select(2)

NAME

select(), pselect(), FD_CLR(), FD_ISSET(), FD_SET(), FD_ZERO() — synchronous I/O multiplexing

SYNOPSIS

#include <sys/select.h>

int pselect(int nfds, fd_set *__restrict readfds,

fd_set *__restrict writefds, fd_set *__restrict errorfds,
const struct timespec *__restrict timeout,
const sigset_t *__restrict sigmask);

int select(int nfds, fd_set *__restrict readfds,

fd_set *__restrict writefds, fd_set *__restrict errorfds
struct timeval *__restrict timeout);

void FD_CLR(int fd, fd_set *fdset);

int FD_ISSET(int fd, fd_set *fdset);

void FD_SET(int fd, fd_set *fdset);

void FD_ZERO(fd_set *fdset);

For Backward Compatibility Only: (_XOPEN_SOURCE_EXTENDED not defined)

#include <time.h>

int select(size_t nfds, int *readfds, int *writefds,

int *exceptds, const struct timeval *timeout); 

DESCRIPTION

The pselect() and select() functions indicate which of the specified file descriptors is ready for reading, ready for writing, or has an error condition pending. If the specified condition is false for all of the specified file descriptors, pselect() and select() block, up to the specified timeout interval, until the specified condition is true for at least one of the specified file descriptors.

The pselect() and select() functions support regular files, terminal and pseudo-terminal devices, STREAMS-based files, FIFOs and pipes. The behaviour of pselect() and select() on file descriptors that refer to other file types is unspecified.

The nfds argument specifies the range of file descriptors to be tested. The pselect() and select() functions test file descriptors in the range of 0 to nfds -1. File descriptor f is represented by the bit 1<<f in the masks. More formally, a file descriptor is represented by:

fds[(f / BITS_PER_INT)] & (1 << (f % BITS_PER_INT)) 

If the readfds argument is not a null pointer, it points to an object of type fd_set that on input specifies the file descriptors to be checked for being ready to read, and on output indicates which file descriptors are ready to read.

If the writefds argument is not a null pointer, it points to an object of type fd_set that on input specifies the file descriptors to be checked for being ready to write, and on output indicates which file descriptors are ready to write.

If the errorfds argument is not a null pointer, it points to an object of type fd_set that on input specifies the file descriptors to be checked for error conditions pending, and on output indicates which file descriptors have error conditions pending.

On successful completion, the objects pointed to by the readfds, writefds, and errorfds arguments are modified to indicate which file descriptors are ready for reading, ready for writing, or have an error condition pending, respectively. For each file descriptor less than nfds, the corresponding bit will be set on successful completion if it was set on input and the associated condition is true for that file descriptor.

If the timeout argument is not a null pointer, it specifies a maximum interval to wait for the selection to complete. The timeout argument points to an object of type struct timespec for pselect(), and to an object of type struct timeval for select(). If the members of these structures are 0, pselect() or select() will not block. If the timeout argument is a null pointer, pselect() or select() will block until an event causes one of the masks to be returned with a valid (non-zero) value. If the time limit expires before any event occurs that would cause one of the masks to be set to a non-zero value, pselect() or select() completes successfully and returns 0.

Implementations may place limitations on the maximum timeout interval supported. On all implementations, the maximum timeout interval supported will be at least 31 days. If the timeout argument specifies a timeout interval greater than the implementation-dependent maximum value, the maximum value will be used as the actual timeout value. Implementations may also place limitations on the granularity of timeout intervals. If the requested timeout interval requires a finer granularity than the implementation supports, the actual timeout interval will be rounded up to the next supported value.

If sigmask is not a null pointer, then the pselect() function shall replace the signal mask of the process by the set of signals pointed to by sigmask before examining the descriptors, and shall restore the signal mask of the process before returning.

If the readfds, writefds, and errorfds arguments are all null pointers and the timeout argument is not a null pointer, pselect() or select() blocks for the time specified, or until interrupted by a signal. If the readfds, writefds, and errorfds arguments are all null pointers and the timeout argument is a null pointer, pselect() or select() blocks until interrupted by a signal.

File descriptors associated with regular files always select true for ready to read, ready to write, and error conditions.

On failure, the objects pointed to by the readfds, writefds, and errorfds arguments are not modified. If the timeout interval expires without the specified condition being true for any of the specified file descriptors, the objects pointed to by the readfds, writefds, and errorfds arguments have all bits set to 0.

Ttys and sockets are ready for reading if a read() would not block for one or more of the following reasons:

input data is available.
an error condition exists, such as a broken pipe, no carrier, or a lost connection.

