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0x200. PROGRAMMING > 0x280. Building on Basics

Building on Basics

Once you understand the basic concepts of C programming, the rest is pretty easy. The bulk of the power of C comes from using other functions. In fact, if the functions were removed from any of the preceding programs, all that would remain are very basic statements.

File Access

There are two primary ways to access files in C: file descriptors and filestreams. File descriptors use a set of low-level I/O functions, and filestreams are a higher-level form of buffered I/O that is built on the lower-level functions. Some consider the filestream functions easier to program with; however, file descriptors are more direct. In this book, the focus will be on the low-level I/O functions that use file descriptors.

The bar code on the back of this book represents a number. Because this number is unique among the other books in a bookstore, the cashier can scan the number at checkout and use it to reference information about this book in the store's database. Similarly, a file descriptor is a number that is used to reference open files. Four common functions that use file descriptors are open(), close(), read(), and write(). All of these functions will return –1 if there is an error. The open() function opens a file for reading and/or writing and returns a file descriptor. The returned file descriptor is just an integer value, but it is unique among open files. The file descriptor is passed as an argument to the other functions like a pointer to the opened file. For the close() function, the file descriptor is the only argument. The read() and write() functions' arguments are the file descriptor, a pointer to the data to read or write, and the number of bytes to read or write from that location. The arguments to the open() function are a pointer to the filename to open and a series of predefined flags that specify the access mode. These flags and their usage will be explained in depth later, but for now let's take a look at a simple note-taking program that uses file descriptors—simplenote.c. This program accepts a note as a command-line argument and then adds it to the end of the file /tmp/notes. This program uses several functions, including a familiar looking error-checked heap memory allocation function. Other functions are used to display a usage message and to handle fatal errors. The usage() function is simply defined before main(), so it doesn't need a function prototype.


  

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