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[Frama-c-discuss] Problem with memset

Hi Jens,

the difference between your function and memset is that memset takes a void pointer to a memory location and returns that pointer. However, the buffer being pointed to is interpreted as an array of bytes.
While this is awkward wrt type safety, a lot of embedded sw-developers use this function. My question is whether this can still be verified with Jessie.


Von: frama-c-discuss-bounces at [mailto:frama-c-discuss-bounces at] Im Auftrag von Jens Gerlach
Gesendet: Mittwoch, 2. Dezember 2009 21:18
An: Frama-C public discussion
Betreff: Re: [Frama-c-discuss] Problem with memset


I think your function is similar to the following one (we called it fill_array).

        requires 0 <= length;
        requires \valid_range(a, 0, length-1);
        assigns a[0..length-1];
        ensures \forall integer i; 0 <= i < length ==> a[i] == value;
void fill_array (int* a, int length,  int value )
         loop invariant 0 <= i <= length;
         loop assigns a[0..i-1];
         loop invariant \forall integer k; 0 <= k < i ==> a[k] == value;
    for (int i = 0; i < length; i++)
        a[i] = value;

Simplify and Z3 can  proof all 16 verification conditions.



On Dec 2, 2009, at 5:13 PM, Hollas Boris (CR/AEY1) wrote:

This is a re-implementation of the memset function. It uses void-pointers as in C.

typedef unsigned int size_t;
typedef unsigned char uint8;

* \brief Standard Libc memset() function.
*      \param[out] pDest       Pointer to target buffer of length
*                              \a num bytes.
*      \param[in] value        Value to set in each byte of \a pDest.
*      \param[in] num          Number of bytes in \a pDest to
*                              set to \a value.
*      \return                 Pointer to target buffer.

/*@ requires (\valid((uint8*)pDest +(0..num-1))) &&
 @          (0 <= value <= 255) &&
 @          (0 < num);
 @ ensures (\forall int i; 0 <= i < num
 @             ==> (((uint8*)pDest)[i] == value)) &&
 @         (\result == pDest);
void *memset( void *pDest, const int value, const size_t num )
   size_t i;

   /*@ loop invariant \forall int k; 0 <= k < i
     @         ==> ((uint8 *)pDest)[k] == (uint8)value;
   for( i=0; i < num; i++ )
               ((uint8 *)pDest)[i] = (uint8)value;

   return pDest;

When I try to verify it, Jessie reports

string.c:31:[jessie] user error: Casting from type struct char_P * to type struct unsigned_char_P * not allowed in logic
string.c:31:[jessie] warning: Dropping definition of function memset
string.c:19:[jessie] user error: Casting from type struct char_P * to type struct unsigned_char_P * not allowed in logic
[jessie] warning: Unsupported feature(s).
                 Jessie plugin can not be used on your code.

This error message puzzles me as there are no structs. Can a function that is equivalent to the memset function built into C be verified with Jessie?


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Dr.-Ing. Jens Gerlach

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