Question

Coding in C:

typedef struct {
   char *string;
   int one;
   int two;
} Example;

… elsewhere:

Example *new = (Example *)malloc(sizeof(Example*));
new->string = strdup(otherstring); //no problem here
new->one = 5; //valgrind says that there is an invalid write of size 4.

My program runs fine, I just can’t make valgrind happy. And that probably means I will have some other error elsewhere.

I guess I don’t have to declare a pointer to the structure, (i.e., I could call “Example new” instead), but I’m stuck finding out how to allocate memory on the heap because I will need to access “new” from other parts of the program.

Am I making a mistake in the above few lines? Or is my error likely to be somewhere else? This is the first memory error that valgrind reports.

EDIT: accidentally had int *s instead of ints in my struct. Fixed.

Answer 1

I see various problems here. First of all, this:

Example *new = (Example *)malloc(sizeof(Example*));

doesn’t allocate the right amount of memory (and you don’t need the cast). You want this:

Example *new = malloc(sizeof(Example));

Then, you say this:

new->one = 5;

and that’s assigning an int to an int*; that’s not a good idea and valgrind rightly complains about it. If your struct is properly declared, then you want this:

new->one = malloc(sizeof(int)); /* The other malloc doesn't allocate this, you have to. */
*(new->one) = 5;

I suspect (as you say that everything works) that you really mean to declare your struct like this:

typedef struct {
   char *string;
   int one;
   int *two;
} Example;

but there isn’t enough information to be certain. Then you probably still have similar issues with new->two.