[Solved] String callocate/reallocate fails with FB64

[noop]

Hi,

I don't see why the following doesn't work (FB 64-bit on 64-bit Win7 using "-w pedantic -exx -g"):

Code: Select all

dim as string ptr x = callocate(1,sizeof(string))
if x <> 0 then
    x[0] = "a"
    for i as integer = 2 to 10
        x = reallocate(x,i*sizeof(string))
        if x <> 0 then x[i-1] = "b"
    next i
end if

It crashes with the error

c0000374

Something stack related. Seems to work fine with FB-32-bit.

[fxm]

There is a bug in the code.
That works with fbc 32-bit by chance.

When the memory is reallocated, the added memory zone is not cleared (at opposite of Callocate).
So the added string descriptor is not cleared and the instruction 'x[i-1]' begins to try to deallocate the previous string (because the descriptor is not cleared) => possible crash.

Before use the new allocated string, one must clear its descriptor:

Code: Select all

dim as string ptr x = callocate(1,sizeof(string))
if x <> 0 then
    x[0] = "a"
    for i as integer = 2 to 10
        x = reallocate(x,i*sizeof(string))
        if x <> 0 then
            clear x[i-1], 0, sizeof(string)
            x[i-1] = "b"
        end if
    next i
end if

[noop]

Thanks fxm!

[fxm]

About this mistake, there is already a note in documentation at page KeyPgReallocate:

.....
When manually allocating memory for String descriptors (or UDTs that contain one), if count is larger than the original size of the memory block, the new extra memory range must be explicitly cleared to zeroes before the first string use (for example, using Clear). Otherwise accessing the string will cause undefined results (trying to write or read at a random place in memory, or trying to deallocate a random pointer).
.....