Behind the Scenes with Standard IO

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Behind the Scenes with Standard I/O

Now that you've seen some of the features of the standard I/O package, let's examine a representative conceptual model to see how standard I/O works.

Normally, the first step in using standard I/O is to use fopen() to open a file. (Recall, however, that the stdin , stdout , and stderr files are opened automatically.) The fopen() function not only opens a file but sets up a buffer (two buffers for read-write modes), and it sets up a data structure containing data about the file and about the buffer. Also, fopen() returns a pointer to this structure so that other functions know where to find it. Assume that this value is assigned to a pointer variable named fp . The fopen() function is said to open a stream. If the file is opened in the text mode, you get a text stream, and if the file is opened in the binary mode, you get a binary stream.

The data structure typically includes a file position indicator to specify the current position in the stream. It also has indicators for errors and end-of-file, a pointer to the beginning of the buffer, a file identifier, and a count for the number of bytes actually copied into the buffer.

Let's concentrate on file input. Usually, the next step is to call on one of the input functions declared in stdio.h , such as fscanf() , getc() , or fgets() . Calling any one of these functions causes a chunk of data to be copied from the file to the buffer. The buffer size is implementation dependent, but it typically is 512 bytes or some multiple thereof. In addition to filling the buffer, the initial function call sets values in the structure pointed to by fp . In particular, the current position in the stream, and the number of bytes copied into the buffer are set. Usually the current position starts at byte 0.

After the data structure and buffer are initialized , the input function reads the requested data from the buffer. As it does so, the file position indicator is set to point to the character following the last character read. Because all the input functions from the stdio.h family use the same buffer, a call to any one function resumes where the previous call to any of the functions stopped .

When an input function finds that it has read all the characters in the buffer, it requests that the next buffer- sized chunk of data be copied from the file into the buffer. In this manner, the input functions can read all the file contents up to the end of the file. After a function reads the last character of the final buffer's worth of data, it sets the end-of-file indicator to true. The next call to an input function then returns EOF .

In a similar manner, output functions write to a buffer. When the buffer is filled, the data is copied to the file.

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