CSc 352 Freeing Dynamically Allocated Memory Saumya Debray Dept. of Computer Science The University of Arizona, Tucson

Motivation Dynamically allocated memory should be freed when no longer needed – The C runtime system does not automatically reclaim memory – memory leaks increase the program’s memory footprint this can lead to slow performance, program crashes This needs to be done with care: – too-aggressive freeing can lead to program errors – too-passive freeing can miss memory allocated “behind the scenes”, e.g., through strdup(). 2

Issues Understanding when dynamic memory allocation happens – explicit (by the programmer) – implicit (by library routines) Through the course of the program: – identifying when memory is no longer needed – being aware of multiple pointers to a block of memory Identifying and fixing memory leaks 3

Understanding dynamic memory allocation Dynamic allocation can be explicit or implicit – explicit: when the programmer invokes malloc, calloc, etc. – implicit: when the memory allocation happens “behind the scenes” in a library routine getline strdup GNU extension to scanf fopen … In general, if memory appears “by magic” at runtime, chances are it’s being allocated implicitly. 4

Freeing up memory Use free(p) when a memory block is no longer needed – p is a pointer to a block of memory previously allocated through malloc or calloc 5 Pitfalls: –accessing a previously- freed block is an error –freeing a block of memory twice is an error pointer malloc’d memory block watch out for multiple ways to reach a memory block

Freeing up memory General approach: – free up memory “hanging off” a block of memory – grab any pointers you need to traverse the rest of the data structure – then free the block itself 6 1 deallocate memory “hanging off” this node 2 grab this pointer 3 free this block

Finding memory leaks 1 7 use valgrind to determine whether the program has memory leaks

Finding memory leaks 2 8 indicates where the lost memory was allocated

Finding memory leaks 2 9 forgot to fclose() after reading the file!

After fclose-ing the input stream 10 less leakage than before, but still quite a bit need to free the linked list of states

Freeing up a linked list 1 11 naïve code to free the nodes in a linked list

Freeing up a linked list 2 12 st0st1stateList

Freeing up a linked list 3 13 no memory errors less memory being leaked but some leakage persists! reason: structures “hanging off” states not being freed // a DFA state typedef struct vertex { char *name; struct edge *transitions; bool isFinal; struct vertex *next; } state; // a transition between states typedef struct edge { char ch; struct vertex *from, *to; struct edge *next; } edge;

Freeing up a linked list 4 14 Solution: free up the edges “hanging off” each state before freeing hat state

Freeing up a linked list 4 15 a lot better! but some leakage still left!

Freeing up a linked list 4 16 // a DFA state typedef struct vertex { char *name; struct edge *transitions; bool isFinal; struct vertex *next; } state;

Freeing up a linked list 5 17

Summary Should free up dynamically allocated memory when done – Use valgrind to identify memory leak sources! Needs to be done carefully – freeing up “live” memory will result in program errors Pay attention to implicitly-allocated memory, e.g.: – file pointers – strings allocated through strdup() – when freeing up a struct, first free up any unused memory “hanging off” the struct 18