(gdb) bt
#0  0x080a2676 in ast_frame_slinear_sum (f1=0xb7d2aad0, f2=0xb7d24288)
    at /usr/src/asterisk-1.4/include/asterisk/utils.h:192
#1  0x080878a0 in ast_channel_spy_read_frame (spy=0x95bcf50, samples=160)
    at channel.c:4362
#2  0x00458f76 in mixmonitor_thread (obj=0x95bcf50) at app_mixmonitor.c:166
#3  0x080faf40 in dummy_start (data=0x957abf0) at utils.c:544
#4  0x0097b371 in start_thread () from /lib/tls/libpthread.so.0
#5  0x0080cffe in clone () from /lib/tls/libc.so.6
(gdb) bt full
#0  0x080a2676 in ast_frame_slinear_sum (f1=0xb7d2aad0, f2=0xb7d24288)
    at /usr/src/asterisk-1.4/include/asterisk/utils.h:192
        count = 0
        data1 = (short int *) 0xb7d2ab50
        data2 = (short int *) 0x0
#1  0x080878a0 in ast_channel_spy_read_frame (spy=0x95bcf50, samples=160)
    at channel.c:4362
        result = (struct ast_frame *) 0x0
        read_buf = {0, 0}
        write_buf = {0, 0}
        read_frame = (struct ast_frame *) 0xb7d2aad0
        write_frame = (struct ast_frame *) 0xb7d24288
        need_dup = 0
        stack_read_frame = {frametype = AST_FRAME_VOICE, subclass = 64,
  datalen = 320, samples = 160, mallocd = 0, mallocd_hdr_len = 0, offset = 0,
  src = 0x0, data = 0xb703b160, delivery = {tv_sec = 0, tv_usec = 0},
  frame_list = {next = 0x0}, has_timing_info = 0, ts = 0, len = 0, seqno = 0}
        stack_write_frame = {frametype = AST_FRAME_VOICE, subclass = 64,
  datalen = 320, samples = 160, mallocd = 0, mallocd_hdr_len = 0, offset = 0,
  src = 0x0, data = 0xb703b010, delivery = {tv_sec = 0, tv_usec = 0},
  frame_list = {next = 0x0}, has_timing_info = 0, ts = 0, len = 0, seqno = 0}
#2  0x00458f76 in mixmonitor_thread (obj=0x95bcf50) at app_mixmonitor.c:166
        next = (struct ast_frame *) 0x0
---Type <return> to continue, or q <return> to quit---
        write = 1
        mixmonitor = (struct mixmonitor *) 0x95bcf50
        f = (struct ast_frame *) 0x0
        __PRETTY_FUNCTION__ = "mixmonitor_thread"
#3  0x080faf40 in dummy_start (data=0x957abf0) at utils.c:544
        _buffer = {__routine = 0x80685e8 <ast_unregister_thread>,
  __arg = 0xb703bbb0, __canceltype = 0, __prev = 0x0}
        ret = (void *) 0x86a6f4
        a = {start_routine = 0x458eaf <mixmonitor_thread>, data = 0x95bcf50,
  name = 0x95bd098 "mixmonitor_thread    started at [  295] app_mixmonitor.c launch_monitor_thread()"}
#4  0x0097b371 in start_thread () from /lib/tls/libpthread.so.0
No symbol table info available.
#5  0x0080cffe in clone () from /lib/tls/libc.so.6
No symbol table info available.