semctl

Perform various control operations on a semaphore set

Declaration

Source position: ipc.pp line 846

Description

semctl performs various operations on the semaphore semnum w ith semaphore set id ID.

The arg parameter supplies the data needed for each call. This is a variant record that should be filled differently, according to the command:

Type
  TSEMun = record
   case longint of
      0 : ( val : longint );
      1 : ( buf : PSEMid_ds );
      2 : ( arr : PWord );
      3 : ( padbuf : PSeminfo );
      4 : ( padpad : pointer );
   end;

Which operation is performed, depends on the cmd parameter, which can have one of the following values:

IPC_STAT

In this case, the arg record should have it's buf field set to the address of a TSEMid_ds record. The semctl call fills this TSEMid_ds structure with information about the semaphore set.

IPC_SET

In this case, the arg record should have it's buf field set to the address of a TSEMid_ds record. The semctl call sets the permissions of the queue as specified in the ipc_perm record.

IPC_RMID

If this is specified, the semaphore set is removed from from the system.

GETALL

In this case, the arr field of arg should point to a memory area where the values of the semaphores will be stored. The size of this memory area is \var{SizeOf(Word)* Number of semaphores in the set}. This call will then fill the memory array with all the values of the semaphores.

GETNCNT

This will fill the val field of the arg union with the number of processes waiting for resources.

GETPID

semctl returns the process ID of the process that performed the last semop call.

GETVAL

semctl returns the value of the semaphore with number semnum.

GETZCNT

semctl returns the number of processes waiting for semaphores that reach value zero.

SETALL

In this case, the arr field of arg should point to a memory area where the values of the semaphores will be retrieved from. The size of this memory area is \var{SizeOf(Word)* Number of semaphores in the set}. This call will then set the values of the semaphores from the memory array.

SETVAL

This will set the value of semaphore semnum to the value in the val field of the arg parameter.

The function returns -1 on error.

Errors

The function returns -1 on error, and IPCerror is set accordingly.

See also

Example

Program semtool;

{ Program to demonstrate the use of semaphores }

Uses ipc,baseunix;

Const MaxSemValue = 5;

Procedure DoError (Const Msg : String);
var
  error: cint;
begin
  error:=fpgeterrno;
  Writeln ('Error : ',msg,' Code : ',error);
  Halt(1);
end;

Function getsemval (ID,Member : longint) : longint;

Var S : TSEMun;

begin
  GetSemVal:=SemCtl(id,member,SEM_GETVAL,S);
end;

Procedure DispVal (ID,member : longint);

begin
  writeln ('Value for member ',member,' is ',GetSemVal(ID,Member));
end;

Function GetMemberCount (ID : Longint) : longint;

Var opts : TSEMun;
    semds : TSEMid_ds;

begin
  opts.buf:=@semds;
  If semctl(Id,0,IPC_STAT,opts)<>-1 then
    GetMemberCount:=semds.sem_nsems
  else
    GetMemberCount:=-1;
end;

Function OpenSem (Key : TKey) : Longint;

begin
  OpenSem:=semget(Key,0,438);
  If OpenSem=-1 then
    DoError ('OpenSem');
end;

Function CreateSem (Key : TKey; Members : Longint) : Longint;

Var Count : Longint;
    Semopts : TSemun;

begin
// the semmsl constant seems kernel specific
{  If members>semmsl then
    DoError ('Sorry, maximum number of semaphores in set exceeded');
}
  Writeln ('Trying to create a new semaphore set with ',members,' members.');
  CreateSem:=semget(key,members,IPC_CREAT or IPC_Excl or 438);
  If CreateSem=-1 then
    DoError ('Semaphore set already exists.');
  Semopts.val:=MaxSemValue; { Initial value of semaphores }
  For Count:=0 to Members-1 do
    semctl(CreateSem,count,SEM_SETVAL,semopts);
end;

Procedure lockSem (ID,Member: Longint);

