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Let s walk through the code and see how we ve implemented the data structures and decision procedures required to implement the password database administration interface. We start by declaring three functions: printMenu(), getInput()I, and enterData(FILE *fp). These will be used to print the application menu to standard output, process user menu selections, and solicit user data, respectively. Next, we define a struct called dbRecord, which resembles the user record type employed by the /etc/passwd file. This struct contains the following variables:
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int uid int gid char username[8] char homeDirectory[64] char shell[64] char comment[64]
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Stores the user s ID Stores the user s primary group ID Defines the login for the user Stores the full path to the user s home directory Contains the full path to the user s default shell Stores the user s full name and optionally a description of some kind
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Next, we introduce the main body of the program, beginning with the declaration of a file handle dbFile. This is a file in which user data will be stored using a binary format, and its name is retrieved from argv[1] (i.e., passed on the command line). After the file is opened for appending and reading, as signified by the permission string a+, a do...while loop is constructed. The loop iterates until menuChoice, as entered by the user after the menu is printed, is q. In practice, this condition is never reached, because the q is caught by the switch statement, and the case q immediately exits from the program. If the case e is encountered, the function enterData(FILE *fp) is called. This function proceeds by asking the user to enter all data elements that are defined for dbRecord. Each entry is read in by using the scanf function. Once the data has been collected, a record is written to the specified file by using the fwrite() function. Let s see how a data operation performs in practice:
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$ ./database database.txt Database Main Menu -----------------(e)nter new record (q)uit Your Choice:
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After selecting e, the Data Entry menu is displayed, and a new record can be inserted:
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Data Entry---------Enter UID: 1001 Enter GID: 100 Enter username: pwatters Enter full name: Paul Enter shell: /bin/sh Enter home directory: /home/paul
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After the record has been inserted, the main menu is displayed once again. Further records can be inserted, or you can simply quit the application:
Database Main Menu -----------------(e)nter new record (q)uit Your Choice:q Session terminated
Here, we started the database application by passing the filename database.txt, which is to contain the user data entered through the application. After the welcome banner is printed, we enter e to go to the data entry screen. Here, we enter data for the user pwatters, including UID 1001, GID 100, full name Paul, shell /bin/sh, and home directory /home/paul. After all of the data has been accepted and the entry has been written to the database file, we are returned to the main menu.
Low-Level Input/Output
The open() system call is used to open a file using a low-level call. The file remains open until it is closed with a close() system call. When the open() system call is called, a file descriptor is returned, which is a unique integer that distinguishes the current open file from other opened files. A pool of available file descriptor integers is maintained, and the next integer in the queue is selected. Recall that there are three file descriptors that are defined by the low-level interface: standard input (0), standard output (1), and standard error (2). The named file is always opened at its beginning, so subsequent operations are operating sequentially on the data contained in the file. The open() function opens the file named in the string pathname, with the permissions specified by primary flags. These flags include the following: O_RDONLY O_WRONLY O_RDWR Opens the file read-only. Opens the file write-only.
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