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This call temporarily disables or enables signal processing for the current process.


DosHoldSignal (ActionCode)


ActionCode (USHORT) - input 
Disables or enables signals intended for the current process.
0 - Signals are enabled
1 - Signals are disabled.

Return Code

rc (USHORT) - return
Return code descriptions are:
  • 0 NO_ERROR


DosHoldSignal with ActionCode = 1 causes signal processing (except central processing errors and numeric processor errors) to be postponed until a DosHoldSignal with ActionCode = 0 is issued. Any signals that occur while processing is disabled are recognized, but not accepted until signal recognition is enabled.

To allow for nesting of requests, a count of the number of outstanding DosHoldSignal requests with ActionCode = 1 are maintained.

DosHoldSignal is used by library routines, subsystems, and similar code that lock critical sections or temporarily reserve resources needed to prevent a signal from terminating a task. A process can have only one signal handling address for each signal. Dynalink routines should not have a signal handler (which might override a handler established by a calling process).

Signals can be held for a short period and should be released and re-held, if necessary. Their guidelines for proper use are similar to hardware interrupt counterparts such as the CLI/STI instructions.

Family API Considerations

Some options operate differently in the DOS mode than in the OS/2 mode. Therefore, the following restriction applies to DosHoldSignal when coding for the DOS mode:

The only signal recognized in the DOS mode is SIGINTR (Ctrl-C) and SIGBREAK. Only SIGINTR and SIGBREAK are turned off by this call.




USHORT  rc = DosHoldSignal(ActionCode);

USHORT  ActionCode;    /* Indicate to Disable/Enable Signals */
USHORT  rc;            /* return code */


EXTRN  DosHoldSignal:FAR

PUSH   WORD    ActionCode    ;Indicate to Disable/Enable Signals
CALL   DosHoldSignal

Returns NONE


The following example illustrates the use of the Ctrl-C (SIGINTR) signal to signal time-critical events. Process1 invokes process2, which establishes a signal handler named CtrlC_Handler() and waits, by blocking on a reserved RAM semaphore, for a signal from process1. A portion of process2 is immune to signalling.


#include <os2.h>

#define SLEEPTIME       200L              /* Sleep interval */
#define START_PROGRAM   "process2.exe"    /* Program name */

  CHAR          ObjFail[50];
  PSZ           Args;
  PSZ           Envs;
  RESULTCODES   ReturnCodes;
  USHORT        rc;

  /* Start process2 and check its PID */
  if(!(DosExecPgm(ObjFail,               /* Object name buffer */
                  sizeof(ObjFail),       /* Length of obj. name buffer */
                  EXEC_ASYNC,            /* Execution flag */
                  Args,                  /* Ptr. to argument string */
                  Envs,                  /* Ptr. to environment string */
                  &ReturnCodes,          /* Ptr. to resultcodes struct.*/
                  START_PROGRAM)))       /* Name of program file */
    printf("Process2 started.\n");
  printf("Process2 ID is %d\n", ReturnCodes.codeTerminate);

  /* Sleep to give time slice to process2 */
  DosSleep(SLEEPTIME);                   /* Sleep interval */

  /*** After process2 sets signal handler, send process2 a signal ***/
  if(!(rc = DosSendSignal(ReturnCodes.codeTerminate,  /* PID of process2 */
                          SIG_CTRLC)))                /* Signal to send*/
    printf("Ctrl-C signal sent from Process1 to Process2.\n");
/* ----- process2.c ----- */


#include <os2.h>

#define SLEEPTIME         50L
#define TIMEOUT           5000L

VOID APIENTRY CtrlC_Handler(arg1, arg2)    /** Define signal handler **/
  USHORT      arg1;
  USHORT      arg2;
  printf("Handler for Ctrl-C now running.\n");

  ULONG        RamSem = 0L;  /* Allocate and initialize Ram Semaphore */
  ULONG far   *RamSemHandle = &RamSem;  /* Ram Semaphore handle */
  USHORT       rc;

  /* Establish signal handler */
  if(!(rc=DosSetSigHandler((PFNSIGHANDLER) CtrlC_Handler,
                           NULL,          /* Previous handler - ignored */
                           NULL,          /* Previous action - ignored */
                           SIGA_ACCEPT,   /* Request type */
                           SIG_CTRLC)))   /* Signal number */
    printf("Process2 has set Ctrl-C handler.\n");

    /* Error processing on rc */;
  /* Get semaphore for first time */
  if(!(rc=DosSemRequest(RamSemHandle,       /* Semaphore handle */
                        TIMEOUT)))          /* Timeout interval */
    printf("Semaphore obtained.\n");

  /*** Disable and then enable signal-handling ***/
  if(!(rc=DosHoldSignal(HLDSIG_DISABLE)))   /** Action code - disable **/
    printf("Signalling DISABLED.\n");

    /* Do signal-proof work here */
    if(!(rc=DosHoldSignal(HLDSIG_ENABLE)))  /** Action code - enable **/
      printf("Signalling ENABLED.\n");
  /* At this point, process1 may have sent a Ctrl-C signal. */
  /* Try to obtain semaphore again -- resulting in Timeout. */
  /* The Timeout, however, may be interrupted by the signal. */

  printf("Process2 will now wait on a Ramsem for a while.\n");
  if((rc=DosSemRequest(RamSemHandle,     /* Semaphore handle */
                       TIMEOUT))         /* Timeout interval */
  printf("Process2 interrupted while waiting, rc is %d.\n", rc);