 1 · answered Mar 14 '17 at 07:28

It doesn't "work", what you observe is an outcome of undefined behavior.

Quoting C11, chapter §7.21.6.1/p9

[...] If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.

In your case, the conversion specifier is %d which expects an argument of type int but you're supplying a char*, and they are not the compatible type, hence the UB.

score 4 · Answer 2 · answered Mar 14 '17 at 07:32

well value of "computer" is memory address where that string is stored. It could be value of that address is: 3571712 (But you should not rely on this - see below).

But to print memory address void* you should use %p format specifier. And using incorrect format specifiers is undefined behavior.

score 2 · Accepted Answer · answered Mar 14 '17 at 07:50

In an Interview, I was asked the output of the following code snippet [...]

Your answer could have been:

a. "This is undefined behavior"

b. "A number will most likely be printed, but I cannot tell you which number because I do not know where the string is stored, as the string's address will be attempted to be interpreted as an integer by the printf function".

c. "Which platform? Which compiler?"

score 0 · Answer 4 · answered Mar 14 '17 at 07:31

0

It will not work it's giving the garbage value of these variables. And the main reason is that we use "%d" for the output of int not for string.

answered Mar 14 '17 at 07:31

Iqbal Butt

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How the printf function works in c when format specifier is not correct?

4 Answers4