Calls to scanf are superficially similar to calls to
printf in that arbitrary arguments are read under the control of
a template string. While the syntax of the conversion specifications in
the template is very similar to that for printf, the
interpretation of the template is oriented more towards free-format
input and simple pattern matching, rather than fixed-field formatting.
For example, most scanf conversions skip over any amount of
“white space” (including spaces, tabs, and newlines) in the input
file, and there is no concept of precision for the numeric input
conversions as there is for the corresponding output conversions.
Ordinarily, non-whitespace characters in the template are expected to
match characters in the input stream exactly, but a matching failure is
distinct from an input error on the stream.
Another area of difference between scanf and printf is
that you must remember to supply pointers rather than immediate values
as the optional arguments to scanf; the values that are read are
stored in the objects that the pointers point to. Even experienced
programmers tend to forget this occasionally, so if your program is
getting strange errors that seem to be related to scanf, you
might want to double-check this.
When a matching failure occurs, scanf returns immediately,
leaving the first non-matching character as the next character to be
read from the stream. The normal return value from scanf is the
number of values that were assigned, so you can use this to determine if
a matching error happened before all the expected values were read.
The scanf function is typically used for things like reading in
the contents of tables. For example, here is a function that uses
scanf to initialize an array of double:
void
readarray (double *array, int n)
{
int i;
for (i=0; i<n; i++)
if (scanf (" %lf", &(array[i])) != 1)
invalid_input_error ();
}
The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.
If you are trying to read input that doesn’t match a single, fixed
pattern, you may be better off using a tool such as Flex to generate a
lexical scanner, or Bison to generate a parser, rather than using
scanf. For more information about these tools, see Flex: The Lexical Scanner Generator, and The Bison Reference Manual.