Finding memory leaks on Solaris systems

While perusing the opensolaris-dtrace discussion list this week, I came across Jonathon Adams recommendation to LD_PRELOAD the Solaris libumem library to find the source of a memory leak. Since I have never taken the opportunity to play with libumem, I decided to test it out this weekend. To get started, I created a C program that leaked memory. Once I verified the program was working as expected, I preloaded the libumem shared library, and executed the program:

$ UMEM_DEBUG=default ./leaky
Leaking 10 bytes of memory
Leaking 10 bytes of memory
Leaking 10 bytes of memory
Leaking 10 bytes of memory
Leaking 10 bytes of memory
Leaking 10 bytes of memory

After the program leaked a few bytes of memory, I used the Solaris gcore utility to generate a core file with the programs memory state:

$ gcore `pgrep leaky`
gcore: core.10973 dumped

This will produce a core file with memory debugging data, which can be analyze by the Solaris modular debugger “::findleaks” dcmd:

$ mdb core.10973

Loading modules: [ ]

> ::findleaks -dv
findleaks:                maximum buffers => 22
findleaks:                 actual buffers => 9
findleaks:             potential pointers => 23567
findleaks:                     dismissals => 17080         (72.4%)
findleaks:                         misses => 6209          (26.3%)
findleaks:                           dups => 274           ( 1.1%)
findleaks:                        follows => 4             ( 0.0%)
findleaks:              elapsed wall time => 0 seconds
00029c08       5 0003e000 leakingmemory+0xc
   Total       5 buffers, 120 bytes

umem_alloc_24 leak: 5 buffers, 24 bytes each, 120 bytes total
            ADDR          BUFADDR        TIMESTAMP           THREAD
                            CACHE          LASTLOG         CONTENTS
           3e000            37fc0    33ec48f6751b4                1
                            29c08                0                0

In this example you can see that we leaked 5-buffers at address leakingmemory+0xc, which happens to be the location where my program is leaking memory (leakingmemory is called from a loop in main):

void leakingmemory(int bytes)
   char *oof = (char *)malloc(bytes);
   int i;

   printf("Leaking %d bytes of memory\n",bytes);
   for (i = 0; i < bytes; i++) {
        oof[i] = 1;

This is some amazingly useful stuff, and from the documentation I read it can be used to detect problems on production systems. Libumem can also provide some substantial performance benefits, which are covered in the USENIX paper Magazines and Vmem: Extending the Slab Allocator to Many CPUs and Arbitrary Resources.

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