Total beginner questions

Good day all - this is my first post.

I've recently started to learn C and I love to be so close to the hardware. However, I have a question about char *. Does the following code cause a memory leak? I'm almost sure it does and sends shivers down the spine of any respectable c-programmer! :-)

What is the best way to do what I'm trying to achieve? I'm thinking of defining a char-array and passsing the address of tha to the doSomeSillyThing function.

Code:

---

int doSomeSillyThing(char **msg)
{
  *msg = "Are you sure this is not causing a memory leak?"
  return 10;
}

int main()
{
  char **msg;
  int code = doSomeSillyThing(msg);
}

---

Thanks for the help...

No, it doesn't cause a memory leak, but only because you're trashing memory your program most likely doesn't own. ;)

To allocate memory dynamically use something like malloc().

Code:

---

int doSomeSillyThing(char **msg)
{
  static char *bleh = "Are you sure this is not causing a memory leak?";
  *msg = malloc(strlen(bleh) + 1);
  strcpy(*msg, bleh);
  return 10;
}

---

Note: You must free() the memory you allocate. However, you have a different problem to address first. You should be declaring msg in main() as a char *, and then passing its address. So in main()....:

Code:

---

int main()
{
  char *msg;
  int code = doSomeSillyThing(&msg);
  printf("msg = %s\n", msg);
  free(msg);
  return 0;
}

---

Note all the changes. They are important. ;)

Edit: And don't forget to include the proper header files. stdio.h is needed for printf() and stdlib.h is needed for malloc() and free().

Complex thing in C I guess :)

Incidentally, bleh should be a const char*. You could declare it as a const char[] instead, upon which you could use sizeof instead of strlen(), e.g.,

Code:

---

static const char bleh[] = "Are you sure this is not causing a memory leak?";
*msg = malloc(sizeof bleh);
strcpy(*msg, bleh);

---

Quote:

---

And don't forget to include the proper header files. stdio.h is needed for printf() and stdlib.h is needed for malloc() and free().

---

Also, <string.h> should be included for strcpy() and strlen().

Quote:

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Originally Posted by audinue

Complex thing in C I guess :)

---

Not really. In fact, your compiler may already be warning you about using an un-initialized variable.

What you do is define a pointer-to-C-string, then dereference it before giving it a value.

You don't need to allocate it manually, as long as you don't plan on changing the contents of msg later. This is perfectly fine:

Code:

---

char *msg = "Some string";

---

so this is too:

Code:

---

void setMsg(char **msg)
{
    *msg = "Some string";
}

int main(void)
{
    char *msg;
    setMsg(&msg);
}

---

You should probably throw in a few const specifiers to make it clear that you're not supposed to try and alter the contents.

--
Computer Programming: An Introduction for the Scientifically Inclined

More correctly:

Code:

---

void setMsg(const char** msg)
{
    *msg = "Some string";
}

int main(void)
{
    const char* msg;
    setMsg(&msg);
}

---

When assigning string literals, always use const. They are unmodifiable (most of the time anyway).
However, when applying const here, we see the limit of this approach: we can't actually modify the msg string later due to it being const.
If you need to change it, you'll have to retort to something along the lines of the solutions above.