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Tip of the Day
Language: C++
Expertise: Beginner
Apr 4, 2000

Tackling a Common Bug With scanf()

In legacy code and environments that support C exclusively, using scanf() is still a widespread method of getting input from a user (or a file, when using fscanf()). When using this function, beware of a common bug that results from C's weak type-checking:

 
  #include <stdio.h>
  int main()
  {
    int n;
    scanf("%d", n); /* a bug; should read '&n' */
  }

The first parameter of scanf() is a format string that describes the type and length of the data to be read. The following arguments must be one ore more pointers to buffers into which the input is written. Unlike C++, C enforces weak type checks. Therefore, a C compiler doesn't detect that the second argument passed to scanf() is an int rather than a pointer to int. Consequently, the program's behavior is undefined—the input is written to a random memory address, not to n. The correct form should be:

 
  scanf("%d", &n); /* now OK */
Danny Kalev
 
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