Re: Issue with free() for linked list implementation
On 4/3/15 6:08 PM, Kitt wrote: On Friday, 3 April 2015 at 22:06:06 UTC, Namespace wrote: On Friday, 3 April 2015 at 22:02:13 UTC, Kitt wrote: Hello. I’m trying to write my own version of a list that doesn’t rely on the garbage collector. I’m working on a very bare bones implementation using malloc and free, but I’m running into an exception when I attempt to call free. Here is a very minimal code sample to illustrate the issue: // Some constant values we can use static const int two = 2, ten = 10; // Get memory for two new nodes Node* head = cast(Node*)malloc(two.sizeof); Node* node1 = cast(Node*)malloc(ten.sizeof); // Initialize the nodes node1.value = ten; node1.next = null; head.value = two; head.next = node1; // Attempt to free the head node Node* temp = head.next; head.next = null; free(head); // Exception right here head = temp; Note, if I comment out the line ‘head.next = node1’, this code works. Does anyone know what I’m doing wrong with my manual memory management? Why did you allocate only 2 / 10 bytes and not Node.sizeof bytes? Since your Node struct has at least one pointer (nexT) and a value (I assume of type int) you must allocate at least 8 bytes for one Node. I'm sure that is at least one of your problems. Wow, I can't even begin to explain how red my cheeks are right now. You're completely right; I have no idea what my head was thinking. Sure enough, call malloc with the correct type, and the error goes away =P Thanks for the help =) I guess I've been in C# land at work for way too long now, my low level C skills are evaporating! I'm not here to redden your cheeks any further, but I did want to make sure you understood what actually was happening above: 1. you have established 2 integers named 'two' and 'ten'. These are simply integers. 2. When you malloc, you use 'two.sizeof' and 'ten.sizeof'. Integers are 4 bytes, so you were allocating 4 bytes for each of these (not 2 or 10 bytes as is alluded to above). 3. Then you are casting the resulting pointer as pointing at a Node *. I'm assuming, having implemented linked lists many times and seeing your usage of Node, that it has at least a pointer and a value. Best case, this needs at least 8 bytes of space (32-bit CPU), and worst case 16 bytes (64-bit CPU). 4. When you access the Node * flavored pointer to your 4-byte block, you were corrupting memory in any case. Why does the free fail? Probably due to corrupted memory, be careful when using casts and C malloc. -Steve
Re: Issue with free() for linked list implementation
2. When you malloc, you use 'two.sizeof' and 'ten.sizeof'. Integers are 4 bytes, so you were allocating 4 bytes for each of these (not 2 or 10 bytes as is alluded to above). Yeah, my mistake. I saw the mistake but could not describe it correctly. :)
Re: Issue with free() for linked list implementation
Namespace: I've written a straight forward linked list implementation here: https://github.com/nomad-software/etcetera/blob/master/source/etcetera/collection/linkedlist.d Even though I'm using the GC to manage memory, maybe it will help you. Good idea to link to some existing code. Here is mine: https://github.com/Dgame/m3/blob/master/source/m3/List.d In 99%+ of cases it's a bad idea to use a linked list. Bye, bearophile
Re: Issue with free() for linked list implementation
On Saturday, 4 April 2015 at 09:05:03 UTC, bearophile wrote: Namespace: I've written a straight forward linked list implementation here: https://github.com/nomad-software/etcetera/blob/master/source/etcetera/collection/linkedlist.d Even though I'm using the GC to manage memory, maybe it will help you. Good idea to link to some existing code. Here is mine: https://github.com/Dgame/m3/blob/master/source/m3/List.d In 99%+ of cases it's a bad idea to use a linked list. Bye, bearophile The thread creator wanted to use it.
Re: Issue with free() for linked list implementation
On Friday, 3 April 2015 at 22:08:52 UTC, Kitt wrote: Thanks for the help =) I guess I've been in C# land at work for way too long now, my low level C skills are evaporating! I've written a straight forward linked list implementation here: https://github.com/nomad-software/etcetera/blob/master/source/etcetera/collection/linkedlist.d Even though I'm using the GC to manage memory, maybe it will help you.
Re: Issue with free() for linked list implementation
On Friday, 3 April 2015 at 22:02:13 UTC, Kitt wrote: Hello. I’m trying to write my own version of a list that doesn’t rely on the garbage collector. I’m working on a very bare bones implementation using malloc and free, but I’m running into an exception when I attempt to call free. Here is a very minimal code sample to illustrate the issue: // Some constant values we can use static const int two = 2, ten = 10; // Get memory for two new nodes Node* head = cast(Node*)malloc(two.sizeof); Node* node1 = cast(Node*)malloc(ten.sizeof); // Initialize the nodes node1.value = ten; node1.next = null; head.value = two; head.next = node1; // Attempt to free the head node Node* temp = head.next; head.next = null; free(head); // Exception right here head = temp; Note, if I comment out the line ‘head.next = node1’, this code works. Does anyone know what I’m doing wrong with my manual memory management? Why did you allocate only 2 / 10 bytes and not Node.sizeof bytes? Since your Node struct has at least one pointer (nexT) and a value (I assume of type int) you must allocate at least 8 bytes for one Node. I'm sure that is at least one of your problems.
Re: Issue with free() for linked list implementation
On Friday, 3 April 2015 at 22:06:06 UTC, Namespace wrote: On Friday, 3 April 2015 at 22:02:13 UTC, Kitt wrote: Hello. I’m trying to write my own version of a list that doesn’t rely on the garbage collector. I’m working on a very bare bones implementation using malloc and free, but I’m running into an exception when I attempt to call free. Here is a very minimal code sample to illustrate the issue: // Some constant values we can use static const int two = 2, ten = 10; // Get memory for two new nodes Node* head = cast(Node*)malloc(two.sizeof); Node* node1 = cast(Node*)malloc(ten.sizeof); // Initialize the nodes node1.value = ten; node1.next = null; head.value = two; head.next = node1; // Attempt to free the head node Node* temp = head.next; head.next = null; free(head); // Exception right here head = temp; Note, if I comment out the line ‘head.next = node1’, this code works. Does anyone know what I’m doing wrong with my manual memory management? Why did you allocate only 2 / 10 bytes and not Node.sizeof bytes? Since your Node struct has at least one pointer (nexT) and a value (I assume of type int) you must allocate at least 8 bytes for one Node. I'm sure that is at least one of your problems. Wow, I can't even begin to explain how red my cheeks are right now. You're completely right; I have no idea what my head was thinking. Sure enough, call malloc with the correct type, and the error goes away =P Thanks for the help =) I guess I've been in C# land at work for way too long now, my low level C skills are evaporating!
Re: Issue with free() for linked list implementation
Wow, I can't even begin to explain how red my cheeks are right now. You're completely right; I have no idea what my head was thinking. Sure enough, call malloc with the correct type, and the error goes away =P Thanks for the help =) I guess I've been in C# land at work for way too long now, my low level C skills are evaporating! Sometimes you wonder how simple such a problem can be. :D My pleasure.
Re: Issue with free() for linked list implementation
On Friday, 3 April 2015 at 22:38:00 UTC, Gary Willoughby wrote: On Friday, 3 April 2015 at 22:08:52 UTC, Kitt wrote: Thanks for the help =) I guess I've been in C# land at work for way too long now, my low level C skills are evaporating! I've written a straight forward linked list implementation here: https://github.com/nomad-software/etcetera/blob/master/source/etcetera/collection/linkedlist.d Even though I'm using the GC to manage memory, maybe it will help you. Good idea to link to some existing code. Here is mine: https://github.com/Dgame/m3/blob/master/source/m3/List.d
