i cannot figure out why NULL is not printed
#include "stdio.h"
#include "stddef.h"
typedef struct{
int info;
struct DEMO1* next;
} DEMO1;
int main()
{
int temp;
DEMO1 *head ;
if(head==NULL)
printf("NULL");
}
3 Answers
Memory is not initialized upon allocation. You cannot expect it to have a particular value until you set it.
5 Comments
io10
I thought that upon declaration it holds null value
aardvarkk
Nope. I don't know what else to say... that's just not the case. It holds random data.
io10
what i am supposed to do ?
aardvarkk
Set it to
NULL when you create it! head = NULL;
Vatine
The initial value depends, among other things, on the storage class. Something that is at the top level of a compilation unit will be zero-initialized, but something that is declared in local scope in a function will not.
Your problem is that you haven't initialized the head pointer to any value.. It just contains whatever bytes were stored there previously (unless the OS was nice and did some clean up). You need to initialize head to NULL if you would like that if-statement to evaluate to true:
int main()
{
DEMO1 *head = NULL;
if(head==NULL)
printf("NULL");
}
Comments
The real life lesson here is that your compiler probably had the ability to inform you that head was used before it was initialized. If you did not see such a message, then either you have a fairly poor compiler, or you have not asked the compiler to warn you about all possible problems. Find out how to get all the help your compiler is capable of offering and you could save a lot of time in your programming.
3 Comments
valdo
Do you really think he/she would notice a compiler warning?
ShinTakezou
hence he must not give this suggestion that turns out to be useful?
io10
I compiled the code using codeblocks IDE , there were no warnings and i did not turn off warnings
System.ALLCAPSEXCEPTIONheadoutside of any function, or had put the keywordstaticin front of it, then your assumption that it would be initialized to 0/NULL would have been correct. That default is not true for stack variables, mainly in the name of efficiency (unlike static variables, that possibly unneeded initialization would have to be done every single time the function was entered).