why this program give "Invalid memory reference" error explain it?

Question

//gcc 5.4.0

#include<stdio.h>
int  main()
{
int i=3;
    float f=3.50,*ptr;
    float *multy(int ,float);
    ptr=multy(i,f);
   printf("\n%u %f",ptr,*ptr);
return 0;
}
float * multy(int ii,float jj)
{
float p=ii*jj;
    printf("%u  %f",&p,p);
return (&p);
}

Comments

No error , program should run properly......

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it is returning address of local variable which is not meant to be used outside local scope.

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http://rextester.com/l/c_online_compiler_gcc

i am using this online compiler and this compiler gives error.

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Try here 

Answer 1

#include<stdio.h>
int  main()
{
    int i=3;
    float f=3.50,*ptr;
    float *multy(int ,float);
    ptr=multy(i,f);                          // Address Return
    printf("\n%u %f",ptr,*ptr);              // Printing
    return 0;
}

float * multy(int ii,float jj)
{
    float p=ii*jj;                           // Memory Allocation
    printf("%u  %f",&p,p);
    return (&p);
}

The function stack would look as under:

"Memory Allocation" happens in function multy(..). This memory is named as 'p'. The print inside the multy(..) works fine and prints the address and value of p as 'p' exists in that stack.

When address is returned from multy(..), the control returns to main function. The address returned is stored in 'ptr'. The memory which was allocated inside the stack of multy(..) is not valid to be used outside the scope of multy(..)

So, when "Printing" happens in main(..), the address is illegal (not meant to be used outside scope of multi(..) ), so you get invalid memory access / segmentation fault.

Comments

"The stack record of multy(..) is destroyed."

This may or may not happen. So, invalid memory access is not a guarantee here.

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Ok Sir, I will edit the answer to reflect the same..

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sir if this type of question will come in gate than what will be our output?

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Whatever should be our answer will be among the choices for an exam like GATE.

Answer 2

#include<stdio.h>
int  main()
{
    int i=3;                  1 memory created for integer variable i and 3 is assigned to it.
    float f=3.50, *ptr;       2 memory created for float variable f and 3.50 is assigned to it.
    float *multy(int ,float); 3 prototype of function multy
    ptr=multy(i,f);     4 call for function multy and multy function return to float pointer ptr. 
    printf("\n%u %f",ptr,*ptr);  9. here error may come or may not come since when multy 
function is over its all local variable are free i.e. not pointed by p but memory is 
pointed by ptr (may be it happen that location assigned to some other variable), gives 
error some time, if memory not assigned to anyone then no error.
return 0; 
}
float * multy(int ii,float jj) 5 call comes here and assigned 3 to int ii and f to  float jj.
{
float p=ii*jj;             6 memory created locally for float variable p and 3.0 *3.05 here 
not ii have type casted internaly not explicitly.
    printf("%u  %f",&p,p); 7. here address of p and value of p is printed.
return (&p);               8. return address of local variable p to ptr of main().
}