[Gambas-user] Segfault when freeing pointer used for Extern

[Benoît Minisini]

Le 06/11/2013 01:07, Benoît Minisini a écrit :
>
> What is the exact definition of 'uuid_t' ?
>
> Also, you can use valgrind to know when exactly the faulty memory
> access occurs. This will give better clues.
>

typedef unsigned char uuid_t[16];

You get it in the '/usr/include/uuid.h', that you must read before
writing any external definition.

So, 'uuid_t' is a pointer to 16 'unsigned char', i.e. 16 bytes.

Now your code:

----------------------------------------------------------------------
' Gambas module file

' void uuid_generate(uuid_t out);
Extern UUID_Gen(op As Pointer) As Pointer In "libuuid:1" Exec
"uuid_generate"

' void uuid_unparse(uuid_t uu, char *out)
Extern UUID_ToStr(ip As Pointer, op As Pointer) As Pointer In
"libuuid:1" Exec "uuid_unparse"

Public Sub Main()

   Dim OP As Pointer    ' ptr to the uuid returned by uuid_generate
   Dim SP As Pointer    ' ptr to the string returned by uuid_unparse

   Dim sOP As String    ' our result (UUID as a string)
   Dim rtn As Integer   ' int rtn code from extern funcs
   Dim sArch As String  ' system architecture (selects pointer length)

   sArch = System.Architecture
   Error "Arch=" & sArch

   OP = Alloc(IIf(sArch = "x86", 4, 8))
   SP = Alloc(IIf(sArch = "x86", 4, 8))

----> Don't do that, use the SizeOf() function.

   rtn = UUID_Gen(OP)

----> OP must points at a 16 bytes allocation. So you failed! The 
library will erase the memory past the allocation, making everything 
crash sooner or later.

   Error "UUID_Gen=" & rtn

   rtn = UUID_ToStr(OP, SP)

----> That function returns nothing. I don't know why you think Gambas 
will make it return something. Moreover, if you read the doc, it tells 
you that SP must point at a 37 bytes allocation. So you failed again, 
and the library will erase the memory randomly a second time.

You really must know C and how a CPU works to deal with extern 
functions. Alas, Gambas can't help there! Or, at least, it does as much 
as he can.

-- 
Benoît Minisini