Comments on: What’s the difference between printing to standard output and printing to standard error? http://www.colonial-printing.com/printing/whats-the-difference-between-printing-to-standard-output-and-printing-to-standard-error Custom Label Printing Fri, 03 Dec 2010 12:23:59 +0000 hourly 1 http://wordpress.org/?v=3.1.2 By: Ratchetr http://www.colonial-printing.com/printing/whats-the-difference-between-printing-to-standard-output-and-printing-to-standard-error/comment-page-1#comment-669 Ratchetr Wed, 03 Feb 2010 06:45:59 +0000 http://www.colonial-printing.com/printing/whats-the-difference-between-printing-to-standard-output-and-printing-to-standard-error#comment-669 When a C program starts up, the run time automatically opens 3 file handles for it:stdin, stdout and stderr. These really are just file handles, as in FILE *. Normally, stdin is connected to the keyboard, and stdout and stderr are connected to the display console. But if you redirect stdout to a file, then stdout goes to the file, while stderr will still go to the screen. Since they really are just FILE * file handles, you can use them in any function that takes a FILE * parameter. printf doesn't take a FILE *, but fprintf does. These 2 function calls are identical: printf("Hello World\n"); fprintf(stdout,"Hello World\n"); For most functions that read the keyboard or write to the screen, there is another function of the same name, but starting with f... puts() --> fputs(). gets() --> fgets(), etc. If you were to do this: fprintf(stderr,"Saying Hello World\n"); printf("Hello World\n"); and run it as: sayhello > hello.txt Then you should see 'Saying Hello...' on the screen, and have 'Hello World' in hello.txt file. Sorry if that was too much detail ;-)<br><b>References : </b><br> When a C program starts up, the run time automatically opens 3 file handles for it:stdin, stdout and stderr. These really are just file handles, as in FILE *. Normally, stdin is connected to the keyboard, and stdout and stderr are connected to the display console. But if you redirect stdout to a file, then stdout goes to the file, while stderr will still go to the screen.

Since they really are just FILE * file handles, you can use them in any function that takes a FILE * parameter.
printf doesn’t take a FILE *, but fprintf does. These 2 function calls are identical:
printf("Hello World\n");
fprintf(stdout,"Hello World\n");

For most functions that read the keyboard or write to the screen, there is another function of the same name, but starting with f… puts() –> fputs(). gets() –> fgets(), etc.

If you were to do this:
fprintf(stderr,"Saying Hello World\n");
printf("Hello World\n");
and run it as:
sayhello > hello.txt

Then you should see ‘Saying Hello…’ on the screen, and have ‘Hello World’ in hello.txt file.

Sorry if that was too much detail ;-)
References :

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