Question

I’m running into a strange situation with passing a pointer to a structure with a very large array defined in the struct{} definition, a float array around 34MB in size. In a nutshell, the psuedo-code looks like this:

typedef config_t{
  ...
  float values[64000][64];
} CONFIG;


int32_t Create_Structures(CONFIG **the_config)
{
  CONFIG  *local_config;
  int32_t number_nodes;

  number_nodes = Find_Nodes();

  local_config = (CONFIG *)calloc(number_nodes,sizeof(CONFIG));
  *the_config = local_config;
  return(number_nodes);
}


int32_t Read_Config_File(CONFIG *the_config)
{
    /* do init work here */
    return(SUCCESS);
}


main()
{
    CONFIG *the_config;
    int32_t number_nodes,rc;

    number_nodes = Create_Structures(&the_config);

    rc = Read_Config_File(the_config);
    ...
    exit(0);
}

The code compiles fine, but when I try to run it, I’ll get a SIGSEGV at the { beneath Read_Config_File().

(gdb) run
...
Program received signal SIGSEGV, Segmentation fault.
0x0000000000407d0a in Read_Config_File (the_config=Cannot access memory at address 0x7ffffdf45428
) at ../src/config_parsing.c:763
763 {
(gdb) bt
#0  0x0000000000407d0a in Read_Config_File (the_config=Cannot access memory at address 0x7ffffdf45428
) at ../src/config_parsing.c:763
#1  0x00000000004068d2 in main (argc=1, argv=0x7fffffffe448) at ../src/main.c:148

I’ve done this sort of thing all the time, with smaller arrays. And strangely, 0x7fffffffe448 – 0x7ffffdf45428 = 0x20B8EF8, or about the 34MB of my float array.

Valgrind will give me similar output:

==10894== Warning: client switching stacks?  SP change: 0x7ff000290 --> 0x7fcf47398
==10894==          to suppress, use: --max-stackframe=34311928 or greater
==10894== Invalid write of size 8
==10894==    at 0x407D0A: Read_Config_File (config_parsing.c:763)
==10894==    by 0x4068D1: main (main.c:148)
==10894==  Address 0x7fcf47398 is on thread 1's stack

The error messages all point to me clobbering the stack pointer, but a) I’ve never run across one that crashes on entry of the function and b) I’m passing pointers around, not the actual array.

Can someone help me out with this? I’m on a 64-bit CentOS box running kernel 2.6.18 and gcc 4.1.2

Thanks!

Matt

Answer 1

You’ve blown up the stack by allocating one of these huge config_t structs onto it. The two stack pointers on evidence in the gdb output, 0x7fffffffe448 and 0x7ffffdf45428, are very suggestive of this.

$ gdb
GNU gdb 6.3.50-20050815 ...blahblahblah...
(gdb) p 0x7fffffffe448 - 0x7ffffdf45428  
$1 = 34312224

There’s your ~34MB constant that matches the size of the config_t struct. Systems don’t give you that much stack space by default, so either move the object off the stack or increase your stack space.