Memory Management III:
Perils and pitfalls
Oct 15, 1998
Topics
•Review of C pointer references
•Memory-related bugs
•Debugging versions of malloc
•Binary translation
•Garbage collection
class16.ppt
15-213
Introduction to Computer Systems
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Memory Management III: Perils and Pitfalls - Memory-related Bugs and Debugging Techniques, Slides of Introduction to Computers
A class presentation from a computer systems course (cs 213) focusing on memory management in c programming. It covers various memory-related bugs, such as dereferencing bad pointers, reading uninitialized memory, overwriting memory, and referencing nonexistent variables. The document also discusses debugging techniques, including using conventional debuggers, debugging versions of malloc, binary translation, and garbage collection.
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Memory Management III:
Perils and pitfalls
Oct 15, 1998
Topics
Review of C pointer references
Memory-related bugs
Debugging versions of malloc
Binary translation
Garbage collection
Introduction to Computer Systems
- 2 –
class16.ppt
C operators
Operators
Associativity
[]
left to right
(type)
sizeof
right to left
left to right
left to right
left to right
left to right
left to right
left to right
^
left to right
left to right
left to right
left to right
right to left
^=
right to left
left to right
Note: Unary +, -, and * have higher precedence than binary forms
- 4 –
class16.ppt
Memory-related bugs
Failing to free blocksReferencing freed blocksFreeing blocks multiple timesReferencing nonexistent variablesOverwriting memoryReading uninitialized memoryDereferencing bad pointers
- 5 –
Dereferencing bad pointers
The classic scanf bug scanf(“%d”, val);
- 7 –
Allocating the wrong sized object Overwriting memory
for (i=0; i<N; i++) {p = malloc(N*sizeof(int)); int **p; p[i] =
(^) malloc(M*sizeof(int));
- 8 –
Overwriting memory
Off-by-one
for (i=0; i<=N; i++) {p = malloc(N*sizeof(int *)); int **p; p[i] =
(^) malloc(M*sizeof(int));
- 10 –
Forgetting that strings end with ‘/0’ Overwriting memory
strcpy(t, s);char s[8] = “1234567”; char t[7];
- 11 –
Not checking the max string size^ Overwriting memory
gets(s);int i; char s[8];
/* reads “123456789” from stdin */
- 13 –
search(int *array,^ Misunderstanding pointer arithmetic^ Overwriting memory
(^) int val) {
while (*p && *p !=
(^) val)
p += (^) sizeof(int);
- 14 –
class16.ppt^ Referencing nonexistent variables
Forgetting that local variables disappear when a
function returns
int *foo () { return &val;int val;
- 16 –
Referencing freed blocks
Evil!
for (i=0; i<M; i++)y = malloc(Msizeof(int));...free(x);
- 17 –
Failing to free blocks
(memory leaks)
slow, long-term killer!
foo() { int (^) x = malloc(Nsizeof(int));
return;...
- 19 –
class16.ppt
Dealing with memory bugs
Conventional debugger (gdb)
good for finding bad pointer dereferences
hard to detect the other memory bugs
Debugging malloc (CSRI UToronto malloc)
wrapper around conventional malloc
- memory leaks– some instances of freeing blocks multiple times – memory overwrites that corrupt heap structures detects memory bugs at malloc and free boundaries
- referencing freed blocks– freeing block twice that has been reallocated in the interim – overwrites into the middle of allocated blocks Cannot detect all memory bugs
- 20 –
class16.ppt Dealing with memory bugs (cont)
Binary translator (Atom, Purify)
powerful debugging and analysis technique
rewrites text section of executable object file
can detect all errors as debugging malloc
- referencing outside of allocated block– overwriting – bad pointers can also check each individual reference at runtime