Course Recap CS 2022 Introduction to C Instructor Hussam Abu Libdeh Cornell University based on slides by Saikat Guha Fall 2011 Lecture 13 Course Recap CS 2022 Fall 2011 Lecture 13 Hello World include stdio h void print greeting printf Hello World n int main int argc char argv print greeting return 0 Course Recap CS 2022 Fall 2011 Lecture 13 Command Line Arguments I I I When an application launches the operating system can pass it command line arguments Optional and not required int main int argc char argv I I I I argc arguments count argv array of arguments as strings application name counted as an argument so argc is at least 1 int main is also valid if you don t care about command line arguments Course Recap CS 2022 Fall 2011 Lecture 13 Data Types 1 3 Primitives I I I I I I I int integer size is platform dependent int32 t 32 bit integer on all platforms float floating point number char character int 10 array of 10 integers char 10 array of 10 characters a string Course Recap CS 2022 Fall 2011 Lecture 13 Data Types 2 3 structs struct person char 20 name int age char 256 address I I I struct types hold collections of elements struct person both words together is now a data type Declare variables as such struct person john doe I the operator is used to access struct members john doe age Course Recap CS 2022 Fall 2011 Lecture 13 Data Types 3 3 Pointers I Pointers are variables whose contents are interpreted as the memory addresses of other variables Course Recap CS 2022 Fall 2011 Lecture 13 Data Types 3 3 Pointers I I I Pointers are variables whose contents are interpreted as the memory addresses of other variables int value struct person john doe Operators relating to pointers I I I dereference follow pointer and read data value address of get address of a variable usually to store in pointer access element access elements of a struct pointer Equivalent to Course Recap CS 2022 Fall 2011 Lecture 13 Memory I Stack I I I memory allocated statically by compiler memory released back to system automatically after function returns Heap I I I memory allocated dynamically by programmer at run time memory has to be released back to system freed manually by programmer use malloc size and free pointer to allocate and free memory I int ptr int malloc sizeof int free ptr Course Recap CS 2022 Fall 2011 Lecture 13 Frequently Used Libraries I I I I stdio h provides printf scanf fgets and other input output functions stdlib h provides malloc and free functions string h provides strcmp strcpy and other string manipulation functions stdint h provides int32 t uint32 t int64 t uint64 t and other fixed size integers Remember to include the correct library when using something provided by it Course Recap CS 2022 Fall 2011 Lecture 13 Debugging I I Debugging is extremely valuable to determine what is going wrong with your program GDB is an interactive command line debugger I I I I I break function name or line number sets a break point at a function def or a line print variable name or expression prints the value of a variable or an expression on program variables run command line arguments starts running the program with the given command line arguments help find help on more commands Compile your code with the g flag for gcc to be able to debug the program with gdb Course Recap CS 2022 Fall 2011 Lecture 13 Terminal Input Output I Output to screen I I I printf Hi d 5 print formatted text puts Hello World print a string Input from keyboard I I I gets buffer array read a single line from stdin into the buffer array fgets buf 128 stdin read a single line or at most 128 characters from stdin into buf scanf s d buf i read from stdin and parse input according to given format Many more variants that work with any stream type for example good for file I O Course Recap CS 2022 Fall 2011 Lecture 13 File I O Opening and closing files int fd File Descriptor fd open path to file O RDWR O CREAT close fd Reading and Writing char buf 4096 int len len read fd buf 4096 len write fd buf 4096 WARNING Size passed is only a suggestion May read write fewer than requested number of bytes Return value is number of bytes actually read written MUST retry if not fully read written Course Recap CS 2022 Fall 2011 Lecture 13 Network I O Opening and closing network sockets int sock File Descriptor sock socket PF INET SOCK STREAM IPPROTO TCP close sock Internet Addresses struct sockaddr in addr addr sin family AF INET addr sin addr s addr htonl 0x7F000001 addr sin port htons 8080 Fill the address info manually or get the info automatically with getaddrinfo See man getaddrinfo Course Recap CS 2022 Fall 2011 Lecture 13 Bitwise Operations I I I Manipulate individual bits in a variable Useful for many things one of which is serialization Operators I I I I I I a b bitwise AND a b bitwise OR a b bitwise XOR a bitwise one s complement a b bitwise shift left a b bitwise shift right Course Recap CS 2022 Fall 2011 Lecture 13 Threads Starting a thread include pthread h pthread t id err pthread create id NULL entry func arg Body of a thread void entry func void arg Exiting current thread pthread exit void return value Course Recap CS 2022 Fall 2011 Lecture 13 Resources I I Dave s programming in C tutorials http www cs cf ac uk Dave C CE html Linux manual pages man pthread create man stdlib h I I also available online at http linux die net Search online Plenty of resources and tutorials online Course Recap CS 2022 Fall 2011 Lecture 13 Questions Course Recap CS 2022 Fall 2011 Lecture 13 Sample Interview Questions 1 Swap two arbitrary variables without using a third temporary variable 2 A linked list can become corrupt if the last element s next pointer points back to some previous element rather than NULL This can cause functions that traverse the list to loop for ever Write a small C function to determine whether a given linked list is corrupt or not Course Recap CS 2022 Fall 2011 Lecture 13