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Can anyone explain how the following code could be explained, I dont understand it really.

while((c = getchar()) != '\n' && c != EOF);

I know that its used to flush the inputstream and that it could be replaced with the standard C library-function:

fflush(stdin)
Björn Hallström
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    No, it couldn't -- `fflush(stdin);` gives undefined behavior (same if you apply it to any other input stream). – Jerry Coffin Oct 06 '13 at 07:15
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    The basic gist of that line is that is reads in characters until it either reads in a newline or hits the end of a file. As Jerry pointed out, that's not the same as `fflush(stdin);`. – Dennis Meng Oct 06 '13 at 07:17
  • [FAQ-Flush the input buffer](http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?answer=1044873249&id=1043284392) – Grijesh Chauhan Oct 06 '13 at 07:18
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    Basically `while ((ch = getchar()) != '\n' && ch != EOF);` reads all unread char from stdin buffer until EOF or `\n` new-line char found. But Note: "if you call these when there is no data in the input stream, the program will wait until there is, which gives you undesirable results." – Grijesh Chauhan Oct 06 '13 at 07:20

2 Answers2

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while((c = getchar()) != '\n' && c != EOF);

This reads input characters until it reaches either the end of the line (i.e., getchar() returned '\n) or end-of-file or an error condition (i.e., getchar() returned EOF).

If stdin is reading from the keyboard, it discards input until you press Enter.

Leaving off the EOF check could give you an infinite loop if there's an input error, or if you trigger an end-of-file condition (on Unix, by typing Ctrl-D twice).

This could be useful, for example, after using scanf() to read an integer. If you execute scanf("%d", &num); and type 123, it will read those 3 digits (and store the value 123 in n), but leave everything after that waiting to be read. The above line can be used to skip the rest of the input line.

(An alternative, likely a better one, is to read whole lines using fgets() and parse them using sscanf().)

This is not equivalent to fflush(stdin). A far as the C standard is concerned, calling fflush on an input stream has undefined behavior.

Some implementations do define the behavior of fflush(stdin). On systems that use GNU libc, for example (most Linux system):

For input streams, fflush() discards any buffered data that has been fetched from the underlying file, but has not been consumed by the application.

That's not the same as discarding input up to the end of the line. And using it makes your code non-portable.

Keith Thompson
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This code, assuming the file (or some other input) has been opened, will continue to get character by character until it finds a newline (\n) or an end of file (eof).

fflush will just clear the stream for an open file.

Harley
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