can be p[-1] dereferenced?

[sakthimuhuu]

I have my program like this int main() { char *p = "Test" p++; p++; printf("%s", p[-1]); return 0; } what will be printed by printf statment Thanks, Sakthi

[_Superman_]

The statement is perfectly valid "in this case". Using an index will simply add it to the address. So p[-1] will internally become *(p-1). In this case, since you're incrementing p twice, p[-1] should print e. Correction - My earlier comment would be true if you change the statement to printf("%c", p[-1]);

«_Superman_»  _I love work. It gives me something to do between weekends.

_Microsoft MVP (Visual C++) (October 2009 - September 2013)

Polymorphism in C

[Jochen Arndt]

p[-1] returns the character at that position ('e' in your case). When passing this value as address of a string to printf, it will try to print the memory content at the address 0x65 as string.

[CPallini]

While p[-1] is valid (why didn't you try to compile it?), you are using it the wrong way: a char pointer (char *) is expected by the format specifier "%s" while you are passing a char ( p[-1], is like *(p-1), that is a char). You should have written either:

printf("%s\n", &p[-1]);

producing output

est

or

printf("%c\n",p[-1]);

, producing output

e

THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

[sakthimuhuu]

But, when i executed the program in Visual studio i got run time error Thanks, Sakthi

[CPallini]

What program? What error? (Please provide both).

THESE PEOPLE REALLY BOTHER ME!! How can they know what you should do without knowing what you want done?!?! -- C++ FQA Lite

[Satya Chamakuri]

Hi, int main() { char *p = "Test"; p++; p++; printf("%s", p[-1]); getchar(); return 0; } Suppose think the starting address of p will be 0x20000, You are incrementing p two times, so before entering to the highlighted line in above code value of p is 0x20002 and the content in that location as starting address will be st. Then p[-1] means *(p-1), means value at 0x20001 location. means 'e'. In printf, %s is a format specifier wich will try to display the string at the memory location provided in variable list. Here the address provided in variable list is value of *(p-1), i.e, 'e' (internally this is 0x65). So %s dereferences the value at 0x65, so some garbage value it will print finally. otherwise will terminate the program as memory is un referenced. Thanks