Why am I getting segmentation fault with this code?
#include<stdio.h>
typedef struct
{
int val;
} DEVICE;
main()
{
DEVICE *dev_ptr;
dev_ptr->val = 21;
printf(" %d ",dev_ptr->val);
}
I know the correct way of assigning the values. For that we need to add these lines
DEVICE simple;
dev_ptr = &simple;
simple.val = 21;
but I want to know what is wrong with previous code ?
When you assign to val, the dev_ptr is uninitialized, you never set it to point at somewhere valid.
So you're (probably) writing to a "random" location in memory, which (absolutely) gives undefined behavior.
The fix should be something like:
DEVICE my_device;
DEVICE *dev_ptr = &my_device;
dev_ptr->val = 21;
printf("my_device.val=%d\n", my_device.val);
The above will print 21.
In your first code, dev_ptr is used uninitialized. Using uninitialized memory leads to undefined behaviour, with a possible side effect of a segmentation fault.
You need to allocate memory [what you're doing in second snippet] to the variable before using [dereferencing] it.
You've got the compile-time memory allocation and using that address for dev_ptr absolutely correct in your second code snippet. Now, see the below code for dynamic allocation.
Try something like
int main() // use proper signature
{
DEVICE *dev_ptr = malloc(sizeof(DEVICE)); // allocate memory
if (dev_ptr) //check for malloc success
{
dev_ptr->val = 21;
printf(" %d ",dev_ptr->val);
}
free(dev_ptr); //make valgrind happy, prevent memory leak
return 0; // have a return statement
}