scanf - The Unknown Workhorse

Introduction

The scanf family of functions, while quite powerful, is most often used only by beginners (because they do not know other ways of input) or for simple conversions from string to other types (where the strtoX() family of functions is much more appropriate).

Problems of scanf and fscanf, regarding keeping track of position...

Due to the problems associated with scanf and fscanf, we only use sscanf in the following examples. You should do the same in your code.

Pitfalls to avoid

One of the most common problem beginners have with scanf is the need to provide pointers to the variables that ultimately shall hold the parsed values. Since scanf is a function that takes a variable number of arguments, they are not type-checked as usual. Because of this, you can provide non-pointer arguments, pointers to the wrong type, or pointers to strings instead of just strings, and the compiler will not fetch these errors at compile time. If you are lucky, you get a segment violation at run-time; if not, you'll get other behaviour. !!! \% vs. %% !!!

Some compilers (notably, gcc) support type-checking for literal format strings by parsing the format string during compile-time and then applying the appropriate type checks.

The following code shows those common errors and the correct code on consecutive lines:

int i;
short h;
char buf[80];
char* str = buf;

/* providing the variable instead of a pointer to it */
sscanf("%d", i);        /* wrong */
sscanf("%d", &i);       /* right */

/* providing a pointer to the wrong type (or a wrong format
   to the right pointer) */
sscanf("%d", &h);       /* wrong */
sscanf("%hd", &h);      /* right */

/* providing a pointer to a string pointer instead of the
   string pointer itself (some people think "once &, always &) */
sscanf("%s", &str);     /* wrong */
sscanf("%s", str);      /* right */

Another common error is not to check the return type of scanf, which you always should do. scanf returns the number of sucessfully scanned fields, so a proper scanf calls looks like this:

int x, y;
if (2 == sscanf(coord_string, "%d,%d", &x, &y)) {
    plot(x, y);
} else {
    perror("bad coordinates");
}

Parsing

Using character set formats

Conclusion

References

Brian W. Kernighan, Dennis M. Ritchie, The C Programming Language.
Prentice Hall, 1988, 2nd edition, ISBN 0-13-110362-8.