why knownConditionTrueFalse

[Sander Bouwhuis]

I have the following function:

 1 // This function finds the positions of the nibbles in the checksum table
 2 void CDataCodec::FindPosInChecksumTable(uint8 u8Data, int32 *pi32Low, int32 *pi32High) const
 3 {
 4   try
 5   {
 6     CHECK_MEM_LEAKS
 7 
 8     uint8 u8Nibble;
 9 
10     // Set the low nibble
11     u8Nibble = uint8(u8Data & 0xf0);
12 
13     // Find the low nibble in the checksum look-up table
14     for(*pi32Low=0;*pi32Low<16;(*pi32Low)++)
15     {
16       // Do we have the correct low nibble value?
17       if(u8Nibble == uint8(g_u8ChecksumTable[*pi32Low] & 0xf0))
18         break;
19     }
20 
21     // Set the high nibble
22     u8Nibble = uint8(u8Data & 0x0f);
23 
24     // Find the high nibble in the checksum look-up table
25     for(*pi32High=0;*pi32High<16;(*pi32High)++)
26     {
27       // Do we have the correct high nibble value?
28       if(u8Nibble == uint8(g_u8ChecksumTable[*pi32High] & 0x0f))
29         break;
30     }
31   }
32   catch(...) { OnException(nullptr, __FILE__, __LINE__, __FUNCSIG__, __TIMESTAMP__, nullptr); }
33 }

But, I get 2 errors stating:

line 14 : 'Condition '*pi32Low<16' is always true'
line 25 : 'Condition '*pi32High<16' is always true'*

Clearly, the value of pi32Low and pi32High can be [0,MAX_INT32] (after initialization in the for loop)?

[versat]

Seems to be another false positive, i can reproduce this with 1.89 dev.
The values clearly can be from 0 to 16 in the for loops. And the loops are exited when 16 is reached.
Here is a small compilable example that shows the false positive:

#include <stdio.h>

void f(int * pA)
{
    for(*pA = 0; *pA < 10; (*pA)++)
    {
        printf("*pA: %d\n", *pA);
    }
}

int main()
{
    int a;
    f(&a);
    return 0;
}

Output of Cppcheck 1.89 dev:

$ g++ -fsyntax-only fp_knownConditionTrueFalse.cpp && ./cppcheck --enable=style --debug fp_knownConditionTrueFalse.cpp
Checking fp_knownConditionTrueFalse.cpp ...

##file fp_knownConditionTrueFalse.cpp
3: void f ( int * pA@1 )
4: {
5: for ( * pA@1 = 0 ; * pA@1 < 10 ; ( * pA@1 ) ++ )
6: {
7: printf ( "*pA: %d\n" , * pA@1 ) ;
8: }
9: }
10:
11: int main ( )
12: {
13: int a@2 ;
14: f ( & a@2 ) ;
15: return 0 ;
16: }

##Value flow
Line 5
  * possible Uninit
  pA possible Uninit*
  0 always 0
  * always 0
  < always 1
  10 always 10
Line 7
  "*pA: %d\n" always "*pA: %d\n"
Line 14
  & possible {lifetime=a,Uninit*}
  a always Uninit
Line 15
  0 always 0
fp_knownConditionTrueFalse.cpp:5:22: style: Condition '*pA<10' is always true [knownConditionTrueFalse]
    for(*pA = 0; *pA < 10; (*pA)++)
                     ^
fp_knownConditionTrueFalse.cpp:5:13: note: *pA is assigned value 0
    for(*pA = 0; *pA < 10; (*pA)++)
            ^
fp_knownConditionTrueFalse.cpp:5:22: note: Condition '*pA<10' is always true
    for(*pA = 0; *pA < 10; (*pA)++)
                     ^

[versat]

I found a ticket that looks related and added your example: https://trac.cppcheck.net/ticket/9036
Thanks for reporting!