C What you Know Objective To introduce some of the features of C This assumes that you are familiar with C or java and concentrates on the features that are not used in C C does not have the OOP features of C nor does it have as rich a library no classes I O string operations and dynamic memory management done through library calls Not much support for higher level data structures e g STL Commonly lots of pointer manipulation Topics Getting Started pointers character arrays and string functions structs I O dynamic memory malloc and free Hello World include stdio h int main printf Hello World n return 0 gcc o hello hello c hello Hello Pointers To declare a pointer put in front of the variable name int i declare an int int ip declare a pointer to an int char name declare a pointer to a character Use to get the address of a variable and to dereference a pointer ip i i ip assign the pointer variable ip the address of i assign i the value of what ip points to Pointers and Arrays Arrays are related to pointers An array variable is a pointer to the first location in the array declare static character array of size 10 char name 10 can initialize character arrays char name 10 Fred char anotherName anotherName Name You can use pointer arithmetic to access array elements access 4th element of name name 4 name 4 character arrays strings In C there is no built in support for strings Instead strings are represented as null terminated character arrays F name char name 7 name Fred compiler adds 0 name 0 F name 1 r name 2 e name 3 d name 4 0 r e d 0 Library Support for Strings include string h strlen const char s returns length of s not including 0 int strcmp const char s1 const char s2 return 0 if s1 s2 0 if s1 s2 0 if s1 s2 strcat char dest const char src appends src to the end of dest char D 20 D 0 0 strcat D Jeremy strcat D strcat D Johnson strncat char dest const char src int n appends upto n characters of src to the end of dest Library Support for Strings char strcpy char dest const char src copy the string pointed to by src including 0 to a character array pointed to by dest dest must already point to a character array with sufficient space to hold src no error checking for insufficient space char strncpy char dest const char src int n copy at most n characters from the string pointed to by src to a character array pointed to by dest This should be used to prevent overflow errors structs A struct is used to group various data fields in a single type classes without methods typedef can be used to give a name to a particular struct declaration Use operator to access different fields typedef struct Employee Employee struct Employee char name double wage int id Employee worker worker name Fred worker id 1 worker wage 15 25 Arrays of Arrays char names 8 names 0 start names s t a r t 0 m i d d l e e n d 0 0 I O stdio h include stdin stdout stderr standard input standard output and standard error file streams character I O int getchar read a single character from stdin if none available return EOF end of file getchar is a macro that uses getc int getc FILE FP read a single character from the file or stream identified by FP int putchar char c write the character c to stdout if successful return the the character c otherwise EOF int putc char c FILE FP write the character c to the file or stream identified by FP mycat c gcc mycat c o mycat mycat mycat c read character stream from stdin copy to stdout include stdio h int main int c while c getchar EOF putchar char c return 0 Formatted Output printf prints a variable number of arguments using a specified format which is represented by a string int printf const char FORMAT ARG FORMAT is a string of characters including special characters such as n for newline and conversion formats for different types with optional width and precision information man 3 printf for more info d int c character s string f e g floating point number 4 2f 4 indicates width 2 precision i e xx xx x hexadecimal int o octal int Example include stdio h int main typedef struct Employee Employee struct Employee char name double wage int id Employee worker worker name Fred worker id 1 worker wage 15 25 printf s earns 4 2f per hour n worker name worker wage return 0 Example a program illustrating character arrays and pointer arithmetic include stdio h include string h int main char name 20 Fred char anothername anothername name printf name s n name printf Another name for s is s n name anothername printf The d th letter of s c n strlen name name name 3 printf The d th letter of s c n strlen name name name 3 return 0 Formatted Input scanf is used to read a variable number of arguments with input format specified by a fomat string similar to printf int scanf const char FORMAT ARG The arguments to scanf are the addresses to the variables passed this is required since the input variables will be modified There must be sufficient arguments for the number of specified conversion types or unpredictable behavior may result There must be sufficient space in the arguments e g character arrays to hold the input or unpredictable behavior may result Example int main char name MAXNAMELEN fixed size array prompt the user to enter their name and read the string s tells scanf to read a string which it stores in the array name A string is read until whitespace is encountered The address of the name must be passed to scanf printf Enter first name n scanf s name printf Hello s n name return 0 Dynamic Memory Allocation In C dynamic memory is allocated through the use of a library routine called malloc malloc inputs the desired amount of memory in bytes use the sizeof macro to help determine the number of bytes needed and returns a pointer if successful to the newly allocated memory If unsuccessful malloc returns NULL The return type is void which is a generic pointer since what the memory will be used for varies A typecast should be used after calling malloc free and realloc When the memory allocated by malloc is no longer in use it should be freed using free Not doing this causes a memory leak which can cause problems when lots of memory is required freeing memory when it is still being used is a common bug one that may be difficult to find realloc can be used to obtain additional memory when the amount requested is insufficient see grow from the text void realloc void …