C pointers and arrays Warning assignment makes pointer from integer without a cast

Question

I m having some trouble with pointers and arrays in C  Here s the code    include lt stdio h gt    int  ap  int a 5   41 42 43 44 45   int x   int main         ap   a 4       x    ap      printf   d  x       return 0      When I compile and run the code I get this warning       Warning  assignment makes pointer from integer without a cast    enabled by default    For line number 9  ap   a 4    and the terminal crashes  If I change line 9 to not include a position  ap   a   I don t get any warnings and it works  Why is this happening  I feel like the answer is obvious but I just can t see it

User · Answer

int   and int  are represented the same way  except int   allocates  IIRC    ap is a pointer  therefore giving it the value of an integer is dangerous  as you have no idea what s at address 45   when you try to access it  x    ap   you try to access address 45  which causes the crash  as it probably is not a part of the memory you can access

User · Answer

What are you doing   I am using bytes instead of in for better reading   You start with int  ap and so on  so your  your computers  memory looks like this   -------------- memory used by some one else -------- 000    001        098    099    -------------- your memory  -------- 100  something           lt - here is  ap 101  41                  lt - here starts a    102  42 103  43 104  44 105  45 106  something           lt - here waits x   lets take a look waht happens when  print short cut for    print   d         print a 0   - gt  41     no surprise print a     - gt  101     because a points to the start of the array print  a    - gt  41      again the first element of array print a 1   - gt  guess  102 print   a 1     - gt  whats behind 102  42  we all love this number    and so on  so a 0  is the same as  a  a 1      a 1          a n  just reads easier   now  what happens at line 9   ap a 4     we know a 4    a 4  somehow  105    gt   45     warning  converting int to pointer  -------------- your memory  -------- 100  45          lt - here is  ap now 45  x    ap       wow ap is 45 - gt  where is 45 pointing to  -------------- memory used by some one else -------- bang          dont touch neighbours garden   So the  warning  is not just a warning it s a severe error

User · Answer

In this case a 4  is the 5th integer in the array a  ap is a pointer to integer  so you are assigning an integer to a pointer and that s the warning  So ap now holds 45 and when you try to de-reference it  by doing  ap  you are trying to access a memory at address 45  which is an invalid address  so your program crashes   You should do ap    amp  a 4    or ap   a   4   In c array names decays to pointer  so a points to the 1st element of the array In this way  a is equivalent to  amp  a 0

[c] C pointers and arrays: [Warning] assignment makes pointer from integer without a cast

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