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uninitialised value & valgrind

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Hello

I'm working with a legacy code that I link to my application, and run it 
under 'valgrind' tool to catch memory leaks, corrupted memory etc. So here 
is what it looks like:

#define VLOG_MSG_MAX_DATA_SIZE   1024
struct vlog_msg_header
{
  u_int32_t   vmh_vr_id;
  u_int16_t   vmh_msg_type;
  module_id_t vmh_mod_id;
  u_int32_t   vmh_proc_id;
  s_int8_t    vmh_priority;
  u_int32_t   vmh_data_len;
};

struct vlog_msg
{
  struct vlog_msg_header vms_msg_hdr;
  char vms_msg_data[VLOG_MSG_MAX_DATA_SIZE+1];
};

int vlog_client_send_message (struct vlog_msg *msg)
{
  ...
  write (sock, (void *) msg, msg_len);  /* line 103. */
}

int vlog_client_send_ctrl_msg ()
{
  struct vlog_msg_header header;

  /* initialise 'header' .. */

  vlog_client_send_message ((struct vlog_msg *)&header);  /* line 41*/

  return RES_OK;
}

Valgrind reports:

==1634== Syscall param write(buf) points to uninitialised byte(s)
==1634==    at 0x6D2D22E: __write_nocancel (syscall-template.S:82)
==1634==    by 0x8116E68: vlog_client_send_message (vlog_client.c:103)
==1634==    by 0x8116EBF: vlog_client_send_ctrl_msg (vlog_client.c:41)
==1634==    by 0x8116EE6: vlog_client_connect_cb (vlog_client.c:211)
==1634==    by 0x80C9A3F: message_client_start (message.c:597)
==1634==    by 0x8116BA8: vlog_client_start (vlog_client.c:294)
==1634==    by 0x8116DA1: vlog_client_create (vlog_client.c:255)
==1634==    by 0x80C8FF1: openzlog (log.c:116)
==1634==    by 0x805AF95: ldp_start (ldp_main.c:85)
==1634==    by 0x8049F39: main (ldp.c:167)
==1634==  Address 0xfe541aee is on thread 1's stack
==1634==  Uninitialised value was created by a stack allocation
==1634==    at 0x8116E7F: vlog_client_send_ctrl_msg (vlog_client.c:32)

I assume valgrind complains because vlog_client_send_message() expects to 
get object of type 'struct vlog_msg', but instead we pass 'struct 
vlog_msg_header' and everything beyond it (what is supposed to be occupied 
by 'char vms_msg_data[VLOG_MSG_MAX_DATA_SIZE+1]' is undefined. Does it sound 
correct ?

My question is -- is it valid to typecast this way or it involves undefined 
behaviours?
Thanks.

Mark 

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Thread

uninitialised value & valgrind "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-06-26 12:35 -0400
  Re: uninitialised value & valgrind James Kuyper <jameskuyper@verizon.net> - 2012-06-26 13:53 -0400
    Re: uninitialised value & valgrind "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2012-06-26 14:07 -0400
      Re: uninitialised value & valgrind James Kuyper <jameskuyper@verizon.net> - 2012-06-26 15:42 -0400
  Re: uninitialised value & valgrind Johann Klammer <klammerj@NOSPAM.a1.net> - 2012-06-27 02:09 +0200

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