Thread: Dynamic memory allocation

Hello everyone

I do not understand what is dynamic memory allocation in programming. I have read many tutorials and have read the topics given in the books but I am not able to understand clearly.

I know dynamic memory allocation at runtime. It is said that when we do not know how much memory will be required, then we should use dynamic memory. But this is not understandable to me because to store any record we should know about its size.

Can anyone clarify why it say when we don't know how much memory we need then we should use dynamic memory

>"If the size is not know at compilation; but, it going to be know at run-time then it is normal to use dynamic memory allocation."

This is the main key point of my problem

Whenever I start writing a program, it becomes necessary for me, I need to know first how many variables I have to store. Like if I want to store Integer, first thing comes to my mind is how many Integer values ​​I have to store. If i want to store more than one integer i will use array If I want to store 10 integers then the size will be 10

Suppose you are reading data from a file.

The data are in records. Each record includes name, address, postal code, phone, etc., mostly on separate lines.

The first line of the file is an integer that indicates how many records.

At compilation time, you don't know what the filename is, how many records there are, how long the various strings are, or anything else.

This is pretty normal for computer programs. Dynamic allocation is really the default, not the exception.

If you want to see more examples like this, try one of the code gamification sites. They're chock full of "first line contains N, the number of ..." type problems to solve.

I think I almost understood when to use dynamic memory. If there is some mistake in my understanding please correct me.

Dynamic memory is used when we do not know really how much memory we need. If we have allocated more memory than required, we are just wasting memory. If we allocate less memory than required, the system will not be able to save the record.

For example, if i have to store the marks of a student in a school. I should definitely allocate dynamic memory because I don't know how many students are in the school.

If I allocate static memory for 200 students, then It will waste of memory if there are only 100 students in the school. if I allocate static memory for 50 students. It would be fail because I haven't allocate enough memory.

If I did allocate dynamic memory for 50 students and later I come to know that there are 50 more students then I can allocate memory space again for 50 students.

This is all my understanding for dynamic memory allocation i have explain in details. Please clarify if I have misunderstood the basic concepts somewhere.

Thank you all for your great help

Originally Posted by Rahul11

Hello everyone

I do not understand what is dynamic memory allocation in programming. I have read many tutorials and have read the topics given in the books but I am not able to understand clearly.

I know dynamic memory allocation at runtime. It is said that when we do not know how much memory will be required, then we should use dynamic memory. But this is not understandable to me because to store any record we should know about its size.

Can anyone clarify why it say when we don't know how much memory we need then we should use dynamic memory

Short of taking a course in C Programming from a qualified instructor, you need to study a good book on the C Programming Language, cover to cover, and do all the exercises at the end of each chapter! Choose one of the three listed below:

C Programming, A Modern Approach
Author: K. N. King

C Primer Plus, 6th Edition
Stephen Prata

C How to Program, 8/e
Deitel & Deitel

Please avoid any painfully inadequate online YouTube videos, or equally inadequate so-called, "tutorials". You will only get part of what you need to know, or worse, incorrect information.

Originally Posted by awsdert

Just as a side note it is usually best to maintain a list of allocated memory blocks, easier to ensure all are free'd before termination in the event it was missed elsewhere or you simply wanted to save some time by keeping memory allocated and just passing it onto the next thing that want's it.

I strongly disagree with this advice. "Maintaining a lost of allocated memory blocks" is exactly what malloc(), free(), realloc() and so on do on the inside, and usually do it in a far more nuanced way than any of us ever will. For example, large allocations may be "memory mapped" blocks, where small allocations come off of a traditional heap. IIRC the current Linux glibc malloc() runs multiple heaps (called "arenas") allowing multiple threads to perform memory allocation at the same time in a thread-safe manner.

You are far better using memory-safe coding patterns to ensure everything that is allocated gets freed, and using tools like "valgrind" to ensure that you don't have memory leaks, than to implement workarounds.

Well, that's my opinion anyway.

Originally Posted by awsdert

Just as a side note it is usually best to maintain a list of allocated memory blocks, easier to ensure all are free'd before termination in the event it was missed elsewhere or you simply wanted to save some time by keeping memory allocated and just passing it onto the next thing that want's it.

Is it necessary to call free() when you're about to reach program termination? The entire program is about to be unloaded anyway..

Originally Posted by Nwb

Is it necessary to call free() when you're about to reach program termination? The entire program is about to be unloaded anyway..

It is always best to free any memory you have allocated using, malloc(), calloc(), or realloc(), then using valgrind under Linux, you can double check for any allocations you missed, along with other memory errors. I use valgrind on all my code.

The other issue with free is not setting the pointer to NULL after freeing the memory. You CANNOT access the memory after freeing it up!!! A better free() can help if consistently used:

Code:

#include <stdio.h>
#include <stdlib.h>

void *free2(void *ptr);

int main(void)
{
   char *ptr = NULL;

   ptr = malloc(100);

   ptr = free2(ptr);

   /* Can also be called as with free():
      free2(ptr);
      ptr = NULL;
   */

   return 0;
}

void *free2(void *ptr)
{
    free(ptr);
    return NULL;
}

This is the way free() should have been defined by Dennis!

Originally Posted by laserlight

and besides, the amount of memory available on the stack could be pretty limited.

Did you mean to say that memory available in stack is limited compared to heap (I got this impression because the discussion was about allocating memory on the heap)?

If so, why would that be? I had read that the stack and heap share the same memory space.. stack being at the top and growing downwards and heap being at the bottom and grouping upwards until they meet and there is no more memory left. So since they share the same memory space and grow until they can't.. they should have the same memory limit..