C - Cannot take input after allocating dynamic array [closed] - c

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I need someone to tell me what I am doing wrong over here.
void main(void) {
int *arr,n,i;
printf("Enter the number of elements you want to enter=");
scanf("%d",&n);
arr = (int*)calloc(n,sizeof(int));
if(!arr)
return;
for(i=0;i<n;i++)
scanf("%d",&arr[i]);
for(i=0;i<n;i++)
printf("%d ",arr[i]);
free(arr);
}
This is nothing but a simple program to input the number of items in an array then take the input and print the inputted values. But whenever I go to input the values in the array after taking the number of items from the user, my program CRASHES. I don't understand what I am doing wrong over here. Need some help immediately. Please run the program and see if it also crashes on other systems. I executed the program using an online compiler and strangely there it was running fine. I am using the Visual Studio 2013 command line compiler to execute my program.

Answer 1

Compiling this piece of code with gcc I get:
calloc.c: In function ‘main’:
calloc.c:4:5: warning: incompatible implicit declaration of built-in function ‘printf’ [enabled by default]
calloc.c:5:5: warning: incompatible implicit declaration of built-in function ‘scanf’ [enabled by default]
calloc.c:7:17: warning: incompatible implicit declaration of built-in function ‘calloc’ [enabled by default]
calloc.c:18:5: warning: incompatible implicit declaration of built-in function ‘free’ [enabled by default]
using clang:
calloc.c:1:1: error: 'main' must return 'int'
void main(void) {
^
calloc.c:4:5: warning: implicitly declaring C library function 'printf' with type 'int (const char *, ...)'
printf("Enter the number of elements you want to enter=");
^
calloc.c:4:5: note: please include the header <stdio.h> or explicitly provide a declaration for 'printf'
calloc.c:5:5: warning: implicitly declaring C library function 'scanf' with type 'int (const char *, ...)'
scanf("%d",&n);
^
calloc.c:5:5: note: please include the header <stdio.h> or explicitly provide a declaration for 'scanf'
calloc.c:7:17: warning: implicitly declaring C library function 'calloc' with type 'void *(unsigned long, unsigned long)'
arr = (int*)calloc(n,sizeof(int));
^
calloc.c:7:17: note: please include the header <stdlib.h> or explicitly provide a declaration for 'calloc'
calloc.c:18:5: warning: implicit declaration of function 'free' is invalid in C99 [-Wimplicit-function-declaration]
free(arr);
^
4 warnings and 1 error generated.
As clang kindly pointed out, you need to add
#include <stdlib.h>
#include <stdio.h>
to the top of your code, as you are using functions that are declared within them. The functions you are using are in the standard C library, so the code will still compile but you should expect the unexpected. The reason your program crashes is likely to be that the compiler has made an incorrect assumption about the return type of calloc, as it doesn't have the correct declaration. Many systems will let you get away with this but at the very least they should warn you.
On a side note, please see this discussion on casting the return value of malloc (don't cast it). This advice also applies to calloc.
edit
Here's a minimal example which might shed some light on the issue:
a.c
#include <stdio.h>
int main()
{
printf("%f\n", f());
return 0;
}
b.c
float f()
{
return 4.2;
}
These two files can be compiled separately into object files:
gcc -c a.c # creates a.o
gcc -c b.c # creates b.o
No warnings or errors are generated. Switching to C99 mode causes an "implicit function declaration" warning. Note that at compile-time, the function definition isn't needed. These two object files can be linked:
gcc a.o b.o
Now, the linker finds the definitions of the functions so there is no error but it is already too late. The implicit declaration of f() when a.c was compiled causes the output to be incorrect when the program is run (I get 0.000000).
The same thing has happened here as in your question. The function calloc can be found, so there is no error at link-time. However, an incorrect assumption is made about the type that it returns, so it doesn't behave as expected on your system.