The Basics of C Programming

Text files in C are straightforward and easy to understand. All text file functions and types in C come from the stdio library.

When you need text I/O in a C program, and you need only one source for input information and one sink for output information, you can rely on stdin (standard in) and stdout (standard out). You can then use input and output redirection at the command line to move different information streams through the program. There are six different I/O commands in <stdio.h> that you can use with stdin and stdout:

The advantage of stdin and stdout is that they are easy to use. Likewise, the ability to redirect I/O is very powerful. For example, maybe you want to create a program that reads from stdin and counts the number of characters:

#include <stdio.h>
#include <string.h>

void main()
{
    char s[1000];
    int count=0;
     while (gets(s))
        count += strlen(s);
    printf("%d\n",count);
}

Enter this code and run it. It waits for input from stdin, so type a few lines. When you are done, press CTRL-D to signal end-of-file (eof). The gets function reads a line until it detects eof, then returns a 0 so that the while loop ends. When you press CTRL-D, you see a count of the number of characters in stdout (the screen). (Use man gets or your compiler's documentation to learn more about the gets function.)

Now, suppose you want to count the characters in a file. If you compiled the program to an executable named xxx, you can type the following:

xxx < filename

Instead of accepting input from the keyboard, the contents of the file named filename will be used instead. You can achieve the same result using pipes:

cat < filename | xxx

You can also redirect the output to a file:

xxx < filename > out

This command places the character count produced by the program in a text file named out.

Sometimes, you need to use a text file directly. For example, you might need to open a specific file and read from or write to it. You might want to manage several streams of input or output or create a program like a text editor that can save and recall data or configuration files on command. In that case, use the text file functions in stdio: