How to properly use the * text modifier in a printf statement.

Factual Questions
1

I amin a beginning C programming class. While writing a simple program I had to tell a scanf statement to ignore the leading white space:

scanf(" %c" . &char);

I read in the textbook that the * is used as a more general form that would ignore any data type that was in the buffer. There were example programs showing its use. However, when I tried to use the astrix the program would not function. It compiled and ran, but it treated the statement as if the astrix was not even there.

scanf("*%c". &char);

So, did I do something wrong? Is this particular to only some compilers?

2

Woops, I screwed up in the title. I meant to say scanf statement.

3

I have taught beginning C (although in a corporate rather than academic setting) and am not familiar with the * formatting feature. I wonder if it’s ANSI.

I would use getchar for what you’re trying to do.

(Using %c is the only time scanf does not skip past white space.)

P.S. “asterisk”

4

An asterisk may be used following the percent sign to suppress assignment for that match. In other words, saying “%*c” would say, “match what you normally would for %c, but don’t assign it to anything.”

5

I think you likely would want to use the following (untested, but documented in Kernighan and Ritchie):


scanf ("%1s", &myChar);

This specifier will skip whitespace as it’s expecting a string (which ignores whitespace) but the field width is set to 1, so only a single character is read from the input stream.
The * modifier is used to ignore a particular input in the stream (in the case of scanf), and it must be between the % and the conversion specifier. So if someone entered three integers separated by spaces but you only wanted the third integer, you could use:


scanf("%*d %*d %d, &thirdInt);

I also find that some scanf docs which appear to say that if you precede the “c” conversion specifier with a space, the scanf statement will ignore leading whitespace. This may be an implementation difference.
If you only want a character, you might also consider using the getchar() function:


char myInput;
while ( (myInput = getchar ()) != '
' )
{
    /* if input is white space, toss it */
}
6

Example from man(3C) scanf(), Solaris 5.8:

int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);

 with input:

 56789 0123 56a72

 will assign 56 to i, 789.0 to x, skip 123,  and  place  the string  56\0  in  name.  The  next  call to getchar(3C) will return the character a.

-lv