CS 450 MPX P ROJECT A Quick C Review

P OINTERS AND ADDRESSES IN C Check Manual I2 from Dr. Mooney’s website for a complete review of C topics you will need Wikipedia: In computer science, a pointer is a programming language data type whose value refers directly to (or "points to") another value stored elsewhere in the computer memory using its address. A pointer is a variable that holds an address to another memory location, not an actual value. To declare a variable as a pointer, use the *: int *myIntPointer; The myIntPointer variable will hold an address to a memory location that contains a value of type int

P OINTERS IN C To access the value held in a pointer, you need to dereference it int *myIntPointer; //declare pointer *myIntPointer = 30; //assign value to the referenced address *myIntPointer++; //access that value If you code myIntPointer = 30; you are not changing the value referenced by myIntPointer, you are changing the address myIntPointer references To access the address of a variable, use &: int myInt = 8;// new int variable, not a pointer myIntPointer = & myInt; //copy address of myInt into myIntPointer

C OMMON P OINTER E RRORS int *a = 8; a = 19; //should say *a = 19. int b = 20; a = b; //should be *a = b to set the value referenced by ‘a’ equal to the value of ‘b’ OR a = &b to set the address referenced by ‘a’ = the address of ‘b’.

S TRINGS I N C Strings in C are stored as character arrays and must be “null terminated”. This means that the last character in the string should be ‘\0’ Declare a string as a character array and give it a maximum size. Use this method if you do not know what you wish to put in the String at compile time: char[20] myString; Declare a string as a character pointer. Use this method if you know the exact string you wish to store in this character array at compile time and do not plan on storing a longer string in it later: char* myString = “hello world\0”;

C OPYING AND C OMPARING S TRINGS Copy the value of one string to another: Do NOT use myString1 = myString2 or *myString1 = *myString2 Use strcpy: strcpy(myString1, myString2); This will copy the value held in myString2 into myString1 Compare two strings Do NOT use myString1 == myString2, etc. Use strcmp(myString1, myString2); If equal, returns 0, otherwise returns something else.

C S TRINGS Potential Problems: You declare a character array of size X, then store a string of size > X in that array. You declare a character pointer and later try to store a string in it (since you declared it as a * with no initial value, no memory is allocated for the string): char *myCharPointer; …… code …. *myCharPointer = “hello world\0”; Not null terminating your strings- printf, strcpy, strcmp find the end of a string by looking for \0.

C OMPILER D IRECTIVES Use “#” to give specific instructions to the compiler #include: essentially directs the compiler to “copy and paste” the contents of a file into another file #include “MyHeaderFile.H” #include Use “…” for header files you create, for system header files NEVER include “.C” files #define: symbolic constants #define MYCONSTANT 40 Typically in all upper case In this example, every time you type “MYCONSTANT” in your code, the compiler will replace it with “40” Allows you to define at compile time constants that you will use throughout your code Use for array sizes, giving numeric values to categories, and register addresses

H EADER F ILES C programs are comprised of two types of files Source Files (.C) Header Files (.H) Source files contain all of your source code Header files contain prototypes for all of your functions, definitions of structures, symbolic constants, and global variable declarations. For each source (.C) file, there should be a header file (.H) You should link the source files together, and include header files in the source files

P ROPER U SE OF H EADER F ILES If the function prototypes contained in the header files are included more than once, errors will be generated by the compiler. To avoid this problem, wrap the header files in an if…end if statement: #ifndef MYHFILE_ #define MYHFILE_ ….. Header file contents ….. #endif

O NLINE R ESOURCES In depth tutorials and reviews of just about every C topic you will need in MPX, including basic syntax, file I/O, C-strings, pointers, function pointers, structures, linked lists, and bitwise operators. m Pointers and arrays