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CMU CS 15213 - Recitation

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Slide 1AgendaSlide 3General DebuggingGDBValgrindstraceSlide 8System Call Error HandlingWrappersmalloccallocfreeSlide 14System I/O BasicsSystem I/O BasicsSample CodeSample CodeSome Real StuffSome Real StuffSome Real StuffDupSome Real StuffSome Real StuffDupDupFile writing exampleSlide 28File writing examplePractice!!RecitationBy yzhuang, sseshadrAgenda•Debugging practices–GDB–Valgrind–Strace•Errors and Wrappers–System call return values and wrappers–Uninitialization–malloc() related bugs•System IODebugging PracticesGeneral 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 flagGDB•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 WrappersSystem 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 errorsWrappers•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 wrappersmalloc•#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 memoryI/OSystem 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 errorSystem 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 purposesSample 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 0STDOUT:close(0) = 0STDERR:close(0) = -1>> ./a.out 1STDERR:close(1) = -1>> ./a.out 2STDOUT:close(2) = 0Why -1 on the second time??Why no STDOUT output? And why -1 for close return value?Why no STDERR output? And why 0 for close return value this time?Some Real StufFrom Lecture:Some Real StufFrom Lecture:CALL TO fork()Some Real StufFrom 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 StufFrom Lecture:Some Real StufFrom Lecture:CALL TO dup2(4,1)DupFrom LectureDup•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


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CMU CS 15213 - Recitation

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