CODEWITHSUNDEEP
cpointerstype-conversion
Hari
Hari

Reputation: 121

Difference between %d and %p printf format string directives in c language?

I want to get a detailed explanation on the difference between using %d and %p type for printing pointer.

Also Why does %p return hexadecimal? What are the cases when %d and %p return different values? Does datatype only represent the way the user wants the output or it has something to do with the memory locations too?

Upvotes: 4

Views: 8913

Answers (3)

WangYang
WangYang

Reputation: 504

A pointer of variable in C has value just like other type of variable such as the int、char and so on. The %p format string in the printf function just indicates that the type of parameter "i" is pointer to the printf instead of int. Thus printf outputs the Hex value of parameter i because printf seem the parameter i as a pointer type. No matter what the type of variable i is, the value is same----10 or 0xa. The difference between the types of i---int or pointer ----is the different ways to use in C. if type of i is regarded as pointer, you can visit the memory specified by value of pointer i or other operations the pointer type supports. If type of i is int, we can do some operations such as addition or subtraction rather than visiting the memory by using value of i, because the grammar of C do't allow you to do that(just warning). if you know what you want to do, you can do that.

Upvotes: 0

NPE
NPE

Reputation: 500177

For the program to be well-defined, the format specifier must match the type of the argument. Therefore you can use %p but not %d to print out pointers. (The latter might happen to work on some architectures but is technically undefined behaviour.)

The primary reason you can't freely interchange %d and %p is that ints and pointers don't have to have the same size.

The format in which pointers are printed out is architecture-specific (pointers can have different size or indeed different structure). It is, however, common to transcribe memory addresses in hexadecimal, so this is what %p usually does.

Upvotes: 13

wallyk
wallyk

Reputation: 57774

Those conversions are highly architecture dependent. One of the most clear distinctions are with the real mode 8086 where int is 16 bits and a (large model) pointer is 32 bits but has a segment and offset which are always written as segment:offset.

%d    takes 16 bits and displays it as a signed value  123
%p    takes a pointer and display it in address format    0fef:0004

Since %p was introduced relatively recently I don't know of any implementations but a PDP-11 library ought to implement it by display the 16-bit address in octal.

Upvotes: 1

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