fpSelect function

Declaration

function FPSelect(N: cint; readfds: pFDSet; writefds: pFDSet; 
 exceptfds: pFDSet; TimeOut: ptimeval): cint

Description

FpSelect checks one of the file descriptors in the FDSets to see if the following I/O operation on the file descriptors will block.

readfds, writefds and exceptfds are pointers to arrays of 256 bits. If you want a file descriptor to be checked, you set the corresponding element in the array to 1. The other elements in the array must be set to zero. Three arrays are passed : The entries in readfds are checked to see if the following read operation will block. The entries in writefds are checked to see if the following write operation will block, while entries in exceptfds are checked to see if an exception occurred on them.

You can use the functions fpFD_ZERO, fpFD_Clr, fpFD_Set or fpFD_IsSet to manipulate the individual elements of a set.

The pointers can be Nil.

N is the value of the largest file descriptor in one of the sets, + 1. In other words, it is the position of the last bit which is set in the array of bits.

TimeOut can be used to set a time limit. If TimeOut can be two types :

1. TimeOut is of type ptimeval and contains a zero time, the call returns immediately. If TimeOut is Nil, the kernel will wait forever, or until a status changed.
2. TimeOut is of type cint. If it is -1, this has the same effect as a Timeout of type PTime which is Nil. Otherwise, TimeOut contains a time in milliseconds.

When the TimeOut is reached, or one of the file descriptors has changed, the Select call returns. On return, it will have modified the entries in the array which have actually changed, and it returns the number of entries that have been changed. If the timeout was reached, and no descriptor changed, zero is returned; The arrays of indexes are undefined after that. On error, -1 is returned.

The variant with the text file will execute the FpSelect call on the file descriptor associated with the text file T

Errors

On error, the function returns -1. Extended error information can be retrieved using fpGetErrno.

SYS_EBADF

An invalid descriptor was specified in one of the sets.

SYS_EINTR

A non blocked signal was caught.

SYS_EINVAL

N is negative or too big.

SYS_ENOMEM

Select was unable to allocate memory for its internal tables.

See also

• fpFD_ZERO
• fpFD_Clr
• fpFD_Set
• fpFD_IsSet

Example: ex33

Program Example33;

{ Program to demonstrate the Select function. }

Uses BaseUnix;

Var FDS : Tfdset;

begin
  fpfd_zero(FDS);
  fpfd_set(0,FDS);
  Writeln ('Press the <ENTER> to continue the program.');
  { Wait until File descriptor 0 (=Input) changes }
  fpSelect (1,@FDS,nil,nil,nil);
  { Get rid of <ENTER> in buffer }
  readln;
  Writeln ('Press <ENTER> key in less than 2 seconds...');
  Fpfd_zero(FDS);
  FpFd_set (0,FDS);
  if fpSelect (1,@FDS,nil,nil,2000)>0 then
    Writeln ('Thank you !')
    { FD_ISSET(0,FDS) would be true here. }
  else
    Writeln ('Too late !');
end.