Dynamically resizing a struct in C - c

Question

I'm fiddling around with Object oriented programming in C (note! Not C++ or C# - just plain ol' C). Right now, I'm trying to dynamically resize a struct (I'm playing with writing a simple String class). The code builds okay:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct TestClass
{
char *s;
size_t size;
size_t b_size;
void (*CreateString) (struct TestClass*,char*);
};
void TestClassCreateString(struct TestClass *m, char* str)
{
char *buf;
m->size = strlen(str);
if (!m->size)
{
free(m->s);
m->s = malloc(16);
}
else
{
buf = realloc(m->s, m->size);
if (buf) m->s = buf;
}
}
struct TestClass* TestClassCreate()
{
struct TestClass* m = malloc((sizeof(struct TestClass)));
m->CreateString = TestClassCreateString;
return m;
}
int main()
{
struct TestClass* fizz = TestClassCreate();
fizz->CreateString(fizz,"Hello World");
free(fizz);
return 0;
}
…But on running it I get the following error:
malloc: *** error for object 0x5000000000000000: pointer being realloc'd was not allocated
*** set a breakpoint in malloc_error_break to debug
Is anyone able to identify where I've gone wrong? Thanks in advance!

Answer 1

malloc does not zero its memory; it returns garbage, so you get an invalid pointer inside this struct:
struct TestClass* m = malloc((sizeof(struct TestClass)));

Answer 2

When creating a struct TestClass in TestClassCreate() the code misses to properly initialise the freshly allocated struct.
So calling
free(m->s);
tries to free memory at a random address, which invokes undefined behaviour and typically crashes the program.
To fix this modify the code as follows
struct TestClass* TestClassCreate()
{
struct TestClass* m = ...
...
m->s = NULL;
m->size = 0;
m->b_size = 0;
return m;
}
To make things better also add some error checking:
struct TestClass* TestClassCreate()
{
struct TestClass * m = malloc((sizeof(struct TestClass)));
if (NULL != m)
{
m->CreateString = TestClassCreateString;
m->s = NULL;
m->size = 0;
m->b_size = 0;
}
return m;
}
To make the code even more fail-safe apply these last changes:
struct TestClass* TestClassCreate(void)
{
struct TestClass * m = malloc(sizeof *m);
...
Further more the code misses to allocate memory for the C-"string"'s 0-terminator here:
void TestClassCreateString(struct TestClass *m, char* str)
{
...
else
{
buf = realloc(m->s, m->size + 1); /* allocate 1 byte more for the trailing
`0` marking the end of a C-"string". */
...

Answer 3

You are short by 1-byte. You need to add 1 to m->size for the null-terminator if you intend to copy str to m->s. E.g.:
void TestClassCreateString(struct TestClass *m, char* str)
{
char *buf;
m->size = strlen(str);
if (!m->size)
{
free(m->s);
m->s = malloc(16);
}
else
{
buf = realloc(m->s, m->size + 1);
if (buf) m->s = buf;
strncpy (m->s, str, m->size + 1);
}
}
Then you can do something like:
int main()
{
struct TestClass* fizz = TestClassCreate();
fizz->CreateString(fizz,"Hello World");
printf ("\n fizz->s : %s\n\n", fizz->s);
free(fizz);
return 0;
}
and get:
$ ./bin/oo_struct
fizz->s : Hello World