CODEWITHSUNDEEP
cmemory-management
Deepak
Deepak

Reputation: 1545

trying to create region based memory pools in c

I am trying to create a memory pool in this function which returns true if the pool is created successfully. This function is supposed to create a pool of memory with name passed as "*name" and size of "int size".

boolean create_memory_pool(char *name, int size)
{
  boolean task;
  task = (name = (char *) malloc (size));
  if(task == true)
    printf("task is true memory pool created");
  else if(task ==false)
    printf("task is false");
  else
    printf("unexpected");

  return task;
}

Its printing "unexpected" which means something else is happening. I have defined the boolean as:

typedef enum{
  false,
  true
}boolean;

Upvotes: 0

Views: 139

Answers (5)

Yu Hao
Yu Hao

Reputation: 122423

After malloc, name is a non-null pointer if successful, a null pointer otherwise.

So the expression name = (char *) malloc (size) has type char *, you are assigning this value to task, which should give you a warning because you didn't use a cast.

In a successful malloc, the pointer name is non-null, which is not the integer 1, that's why task would be neither equal to true(1), nor false(0)

Actually you don't need task, just test if the value name is a null pointer:

boolean create_memory_pool(char *name, int size)
{
   name = malloc(size);
   if (name != NULL)
   {
       printf("malloc successful\n");
       return true;
   }
   else
   {
       printf("malloc failed\n");
       return false;
   }
}

Upvotes: 2

Harikrishnan
Harikrishnan

Reputation: 3812

malloc returns NULL (ie, 0, in your case false), if it cant allocate memory. It returns the address of newly allocated memory if it successfully allocated a memory, which wont be exactly 1. Your true is set as 1, so all values not equal to 1 will not be 'true'. That's why you are getting unexpected, even it is successful.

What you can do is 

if(task ==false)
  printf("task is false");
else
  printf("task is true memory pool created");

Upvotes: 0

Carl Norum
Carl Norum

Reputation: 224992

The simplest solution to your problem is to make sure that task only gets a 1 or 0, as opposed to your current code, which assigns a pointer value to it instead. The following code will do just that:

task = !!(name = malloc(size));

The first ! operator will convert the result of the malloc call into a 1 if malloc returned NULL and 0 otherwise, and the next ! operator will change it back into the 1 or 0 you care about.

Alternately, use a real boolean type rather than rolling your own:

_Bool task;

or:

 bool task;

if you #include <stdbool.h> first.

Note: I also removed the unnecessary typecast. Your current code should probably be giving you a warning about converting a pointer to an integer without a cast - you should heed such warnings and correct the code appropriately. For example, from clang:

example.c:12:8: warning: incompatible pointer to integer conversion assigning to
      'boolean' from 'char *' [-Wint-conversion]
  task = (name = (char *) malloc (size));
       ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

Upvotes: 1

user1814023
user1814023

Reputation:

To make it work use

  task = ((name = (char *) malloc (size))!=NULL);

Everything in C returns a value. and enum is basically int. when you say 

task = (name = (char *) malloc (size));

The expression (name = (char *) malloc (size)); also return value. The value returned by that expression is pointer.

task gets the pointer value. So it is printing unexpected

Upvotes: 1

Sam Hartman
Sam Hartman

Reputation: 6499

First, when you do something like

task = (name = (char *) malloc(size));

You're saying that task gets the same thing as name. Since name gets the return of malloc, that's going to be a memory address. So task is unlikely to be either true or false. Instead try something like if(task) return true; else return false;

Secondly, name is an input parameter to the function. your caller will not be able to read the output. It's not clear why you call the name input name anyway; it's never used as the name of anything, and in fact never read. Instead, you assign the output of malloc to it. Which is probably not what you want.

Upvotes: 0

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