How is the following line interpreted by GCC compiler:
printf("HELLO"); I want to know this because when I am running following program:
main() { printf(5+"Good Morning"); } The program is printing:
Morning Why is the compiler is starting the printing from the sixth character?
This is an artifact of C pointer-arithmetic; printf is just a red herring.
The type of a string literal (such as "Good morning") is const char *. Your code is equivalent to:
const char *p = "Good morning"; p = p + 5; printf(p); Adding a pointer and an integer produces a pointer to the 5th element in the sequence.
There are a lot of things happening here. As others have said, printf() doesn't 'know' anything about the expression 5+"Good Morning". The value of that expression is determined by the C language.
First, a+b is the same as b+a, so 5+"Good Morning" is the same as "Good Morning"+5.
Now, the type of "Good Morning" (i.e., a string literal) is an "array of char". Specifically, "Good Morning" is a 13-character array (12 "regular" characters, followed by a 0). When used in most expressions, the type of an array in C "decays" to a pointer to its first element, and binary addition is one such case. All this means that in "Good Morning"+5, "Good Morning" decays to a pointer to its first element, which is the character G.
Here is how the memory looks like:
0 1 2 3 4 5 6 7 8 9 0 1 2 +---+---+---+---+---+---+---+---+---+---+---+---+---+ | G | o | o | d | | M | o | r | n | i | n | g | 0 | +---+---+---+---+---+---+---+---+---+---+---+---+---+ The value of the address of G plus 5 is a pointer that points to 5 locations from G above, which is M. So, printf() is getting an address that is at M. printf() prints that till it finds a 0. Hence you see Morning as output.
Is is the same as writing
char *ptr="Good Morning"; followed by
printf( ptr + 5 ); which is &ptr[5] this adress points to "Morning";
Adding an integer n to a pointer results to an adress ptr + n * sizeof(type)
Because "Good Morning" is a pointer to the string (actually to the first byte of it) and adding 5 to that pointer yields a pointer to the 5th character. C strings are null-terminated, so either way, printf runs until it encounters the null at the end.
It advances the input pointer by 5 bytes and hence skips the word "Good".
This is pointer arithmetic in C.
Lets assume base pointer of the string "Good Morning" is p, and 5+p =p+5 and it points to letter M.
Hence the input to printf is the pointer from the letter M.
