how to free memory in C - c

Question

I'm writing a C program but i'm getting some memory errors so i decided to make a small program to see when and how to deallocate memory.
My example :
int main() {
char *a=malloc(5);
a="mama";
char *b=malloc(strlen(a));
printf("%s\n",a);
strcpy(b,a);
free(a);
printf("%s\n",b);
return 0;
}
here i'm getting an error :
* Error in `./a': free(): invalid pointer: 0x0000000000400757 *
but when i change just one line :
int main() {
char *a=malloc(5);
strcpy(a,"mama");
char *b=malloc(strlen(a));
printf("%s\n",a);
strcpy(b,a);
free(a);
printf("%s\n",b);
return 0;
}
My second program is working well, so why i'm getting this error in the first?
i hope that you can explain me why.
In My Real program I'm doing like that :
char * getDate(){
char *date=malloc(5);
//some instructions here
return date;
}
int main() {
char *a=malloc(5);//im sure that a will not exceed 5 bytes
a=getDate();
return 0;
}
It's just a small example , and i don't know where to free memory .so how and where i can free memory , have I to free date or the char pointer?
Edited second Time:
void getDate(char *a){
//some instructions here
strcpy(a,"haha");
}
int main() {
char *a=malloc(5);
getDate(a);
int i= strlen(a);
printf("%s and the size is :%d\n",a,i);
free(a);
return 0;
}
what are the rules i have to follow to avoid memory errors.
many thanks.

Answer 1

In the line:
char *a=malloc(5);
You assign a with the address of a dynamically allocated memory block, but then in the line:
a="mama";
You reassign it with the address of the literal string constant "mama". Discarding the pointer to the dynamically allocated block so that when you call
free(a);
a is no longer a valid heap memory block, and cannot be deallocated.
Strings (and arrays in general) in C are not data first-class types and cannot be assigned. You should replace:
a="mama";
with
strcpy( a, "mama" ) ;
The assignment sets the pointer value - it does not copy "mama" to the memory points to by a. The strcpy() call on teh other hand copies the string to the dynamic memory block referenced by a.
In your second code fragment, you are "leaking" the memory allocated in main() since you have reassigned a without freeing the original block. Although in your example you have not free'd a, it remains freeable, because the allocation in GetDate() is dynamic. As a coding style, dynamically allocating memory in a function and returning its pointer is a bad idea since it makes it the responsibility of the caller to know that the memory was dynamically allocated and must be free'd, and it invites memory leaks.