Recitation By yzhuang sseshadr Agenda Debugging practices GDB Valgrind Strace Errors and Wrappers System call return values and wrappers Uninitialization malloc related bugs System IO Debugging Practices General Debugging printfs are a good start but won t solve everything Remember printfs CHANGE your code And how it s compiled And how it runs Especially for races A lot of the debugging tools should be used with the g compiler flag GDB From bomblab buflab You WILL need it for malloc Demo Valgrind Memory related issues Lots of options man valgrind valgrind leak check full a out Demo strace From the man page In the simplest case strace runs the specified command until it exits It intercepts and records the system calls which are called by a process and the signals which are received by a process The name of each system call its arguments and its return value are printed on standard error or to the file specified with the o option Cool for debugging Errors and Wrappers System Call Error Handling Always handle errors for every system call include errno h Failed system calls almost always return 1 Global integer error number errno Getting error description strerror errno We deduct style points for not handling system call errors Wrappers If a system call is frequently used create a wrapper for it For example pid t Fork void pid t pid if pid fork 0 error handling return pid Proclab always handle errors You can choose whether to use wrappers malloc include stdlib h void malloc size t size Allocates size bytes of memory A pointer is returned Returned memory uninitialized p struct cacheline malloc sizeof cacheline P valid Cachelab using uninitialized valid bit very bad calloc With malloc Either initialize Or use calloc void calloc size t num size t size Allocate num size bytes of memory Initialized to 0 Caveat what if num size causes an overflow Check before calling calloc free Free memory allocated by malloc calloc Common mistakes Freeing memory already freed Freeing memory not allocated Writing to memory already freed Index out of bound accesses of allocated array Not freeing allocated memory I O System I O Basics Four basic operations open close read write What s a file descriptor Returned by open int fd open path to file O RDONLY fd is some positive value or 1 to denote error System I O Basics Every process starts with 3 open file descriptors 0 STDIN 1 STDOUT 2 STDERR Can I close these file descriptors Yes But you shouldn t this next example is just for illustrative purposes Sample Code include stdio h include stdlib h include unistd h int main int argc char argv int fd atoi argv 1 argc argc Keep GCC Happy fprintf stdout STDOUT close d d n fd close fd fprintf stderr ERROR close d d n fd close fd return 1 What are the outputs when run with a out 0 a out 1 and a out 2 Sample Code include stdio h include stdlib h include unistd h int main int argc char argv int fd atoi argv 1 argc argc Keep GCC Happy fprintf stdout STDOUT close d d n fd close fd fprintf stderr STDERR close d d n fd close fd return 1 a out 0 STDOUT close 0 0 STDERR close 0 1 Why 1 on the second time a out 1 STDERR close 1 1 Why no STDOUT output And why 1 for close return value a out 2 STDOUT close 2 0 Why no STDERR output And why 0 for close return value this time Some Real Stuf From Lecture Some Real Stuf From Lecture CALL TO fork Some Real Stuf From Lecture Dup What is file redirection Redirection vs Pipes Redirection has one input of a file pipes can be between tasks Ex cat filename Ex find cat What is dup2 It switches which file a file descriptor is pointing to What is dup It initializes another file descriptor to point to an already existing file Some Real Stuf From Lecture Some Real Stuf From Lecture CALL TO dup2 4 1 Dup From Lecture Dup So what is dup and what is dup2 int fd1 open int fd2 dup fd1 int fd1 open int fd2 dup2 fd1 fd2 Are these the same NO The first is OK the second uses an uninitialized variable Remember it s not dup2 fd1 fd2 File writing example Questions What are all of the possible contents of the file after running this code What is wrong with the style of this code How would you close these file descriptors include stdio h include unistd h int main int fd1 fd2 fd3 parent 0 char fname filename fd1 open fname O CREAT O TRUNC O RDWR S IRUSR S IWUSR write fd1 A 1 fd3 open fname O APPEND O WRONLY 0 write fd3 BBB 3 if parent fork fd2 dup fd1 else fd2 fd3 write fd2 C 1 write fd3 D 1 if parent waitpid 1 NULL 0 return 0 File writing example Answers Possible outputs ACBBDCD ACBBCDD What is wrong with the style of this code Didn t check error codes didn t close anything no comments How would you close these file descriptors fd2 sometimes needs close d sometimes doesn t So don t do both fd2 fd3 and fd2 dup f1 Practice Tons of practice available in past exam 2 s Very likely to be an I O question on the next test
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