Var lock : TSEMbuf;

begin
  With lock do
    begin
    sem_num:=0;
    sem_op:=-1;
    sem_flg:=IPC_NOWAIT;
    end;
   if (member<0) or (member>GetMemberCount(ID)-1) then
     DoError ('semaphore member out of range');
   if getsemval(ID,member)=0 then
     DoError ('Semaphore resources exhausted (no lock)');
   lock.sem_num:=member;
   Writeln ('Attempting to lock member ',member, ' of semaphore ',ID);
   if semop(Id,@lock,1)=-1 then
     DoError ('Lock failed')
   else
     Writeln ('Semaphore resources decremented by one');
   dispval(ID,Member);
end;

Procedure UnlockSem (ID,Member: Longint);

Var Unlock : TSEMbuf;

begin
  With Unlock do
    begin
    sem_num:=0;
    sem_op:=1;
    sem_flg:=IPC_NOWAIT;
    end;
   if (member<0) or (member>GetMemberCount(ID)-1) then
     DoError ('semaphore member out of range');
   if getsemval(ID,member)=MaxSemValue then
     DoError ('Semaphore not locked');
   Unlock.sem_num:=member;
   Writeln ('Attempting to unlock member ',member, ' of semaphore ',ID);
   if semop(Id,@unlock,1)=-1 then
     DoError ('Unlock failed')
   else
     Writeln ('Semaphore resources incremented by one');
   dispval(ID,Member);
end;

Procedure RemoveSem (ID : longint);

var S : TSemun;

begin
  If semctl(Id,0,IPC_RMID,s)<>-1 then
    Writeln ('Semaphore removed')
  else
    DoError ('Couldn''t remove semaphore');
end;


Procedure ChangeMode (ID,Mode : longint);

Var rc : longint;
    opts : TSEMun;
    semds : TSEMid_ds;

begin
  opts.buf:=@semds;
  If not semctl (Id,0,IPC_STAT,opts)<>-1 then
    DoError ('Couldn''t stat semaphore');
  Writeln ('Old permissions were : ',semds.sem_perm.mode);
  semds.sem_perm.mode:=mode;
  If semctl(id,0,IPC_SET,opts)<>-1 then
    Writeln ('Set permissions to ',mode)
  else
    DoError ('Couldn''t set permissions');
end;

Procedure PrintSem (ID : longint);

Var I,cnt : longint;

begin
  cnt:=getmembercount(ID);
  Writeln ('Semaphore ',ID,' has ',cnt,' Members');
  For I:=0 to cnt-1 Do
    DispVal(id,i);
end;

Procedure USage;

begin
  Writeln ('Usage : semtool c(reate) <count>');
  Writeln ('                l(ock) <member>');
  Writeln ('                u(nlock) <member>');
  Writeln ('                d(elete)');
  Writeln ('                m(ode) <mode>');
  Writeln ('                p(rint)');
  halt(1);
end;

Function StrToInt (S : String): longint;

Var M : longint;
    C : Integer;

begin
  val (S,M,C);
  If C<>0 Then DoError ('StrToInt : '+S);
  StrToInt:=M;
end;

Var Key : TKey;
    ID : Longint;


const ipckey='.'#0;  

begin
  If ParamCount<1 then USage;
  key:=ftok(@ipckey[1],ORD('s'));
  Case UpCase(Paramstr(1)[1]) of
   'C' : begin
         if paramcount<>2 then usage;
         CreateSem (key,strtoint(paramstr(2)));
         end;
   'L' : begin
         if paramcount<>2 then usage;
         ID:=OpenSem (key);
         LockSem (ID,strtoint(paramstr(2)));
         end;
   'U' : begin
         if paramcount<>2 then usage;
         ID:=OpenSem (key);
         UnLockSem (ID,strtoint(paramstr(2)));
         end;
   'M' : begin
         if paramcount<>2 then usage;
         ID:=OpenSem (key);
         ChangeMode (ID,strtoint(paramstr(2)));
         end;
   'D' : Begin
         ID:=OpenSem(Key);
         RemoveSem(Id);
         end;
   'P' : begin
         ID:=OpenSem(Key);
         PrintSem(Id);
         end;
  else
    Usage
  end;
end.