(language, compilation and debugging) David 09/16/2011

Content  C language overview  Compilation  debugging

Basic data type  Integer int, short, long, long long and unsigned  Char a byte has the same number of bits of char  _Bool Integer with 1 bit in memory  Floating point float, double, and long double

Other data type  Pointer type *p; store the address of memory location  array type name[dim];  struct struct ex{ int a; char b;};  const int * const p; /*a const pointer to a int*/ const int * p; /*a pointer to a const int*/

Statement  conditionals if switch-case  loops while (do-while)  while(1){} for  break/continue change the flow of execution

Functions  functions written by yourself return-type function-name ( argument-list-if-necessary ) {...local-declarations......statements... return return-value; }  library functions include header files in your code  libraries stdio.h string.h stdlib.h

Example - 1 int max(int a, int b) { if(a > b) return a; else return b; } int factorial(int a) { int ret = 1; if(a < 0) return -1; while(a > 0) ret *= a--; return ret; }  If and while statements

Example - 2 int even_sum (int m) { int sum = 0; while (m > 0) { switch (m%2) { case 1: m--; break; case 0: sum += m--; } return sum; }  switch statement

Example - 3 float thres_sum(float *array, int size, float thres){ int i; float sum = 0; for(i = 0; i < size; i++) { if(array[i] <= thres) continue; else sum += array[i]; } return sum; }  for statement and pointer

Example - 4 #define PI typedef struct cylinder Cy; Cy max_volumn (Cy cy1, Cy cy2) { float vol1, vol2; vol1 = PI * cy1.r * cy1.r * cy1.H; vol2 = PI * cy2.r * cy2.r * cy2.H; if(vol1 > vol2) return cy1; else return cy2; }  Struct and typedef struct cylinder { float r; float H; };

Recursive function call  a function that contains a call to itself. Divide complex problem into identical simple cases. Must have at least one exit condition that can be satisfied. i.e. Towers of Hanoi int factorial (int n) { if(n == 1) return 1; else return n * factorial(n-1); }

Pass by value vs. pass by address  How are parameters passed to a function? pass by value: make a copy of the variable  Simple  Can not change the passed variable pass by address: address of the variable is passed  Efficient  Can change the variable, though the address of the variable is not changed

void swap1(int a, int b) { int c; c = b; b = a; a = c; } void swap2(int *a, int *b) { int c; c = *b; *b = *a; *a = c; } Example - 5  Pass by value and pass by reference

int main() { int a = 3, b = 5; swap1(a,b); printf(“a is %d, b is %d\n”, a, b); swap2(&a, &b); printf(“a is %d, b is %d\n”, a, b); return 0; } Example - 5 (continued)  Pass by value and pass by reference

Dynamic memory allocation  Typical variable definition (known size) Statically defined in program Allocated in the stack i.e., int a[10]  Define a variable that has a varying size? Allocated at run-time, in the heap Use malloc() function call to allocate Use free(*pointer) to free the memory of a variable

int main() { int *array, i, size; printf(“Input the array size:”); scanf(“%d”, &size); array = (int *)malloc(size, sizeof(int)); for(i=0; i < size; i++) array[i] = i; free(array); //otherwise, memory leak return 0; } Example - 6  Dynamic memory allocation

Preprocessing directives  #define Defines a macro.  #include Inserts text from another source file.  #ifdef Conditionally includes source text if a macro name is defined.  #ifndef Conditionally includes source text if a macro name is not defined. #ifndef FILENAME_H #define FILENAME_H … #endif  #else Conditionally includes source text if the previous #ifdef, #ifndef test fails.  #endif Ends conditional text.

I/O capability  FILE *fp;  fp = fopen(name, mode);  fclose(fp);  fprintf(fp, "format string", variable list);  fgets(str_name, length, fp);  feof(fp); while(!feof(fp)) { fgets(line, max_length, fp); … }

Example - 7 int print_triangle(char style, int base, char *filename) { int i,j; FILE *fp = fopen(filename, “w+”); // open file if(fp == NULL) return -1; if(lines % 2 == 0) base--; for(i = 1; i <= base; i=i+2) { for(j = 0; j < (base – 1) / 2; j++) fprintf(fp, ‘ ‘); for(j = 0; j < i; j++) fprintf(fp, “%c”,style); fprintf(fp, “\n”); } fclose(fp); return 0; } * *** ***** ******* **********  File operations

Command-line arguments  main(int argc, char** argv)  a.out -i 2 -g -x 3  argc = 6 argv[0] = "a.out" argv[1] = "-i" argv[2] = "2" argv[3] = "-g" argv[4] = "-x" argv[5] = "3"

Compilation  gcc codename –o programname  make prog = cc –o prog prog.c  options -o filename -g produce extra debugging information -O(0/1/2/3) optimization level -I(i)directory add header file directory -l(L)libraryname link the library

Example - 8 #include int main() { double s3 = sqrt(3); printf(“Square root of 3 is:%lf\n”, s3); return 0; }  Library Usage  Source code: example.c  Compilation gcc –g –o example example.c –lm

Makefile  Multiple files compilation  files main.c factorial.c hello.c  Run make #Makefile_example all: hello hello: main.o factorial.o hello.o gcc main.o factorial.o hello.o -o hello main.o: main.c gcc -c main.c factorial.o: factorial.c gcc -c factorial.c hello.o: hello.c gcc -c hello.c clean: rm -rf *o hello

Another makefile CC=gcc CFLAGS=-c -Wall SOURCES=main.c hello.c factorial.c OBJECTS=$(SOURCES:.c=.o) EXECUTABLE=hello all: $(SOURCES) $(EXECUTABLE) $(EXECUTABLE): $(OBJECTS) $(CC) $(OBJECTS) -o $(CC) $(CFLAGS) $< -o 

Common compilation errors  Typos (C language is case sensitive)  Try to use a variable without definition  Some statements are not ended by ;  Brackets are not paired ( ), { }, [ ]  Number or type of parameters passed to a function does not match the function definition.  Still have errors? copy the error message, Google it.

gdb for debugging  -g in compilation to enable gdb  gdb obj //start gdb with obj program  (gdb) run (arg )  (gdb) s for step execution  (gdb) p for print variables  (gdb) br to set breakpoint  (gdb) set to modify registers or memory set $eax=10  (gdb) help

Most powerful debug tool  printf function print out check points near vulnerable codes to trace down where bug appears  i.e, File operation, control statements... print out value of variables to trace down when bug happens  i.e. invalid array operation, loop does not stop, and etc.

For more information    Google