Thanks both for the help. I feel a bit silly now, as I did read the ffi memory management page... ahh well. Excuses, excuses, eh? Thankfully, in this case, the pointer isn't kept by the c library, as it is just wanting somewhere to store a string.
Thanks again, Neil On 26 January 2012 14:12, Michael W <mwil...@uccs.edu> wrote: > Hey there, Neil! > > When racket allocates something, it lives in memory that's > managed by the garbage collector unless you say otherwise. By > contrast, when the C library allocates something, racket's > garbage collector has no clue about it -- that's what (free) is > for. > > By default, (malloc) allocates a pointer that's managed by the > garbage collector. So, you're (free)ing it yourself, but the > garbage collector comes along later, notices that you haven't > kept that pointer around, and frees it again. > > See the "mode" argument in the malloc documentation: > http://docs.racket-lang.org/foreign/foreign_pointer-funcs.html?q=malloc#(def._((quote._~23~25foreign)._malloc)) > > There are two ways around this. First, you can: > (malloc _pointer 'raw) > and then be extra careful to (free) it yourself later. Or, you > can just (define x (malloc _pointer)) and let the garbage > collector deal with it, being extra careful that the xmms > libraries don't keep that pointer around after racket's garbage > collector frees it. > > This post by Thomas Chust helped me understand: > https://lists.racket-lang.org/users/archive/2011-June/045967.html > (see about halfway down) > > 3 hours ago, Neil Caldwell wrote: >> Hi >> >> I've just started playing with racket, and decided to start with a >> small (that's what I'm telling myself, anyway) project in the form of >> an xmms2 client. To do this, I'm having to interface with >> libxmmsclient. Long story short, I'm having a problem with free, and >> was hoping to get some advice. >> >> If I put the following code into ptr.rkt: >> #################################### >> >> #lang racket >> >> (require ffi/unsafe) >> >> (free (malloc _pointer)) >> >> #################################### >> >> and then run it I get a double free error. >> >> #################################### >> >> $ racket ptr.rkt >> *** glibc detected *** /home/neil/apps/racket-5.2/bin/racket: double >> free or corruption (out): 0xb45d4488 *** >> ======= Backtrace: ========= >> /lib/libc.so.6(+0x6b6d1)[0xb76ed6d1] >> >> #################################### >> >> I haven't had much luck googling, unfortunatly (I could be search the >> wrong things) >> >> Is this a problem with racket (v 5.2), my setup (compiled it myself >> with all the default switches), glibc version incompatibility, or am I >> just doing something plain wrong? >> >> Thanks for the help, >> >> Neil > > -- > Hope that helps, > _mike > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
