■ Attack Lab
■ C Exercises
■ C Conventions
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■ C Debugging
■ Version Control
■ Compilation
■ Demonstration
Attack Lab…
is due Thursday. Start now if you haven’t yet.
Some warnings about C
■ It’s possible to write bad code. Don’t.
■ Watch out for implicit casting.
■ Watch out for undefined behavior.
■ Watch out for memory leaks.
■ Macros and pointer arithmetic can be tricky.
■ K&R is the official reference on how things behave.
int foo(unsigned int u) {
return (u > -1) ? 1 : 0;
int foo(unsigned int u) {
return (u > -1) ? 1 : 0;
-1 is cast to an unsigned int in the comparison, so the comparison that is happening is actually u > int_max. This always returns 0.
int main() {
int* a = malloc(100*sizeof(int));
for (int i=0; i<100; i++) {
a[i] = i / a[i];
return 0; }
int main() {
int* a = malloc(100*sizeof(int));
for (int i=0; i<100; i++) {
a[i] = i / a[i];
return 0; }
No value in a was initialized. The behavior of main is undefined.
int main() {
char w[strlen("C programming")];
strcpy(w,"C programming");
printf("%s\n", w);
int main() {
char w[strlen("C programming")];
strcpy(w,"C programming");
printf("%s\n", w);
strlen returns the length of the string not including the null character, so we end up writing a null byte outside the bounds of w.
struct ht_node {
int data; };
typedef struct ht_node* node;
node makeNnode(int k, int e) {
node curr = malloc(sizeof(node));
curr->key = k;
curr->data = e;
return curr;
struct ht_node {
int data; };
typedef struct ht_node* node;
node is a typedef to a struct ht_node pointer, not the actual struct. So malloc could return 4 or 8 depending on system word
node makeNnode(int k, int e) { size. node curr = malloc(sizeof(node)); curr->key = k;
curr->data = e;
return curr;
char *strcdup(int n, char c) {
char dup[n+1];
for (i = 0; i < n; i++)
dup[i] = c;
dup[i] = ’\0’;
char *A = dup;
char *strcdup(int n, char c) {
char dup[n+1];
for (i = 0; i < n; i++)
dup[i] = c;
dup[i] = ’\0’;
char *A = dup;
strcdup returns a stack- allocated pointer. The contents of A will be unpredictable once the function returns.
#define IS_GREATER(a, b) a > b
inline int isGreater(int a, int b) {
return a > b ? 1 : 0;
int m1 = IS_GREATER(1, 0) + 1;
int m2 = isGreater(1, 0) + 1;
#define IS_GREATER(a, b) a > b
inline int isGreater(int a, int b) {
return a > b ? 1 : 0;
int m1 = IS_GREATER(1, 0) + 1;
int m2 = isGreater(1, 0) + 1;
IS_GREATER is a macro that is not wrapped in parentheses. m1 will actually evaluate to 0, since 1 > 0+1 = 0.
#define NEXT_BYTE(a) ((char*)(a + 1));
int a1 = 54; // &a1 = 0x100
long long a2 = 42; // &a2 = 0x200
void* b1 = NEXT_BYTE(&a1);
void* b2 = NEXT_BYTE(&a2);
#define NEXT_BYTE(a) ((char*)(a + 1));
int a1 = 54; // &a1 = 0x100
long long a2 = 42; // &a2 = 0x200
void* b1 = NEXT_BYTE(&a1);
void* b2 = NEXT_BYTE(&a2);
b1 is a void pointer to the address 0x104. b2 is a void pointer to the address 0x208.
C Workshop
■ If you had trouble with the previous exercises, go!!!
■ Saturday, October 10 in the afternoon. Details TBA. ■ Material:
■ Undefined behavior, casting
■ Structs, pointers
■ Memory management
■ Standard library functions
■ Random stuff: macros, typedefs, function pointers, header guards… and anything else you have questions on!
The C Standard Library
■ #include
■ Use it. It is your friend!
■ Don’t write code that’s already been written!
■ Your work might have a bug or lack features
■ All C Standard Library functions are documented.
■ Use the UNIX man command to look up usage
Robustness
■ Code that crashes is bad.
■ Avoid making bad things!
■ Don’t write code with undefined behavior
■ Check for failed system calls and invalid input
■ Some errors should be recoverable, others not
■ Proxy Lab is an excellent example of this
■ Free memory that you allocate
■ Leaky code will crash (and code that crashes is bad!)
■ Memory leaks will cost you style points
Robustness: Continued
■ CSAPP wrappers check return values of system calls
■ Terminate program when error is encountered
■ Malloc, Free, Open, Close, Fork, etc.
■ Super duper useful for Proxy & Shell Labs
■ Alternatively, check for error codes yourself
■ Useful when you don’t want program to terminate
FILE *pfile; // file pointer
if (!(pfile = fopen(“myfile.txt”, “r”))) {
printf(“Could not find file. Opening default!”);
pfile = fopen(“default.txt”, “r”);
Quick C Tip: getopt
■ Used for parsing command-line arguments
■ Don’t write your own code for this. Not worth it.
■ In fact, we actively discourage it
■ Autograder randomizes argument order
■ Try it: man getopt
Style Points
■ We read and grade your code for style
■ Style guide: http://cs.cmu.edu/~213/codeStyle.html
■ Vim macro to highlight lines longer than 80 cols:
▪ 2mat ErrorMsg ‘\%80v.’
■ Emacs users… :
(setq whitespace-style ‘(trailing lines space-
before-tab indentation space-after-tab)
whitespace-line-column 80)
■ View your annotated code on Autolab
■ Step through C code side-by-side with Assembly
■ Print variables, not just registers and addresses!
■ Break at lines, not just addresses and functions!
■ gdbtui
■ Nice for looking at your code during execution
■ Type layout split to view Assembly alongside
■ Best tool for finding…
■ Memory leaks
■ Other memory errors (like double frees)
■ Memory corruption
■ Use gcc with -g to give you line numbers of leaks
■ Use valgrind –leak-check=full for thoroughness
Version Control: Your Friend
■ You may find it useful to use version control if you are already familiar with it
■ If not, that’s okay, it’s not required. Making regular submissions to Autolab can act as a checkpointing system too.
■ GNU Compiler Collection
■ Is a C compiler, among other things
■ We will give you instructions for compilation in handouts
■ man gcc if you’re having trouble
■ Lab handouts come with a Makefile ■ Don’t modify them
■ You write your own for Proxy Lab
■ Examples for syntax found in previous labs
■ make reads Makefile and compiles your project
■ Easy way to automate tedious shell commands
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