Re: select() call corrupts stack

[Byron Stanoszek]

On Wed, 27 Feb 2002, Artiom Morozov wrote:

> Hello,
> 
> 	Here's a sample program. Try running it and open about 2k of 
> connections to port 5222 (you'll need ulimit -n 10000 or like that). It 
> will segfault. Simple asm like this

> ps: it's okay for 1k of connections or so

printf("%d\n", sizeof(fd_set));

You'll see that it's 128, which is equivalent to 1024 bits, or 1024 possible
open connections that can be held in that variable. I would consider using a
dynamically-resizable memory space like in the code below to do what you want:

    struct timeval timeout;
    static fd_set *input_set, *output_set;
    static int fdset_size;

    extern int top_desc;

    /* dynamically grow file descriptor sets when needed (for >1024 FDs) */
    if(!input_set || (fdset_size-8)*8 <= top_desc) {
      free(input_set);
      free(output_set);
      fdset_size=(top_desc/64+2)*8;
      input_set=malloc(fdset_size);
      output_set=malloc(fdset_size);
    }

    /* clear input/output file descriptor sets */
    memset(input_set, 0, fdset_size);
    memset(output_set, 0, fdset_size);

    [... insert code to set input_set/output_set bits using FD_SET ...]

    select(top_desc+1, input_set, output_set, NULL, &timeout);

 -Byron

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