Question

I am writing a code to read the address of the node in linked list so can some one confirm if %p and %u format in printf is correct. One gives me the hex and other gives me a decimal and conversion to hex or deci are matching. Also it will be great if someone can brief on difference of %p and %u.

struct node {
    int x;
    struct node *next;
};

int main () {
    struct node *root;
    struct node *track;

    root = malloc(sizeof(struct node));
    printf ("Location of root is %p\n", root);
    printf ("Location of root is %u\n", root);
}

Output of execution:

[root@vm c_prog]# ./a.out 
Location of root is 0xdc3010
Location of root is 14430224
[root@vm c_prog]# ./a.out 
Location of root is 0x11fbf010
Location of root is 301723664
[root@vm c_prog]# ./a.out 
Location of root is 0x7e8e010
Location of root is 132702224
[root@vm c_prog]# 

Answer 1

%p is for a pointer, and is correct. Strictly speaking, you should cast to void *, but I’ve never met an implementation that cared. Example:

printf ("Location of root is %p\n", (void *)root);

%u is for an unsigned int and is not correct, unless you play some casting gymnastics and are sure a pointer and an int are the same size. To safely print a pointer value in decimal, you should use PRIuPTR from inttypes.h:

printf ("Location of root is %"PRIuPTR"\n", (uintptr_t)root);