15-213 Recitation 9: 11/04/02Coverage of Exam 2Error HandlingUnix-Style Error HandlingPOSIX-Style Error HandlingDNS-Style Error HandlingExample: WrappersNot All Errors are FatalI/OUnix I/OKernel I/OFunctions – OpenFunctions – Read and WriteFunctions – CloseStandard I/OExample: buffered_io.cUse strace to CheckFile StreamsSo Which Should I Use?Flushing a File SystemFlushing a File System (cont.)strace RevisedUnix Man pagesMan Page Sections (cont.)Man Page SectionsSummary15-213 Recitation 9: 11/04/02Outline•Error Handling•I/O•Linux man pagesAnnie Luoe-mail: [email protected] Hours: Thursday 6:00 – 7:00 Wean 8402ReminderReminder•Exam 2: Tuesday, Nov. 12Review session next Monday evening•L6 Due: Friday, Nov. 19•This week’s lectures•Text book: 10.9Coverage of Exam 2•Similar to last year’s exam 2–Performance optimization–Cache simulation–Cache miss rate–Cache miss analysis–Processes–Signals–Virtual memory•Come to the special recitation –Mon. 11/11 evening–Location TBDError Handling•Should always check return code of system calls–Not only for 5 points in your lab!–There are subtle ways that things can go wrong –Use the status info kernel provides us •Approach in this class: Wrappers•Different error handling styles–Unix-Style–Posix-Style–DNS-StyleUnix-Style Error Handling•Special return value when encounter error (always –1)•Set global variable errno to an error code–Indicates the cause of the error•Use strerror function for text description of errnovoid unix_error(char *msg){ fprintf(stderr, “%s: %s\n“, msg, strerror(errno)); exit(0);}… …if ((pid = wait(NULL)) < 0) unix_error(“Error in wait”);POSIX-Style Error Handling•Return value only indicate success (0) or failure (nonzero)•Useful results returned in function argumentsvoid posix_error(int code, char *msg){ fprintf(stderr, “%s: %s\n“, msg, strerror(code)); exit(0);}… …if ((retcode = pthread_create(…)) != 0) posix_error(retcode, “Error in pthread”);DNS-Style Error Handling•Return a NULL pointer on failure•Set the global h_errno variablevoid dns_error(char *msg){ fprintf(stderr, “%s: DNS error %d\n“, msg, h_errno); exit(0);}… …if ((p = gethostbyname(name)) == NULL) dns_error(“Error in gethostbyname”);Example: Wrappers•Appendix B: csapp.c and csapp.h•Unix-Style, for kill function•Behaves exactly like the base function if no error•Prints informative message and terminates the processvoid Kill (pid_t pid, int signum){ int rc; if((rc = kill(pid, signum)) <0)unix_error(“Kill error”);}Not All Errors are Fatal•Wrappers are not always the correct path–Treat all information as fatal errors–Terminate the program with exit()–Sometimes an error is not fatalvoid sigchld_handler(int signum){ pid_t pid; while((pid = waitpid(-1, NULL, 0)) > 0) printf(“Reaped %d\n”, (int)pid); if(errno != ECHILD) unix_error(“waitpid error”);}I/O•Full coverage in Lecture 24, and Chapter 11 in textbook•But folks were having some issues with the Shell Lab–fflush? printf? fprintf? scanf? fscanf?•And these issues will pop up with the Malloc LabUnix I/O•Why need I/O? –copy data between main memory and external devices•All devices modeled as files in Unix–Disk drives, terminals, networks–A Unix file is a sequence of bytes–Input/Output performed by reading and writing filesKernel I/O•Why Kernel I/O–System level I/O functions provided by the kernel–Understand system concepts (process, VM, etc.)–When impossible/inappropriate to use standard library I/O•Info such as file size, creation time•Networking programming•How kernel I/O works –Kernel maintains all file information–Apps only keep track of file descriptor returned from kernelFunctions – Open•int open(const char *pathname, int flags);–Return value is a small integer – file descriptor •Special file descriptors –Defined in <unistd.h>–Default open with each shell-created process –Standard Input (descriptor = 0)–Standard Output (descriptor = 1)–Standard Error (descriptor = 2)Functions – Read and Write•ssize_t read(int fd, void *buf, size_t count);–Copy count >0 bytes from a file (fd) to memory (buf) –From current position k (maintained by kernel)–Trigger EOF when k is greater than file size–No “EOF character”•ssize_t write(int fd, void *buf, size_t count);–Copy count >0 bytes from memory (buf) to a file (fd)Functions – Close•int close(int fd);–Kernel frees data structures created by file open (if any)–Restores file descriptor to a pool of available descriptors–What if a process terminates? –Kernel closes all open files and free memory (for this proc)Standard I/O•Higher level I/O functions–fopen,fclose, fread,fwrite, fgets,fputs, –scanf and printf: formated I/O–Eventually calls the kernel I/O routines•Models an open file as a stream: –A pointer to FILE–Abstraction for file descriptor and for a stream buffer•Why use stream buffer?–Reduce expensive Unix system calls!Example: buffered_io.c#include <stdio.h>int main(void){ printf("1"); printf("5"); printf("2"); printf("1"); printf("3"); return 0;}•How many system calls (kernel I/O routine) used?Use strace to Checkunix> strace ./buffered_io…write(1, "15213", 515213) = 5…•strace <p r o g r am>–Runs <program> and prints out info about all the system calls•Let’s run strace on buffered_ioFile Streams•C Library equivalent of file descriptors•More formatted I/O routines: fprintf and fscanf–Take an extra first argument FILE*•int printf(const char *format, ...); •int fprintf(FILE *stream, const char *format, ...);–printf(args) == fprintf(stdout, args)–scanf(args) == fscanf(stdin, args)So Which Should I Use?•Use standard I/O routines for disks and terminals–Full duplex: can input and output on the same stream•Nasty restrictions when use stdio on sockets–Input cannot follow output (and vice versa) without fflush, fseek, fsetpos, or rewind–Flush the buffer for the first restriction–Use two streams for the second restriction•So do not use standard I/O functions for network socketsFlushing a File System•stderr is not buffered•stdout is line-buffered when it points to a terminal–Partial lines not appear until fflush or exit–Can produce unexpected results, esp. with debugging output•There may also be input buffering when stdin points to a terminalFlushing a File
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