If errno contains a nonzero number, is there an error?

The global variable errno is used by many standard C library functions to pass back to your program an error code that denotes specifically which error occurred. However, your program should not check the value of errno to determine whether an error occurred.

Usually, the standard C library function you are calling returns with a return code which denotes that an error has occurred and that the value of errno has been set to a specific error number. If no error has occurred or if you are using a library function that does not reference errno, there is a good chance that errno will contain an erroneous value. For performance enhancement, the errno variable is sometimes not cleared by the functions that use it.

You should never rely on the value of errno alone; always check the return code from the function you are calling to see whether errno should be referenced. Refer to your compiler’s library documentation for references to functions that utilize the errno global variable and for a list of valid values for errno.

What is a stream?

A stream is a continuous series of bytes that flow into or out of your program. Input and output from devices such as the mouse, keyboard, disk, screen, modem, and printer are all handled with streams. In C, all streams appear as files—not physical disk files necessarily, but rather logical files that refer to an input/output source.

The C language provides five “standard” streams that are always available to your program. These streams do not have to be opened or closed. These are the five standard streams:

Note that the stdprn and stdaux streams are not always defined. This is because LPT1: and COM1: have no meaning under certain operating systems. However, stdin, stdout, and stderr are always defined. Also, note that the stdin stream does not have to come from the keyboard; it can come from a disk file or some other device through what is called redirection. In the same manner, the stdout stream does not have to appear on-screen; it too can be redirected to a disk file or some other device.

How do you redirect a standard stream?

Most operating systems, including DOS, provide a means to redirect program input and output to and from different devices. This means that rather than your program output (stdout) going to the screen, it can be redirected to a file or printer port. Similarly, your program’s input (stdin) can come from a file rather than the keyboard. In DOS, this task is accomplished using the redirection characters, < and >. For example, if you wanted a program named PRINTIT.EXE to receive its input (stdin) from a file named STRINGS.TXT, you would enter the following command at the DOS prompt:

C:>PRINTIT < STRINGS.TXT

Notice that the name of the executable file always comes first. The less-than sign (<) tells DOS to take the strings contained in STRINGS.TXT and use them as input for the PRINTIT program.

Redirection of standard streams does not always have to occur at the operating system. You can redirect a standard stream from within your program by using the standard C library function named freopen(). For example, if you wanted to redirect the stdout standard stream within your program to a file named OUTPUT.TXT, you would implement the freopen() function as shown here:

...
freopen(“output.txt”, “w”, stdout);
...

Now, every output statement (printf(), puts(), putch(), and so on) in your program will appear in the file OUTPUT.TXT.