1. CS414 C Programming Tutorial Ben Atkin batkin@cs.cornell.edu

2. CS414 C Programming Tutorial2 Why use C?  C is not "safe"  It assumes you know what you're doing  It has a lot of scope for bugs  It's a good systems programming language  Not much is hidden  Faster than Java, more predictable performance  It has all the low-level operations

3. CS414 C Programming Tutorial3 Types  No Boolean type, use int : "false" is 0, true is non-zero  Structures for grouping related data  "Like Java classes, but no methods" typedef int pid_t; /* for clarity */ struct pcb { pid_t pid; file_t openfiles[MAXOPENFILES];... } typedef struct pcb pcb_t; /* rename */

4. CS414 C Programming Tutorial4 Constants #define KB 1024 /* constant */ #define MB (KB*1024) /* note brackets */ /* wrong! ';' causes compile error */ #define WARN(s) fprintf(stderr, s); /* alternative constant definition: PROC_READY==0, PROC_RUNNING==1,... */ enum { PROC_READY, PROC_RUNNING,... }; /* give it a name */ typedef enum { PROC_READY, PROC_RUNNING,... } proc_status_t;

5. CS414 C Programming Tutorial5 Dynamic memory allocation  Memory must be explicitly allocated and freed #include int main() { /* declare and allocate memory */ int *ptr = (int *) malloc(sizeof(int)); *ptr = 4; /* use the memory */ free(ptr); /* free it for re-use */ }

6. CS414 C Programming Tutorial6 Example memory layout 0x0 0x2000 0x50000 code stack heap Dynamic allocation malloc(), free() Function calls, local variables Immutable

7. CS414 C Programming Tutorial7 Pointer initialisation  Always initialise, even if only to NULL #include int main() { int x = 5; int *ptr; x = *ptr; /* unpredictable: initialise! */ ptr = &x; /* good initialisation */ *ptr = 4; /* bad initialisation */ *ptr = NULL; /* good 5 */ }

8. CS414 C Programming Tutorial8 Changing pointer types  Pointers can be converted between types  type_t *x = (type_t *) y;  (void *) equivalent to any type, e.g. implementation of generic collections  malloc() returns a void *  Useful for low-level operations  e.g. networking, memory allocation

9. CS414 C Programming Tutorial9 Parameter passing  Functions can take pointer arguments int function(int *in, int *out);  Two main purposes  So that function can modify the variable (pass-by- reference)  Efficiency: passing " big_struct_t x " is not efficient, passing &x is efficient  Return values are often used to signal errors  e.g. return value 0=="no error", non-zero==error

10. CS414 C Programming Tutorial10 Parameter passing void function(int *arg1, char **arg2) {... } int main(void) { int x = 5; int *y = &x; char string[] = "Hello, world!"; char *ptr = NULL; function(&x, NULL); function(y, &ptr); /* *ptr can be changed */ function(y, &string); /* probably wrong */ }

11. CS414 C Programming Tutorial11 Pointers and functions /* function returning integer */ int func(); /* function returning pointer to integer */ int *func(); /* pointer to function returning integer */ int (*func)(); /* pointer to function returning pointer to integer */ int *(*func)(); Hide this complexity with typedefs! e.g. typedef void (*interrupt_handler)(void *); void an_interrupt_handler(void *arg);

12. CS414 C Programming Tutorial12 Multiple modules #include #include "queue.h" int main() { queue_t queue; queue_init(&queue);... } #include "queue.h" void queue_init(queue_t* q) {... }... typedef struct {... } queue_t; void queue_init(queue_t* q); Standard library

13. CS414 C Programming Tutorial13 External definitions #include #include "defs.h" int main() {... if (tickcount > 5) set_timer(); } #include "defs.h" int tickcount = 0; void set_timer() { printf("Timer at %d.", tickcount); } /* defs.h */... extern int tickcount;...

14. CS414 C Programming Tutorial14 Hints and tips  Avoid bugs in your program:  don't index past the end of an array  don't return pointers to local variables from functions  initialise data properly, especially pointers  check error codes (no exceptions in C)  free() what you malloc()  destroy all pointers to what you free() ... or else you might only find them when you least expect it!

15. CS414 C Programming Tutorial15 Hints and tips  Program safely!  Plan before you start  Pay attention to compiler warnings  Use assert() to check conditions are valid  Indent your code to make it easy to read  Beware of complicated #defines  Use the debugger  If you're serious about C, get K&R  Lots of library functions for you to use