12.3 Predefined Streams

Besides the file pointers which we explicitly open by calling fopen, there are also three predefined streams. stdin is a constant file pointer corresponding to standard input, and stdout is a constant file pointer corresponding to standard output. Both of these can be used anywhere a file pointer is called for; for example, getchar() is the same as getc(stdin) and putchar(c) is the same as putc(c, stdout). The third predefined stream is stderr. Like stdout, stderr is typically connected to the screen by default. The difference is that stderr is not redirected when the standard output is redirected. For example, under Unix or MS-DOS, when you invoke

	program > filename
anything printed to stdout is redirected to the file filename, but anything printed to stderr still goes to the screen. The intent behind stderr is that it is the ``standard error output''; error messages printed to it will not disappear into an output file. For example, a more realistic way to print an error message when a file can't be opened would be
	if((ifp = fopen(filename, "r")) == NULL)
		fprintf(stderr, "can't open file %s\n", filename);
		exit or return
where filename is a string variable indicating the file name to be opened. Not only is the error message printed to stderr, but it is also more informative in that it mentions the name of the file that couldn't be opened. (We'll see another example in the next chapter.)

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