scanf Cppcheck warning

Question

Cppcheck shows the following warning for scanf:

Message: scanf without field width limits can crash with huge input data. To fix this error message add a field width specifier:
    %s => %20s
    %i => %3i

Sample program that can crash:

#include 
int main()
{
    int a;
    scanf("%i", &a);
    return 0;
}

To make it crash:
perl -e 'print "5"x2100000' | ./a.out

I cannot crash this program typing "huge input data". What exactly should I type to get this crash? I also don't understand the meaning of the last line in this warning:

perl -e ...

Answer 1

The last line is an example command to run to demonstrate the crash with the sample program. It essentially causes perl to print 2.100.000 times "5" and then pass this to the stdin of the program "a.out" (which is meant to be the compiled sample program).

First of all, scanf() should be used for testing only, not in real world programs due to several issues it won't handle gracefully (e.g. asking for "%i" but user inputs "12345abc" (the "abc" will stay in stdin and might cause following inputs to be filled without a chance for the user to change them).

Regarding this issue: scanf() will know it should read a integer value, however it won't know how long it can be. The pointer could point to a 16 bit integer, 32 bit integer, or a 64 bit integer or something even bigger (which it isn't aware off). Functions with a variable number of arguments (defined with ...) don't know the exact datatype of elements passed, so it has to rely on the format string (reason for the format tags to not be optional like in C# where you just number them, e.g. "{0} {1} {2}"). And without a given length it has to assume some length which might be platform dependant as well (making the function even more unsave to use).

In general, consider it possibly harmful and a starting point for buffer overflow attacks. If you'd like to secure and optimize your program, start by replacing it with alternatives.

Answer 2

I tried running the perl expression against the C program and it did crash here on Linux (segmentation fault).

Answer 3

Using of 'scanf' (or fscanf and sscanf) function in real-world applications usually is not recommended at all because it's not safe and it's usually a hole for buffer overrun if some incorrect input data will be supplied. There are much more secure ways to input numbers in many commonly used libraries for C++ (QT, runtime libraries for Microsoft Visual C++ etc.). Probably you can find secure alternatives for "pure" C language too.