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This program reads in a set of characters from standard input, stores them in a character array of static size (defined by MAX_SIZE), and then prints out the characters individually to standard output. If the application was executed, and the characters 1234567 were typed in, they would be dutifully printed to standard output. However, if the characters 12345678901234567890 were typed in, the message Segmentation fault (core dumped) would appear (along with a very large core file!). If this application was running as root, or any other privileged user, the unpredictable behavior of the program may have serious security implications, as well as potentially violating the integrity of kernel and user memory. To remove the problem, we simply need to add an appropriate boundary-checking condition to the input routine. In this case, we simply check that the number of characters being read does not exceed the number specified by MAX_SIZE. We don t need to do the same check when printing the characters to standard output, because we know that there will never be an inappropriate number of characters stored in the character buffer in the first place:
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#include <stdio.h> #define MAX_SIZE 16 main() { int character=0, i=0, j=0; char buf1[MAX_SIZE]; do { character=getchar(); if (character!=EOF) { buf1[i]=character; i++; } } while ((character!=EOF)&&(i<MAX_SIZE)); do { printf("%c", buf1[j]); j++; } while (j<i); }
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If the application was executed now, and the characters 1234567 were typed in, they would be dutifully printed to standard output. If the characters 12345678901234567890 were typed in, however, only 1234567890123456 would be displayed, and no core file would be dumped. A core file is an image of memory dumped when a process terminates abnormally. Because some applications can consume many megabytes of RAM, core files can become very large and waste valuable disk space. If you don t intend to use core files for debugging, you can safely remove them.
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Part VI:
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Regardless of whether standard input or another stream is used (such as a file), it is critical to check that boundaries have not been overwritten, especially where arrays and pointers are concerned. We ve so far looked at some simple cases involving text files. However, more complex applications that use structs to create database-like records usually require faster read/ write access provided by binary data streams. Using a binary stream makes it impossible to use cat or grep to examine the contents of a file, but it does allow a valuable abstraction from files on a character-by-character basis. Complex data structures can be easily serialized and written to a binary file. In the following example program, we define a struct called dbRecord, which contains some of the user data typically stored in the password file (/etc/passwd). Many applications use this data for authentication purposes. Imagine that we were going to write a new, improved version of /etc/passwd that uses a binary data format rather than the existing cumbersome (and slow) text format. We d need an administrative interface to allow new records to be easily added, because they could no longer be added by manually editing the /etc/passwd file. Let s have a look at how this could be achieved:
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#include <stdio.h> void printMenu(); char getInput(); void enterData(FILE *fp); struct dbRecord { int uid; int gid; char username[8]; char homeDirectory[64]; char shell[64]; char comment[64]; }; main(int argc, char *argv[]) { FILE *dbFile; char menuChoice; if ((dbFile=fopen(argv[1],"a+"))==NULL) { fprintf(stderr, "Cannot open database file %s\n", argv[1]); exit(1); } do { printMenu(); menuChoice=getInput();
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32:
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switch (menuChoice) { case 'e': enterData(dbFile); break; case 'q': printf("Session terminated\n"); fclose(dbFile); exit(1); break; } } while (menuChoice!='q'); } void printMenu() { printf("Database Main Menu\n"); printf("------------------\n"); printf("(e)nter new dbRecord\n"); printf("(q)uit\n"); } char getInput() { char answer; printf("\nYour Choice: "); answer=getchar(); return answer; } void enterData(FILE *fp) { struct dbRecord user; printf("Data Entry\n"); printf("----------\n\n"); printf("Enter UID: "); scanf("%i",&user.uid); printf("Enter GID: "); scanf("%i",&user.gid); printf("Enter username: "); scanf("%s",user.username); printf("Enter full name: "); scanf("%s",user.comment); printf("Enter shell: "); scanf("%s",user.shell); printf("Enter home directory: "); scanf("%s",user.homeDirectory); fwrite((char *)&user, sizeof(struct dbRecord), 1, fp); }
Part VI:
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