+ 2

Need help with pointers in C

#include<stdio.h> void f(int *p, int *q) { p = q; *p = 3; } int i = 0, j = 10; int main() { f(&i, &j); printf("%d %d \n", i, j); getchar(); return 0; } i thought on execution answer should be 3 3 but,when executed the answer is 0 3 Can someone explain??

2nd Mar 2018, 8:23 AM
Ayush walia
Ayush walia - avatar
2 Respostas
+ 2
At the beginning you have i and j two variables in different area of memory. --------------------- 0x001 | i | 0 --------------------- 0x002 | j | 10 --------------------- Then you call f and you have p pointing to i an q pointing to j --------------------- 0x001 | i | 0 --------------------- 0x002 | j | 10 --------------------- 0x003 | p | 0x001 --------------------- 0x004 | q | 0x002 --------------------- Then p = q so, variable p is pointing to 0x002 (variable j), and now no one is pointing to i --------------------- 0x001 | i | 0 --------------------- 0x002 | j | 10 --------------------- 0x003 | p | 0x002 --------------------- 0x004 | q | 0x002 --------------------- Then you modify the value of the variable pointed by p (*p = 3;) and this variable is allocated in memory 0x002( variable j) so j will be equal to 3 --------------------- 0x001 | i | 0 --------------------- 0x002 | j | 3 --------------------- 0x003 | p | 0x002 --------------------- 0x004 | q | 0x002 --------------------- Then you print i = 0 and j = 3
2nd Mar 2018, 10:52 AM
J Domingo Jiménez J
J Domingo Jiménez J - avatar
+ 1
Start with tell that a pointer is a simple variable that contains adresses... Ok now go to first line of f function body... p=q; Here you tell "assign q value (an adress) to p var".. Now p and q have same value (adress)... *p=3; Here you tell "derefence p (access to memory) and assign 3 as value (at adress/value contained in p). What happen? you change the int value at adress p (and q) which is second adress passed to function... The first adress is untouched and hence unmodified... You can obtain wanted result if: - remove first line (p=q) - add follow statement: *q=3;
2nd Mar 2018, 10:46 AM
KrOW
KrOW - avatar