Similarly, ttys and sockets are ready for writing if a write() would not block for one or more of the following reasons:

output data can be accepted.
an error condition exists, such as a broken pipe, no carrier, or a lost connection.

TCP sockets select true on reads only for normal data. They do not select true on reads if out-of-band data ("urgent" data) arrives.

TCP sockets select true on exceptions for out-of-band data.

AF_CCITT sockets select true on reads for normal and out-of-band data and information, including supervisory frames.

Pipes are ready for reading if there is any data in the pipe, or if there are no writers left for the pipe. Pipes are ready for writing if there is room for more data in the pipe AND there are one or more readers for the pipe, OR there are no readers left for the pipe. select() returns the same results for a pipe whether a file descriptor associated with the read-only end or the write-only end of the pipe is used, since both file descriptors refer to the same underlying pipe. So a select() of a read-only file descriptor that is associated with a pipe can return ready to write, even though that particular file descriptor cannot be written to.

File descriptor masks of type fd_set can be initialized and tested with FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO(). It is unspecified whether each of these is a macro or a function. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with any of these names, the behaviour is undefined.

FD_CLR(fd, &fdset): Clears the bit for the file descriptor fd in the file descriptor set fdset.
FD_ISSET(fd, &fdset): Returns a non-zero value if the bit for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.
FD_SET(fd, &fdset): Sets the bit for the file descriptor fd in the file descriptor set fdset.
FD_ZERO(&fdset): Initializes the file descriptor set fdset to have zero bits for all file descriptors. The behaviour of these macros is undefined if the fd argument is less than 0 or greater than or equal to FD_SETSIZE.

The use of a timeout does not affect any pending timers set up by alarm(), ualarm(), or setitimer().

On successful completion, the object pointed to by the timeout argument of select() may be modified.

The FD_SETSIZE is used in the definition of fd_set structure. It is set to a value of 2048 to accommodate 2048 file descriptors. Any user code that uses FD_SETSIZE or the structure fd_set should redefine FD_SETSIZE to a smaller value (greater than or equal to the number of open files the process will have) in order to save space and execution time. Similarly, any user code that wants to test more than 2048 file descriptors should redefine FD_SETSIZE to the required higher value.

The user can also allocate the space for fd_set structure dynamically, depending upon the number of file descriptors to be tested. The following code segment illustrates the basic concepts.

     int num_of_fds,s; 
     struct fd_set *f; 

     /* 
      * Set num_of_fds to the required value. 
      * User can set it to the maximum possible value the kernel is 
      * configured for, by using sysconf(_SC_OPEN_MAX). 
      * Note that, if you are not using this many files, you are 
      * wasting too much space. 
      */ 

     num_of_fds = sysconf(_SC_OPEN_MAX); 
     s = sizeof(long); 

     /* 
      * howmany is a macro defined in sys/types.h 
      */ 

     f = (struct fd_set *)malloc(s*howmany(num_of_fds, s*8); 

     /* 
      * Use f wherever struct fd_set * is used. 
      * It can be used to test num_of_fds file descriptors. 
      */ 

RETURN VALUE

FD_CLR(), FD_SET(), and FD_ZERO() return no value. FD_ISSET() returns a non-zero value if the bit for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.

On successful completion, pselect() and select() return the total number of bits set in the bit masks. Otherwise, -1 is returned, and errno is set to indicate the error.

ERRORS

Under the following conditions, pselect() and select() fail and set errno to:

EBADF: One or more of the file descriptor sets specified a file descriptor that is not a valid open file descriptor. This could happen either if the file descriptor sets are not initialized or nfds argument is greater than FD_SETSIZE.
EINTR: The function was interrupted before any of the selected events occurred and before the timeout interval expired. If SA_RESTART has been set for the interrupting signal, it is implementation-dependent whether the function restarts or returns with EINTR.
EFAULT: One or more of the pointers was invalid. The reliable detection of this error is implementation dependent.
EINVAL: An invalid timeout interval was specified. Valid values for the number of nanoseconds (in struct timespec) should be a non-negative integer less than 1,000,000,000. For the number of microseconds (in struct timeval), a non-negative integer less than 1,000,000 should be used. Seconds should be specified using a non-negative integer.
EINVAL: The nfds argument is less than 0, or is greater than or equal to the value of MAXFUPLIM, which specifies the absolute maximum number of files a process can have open at one time. If the resource limit RLIMIT_NOFILE for a process is less than or equal to 2048, MAXFUPLIM is considered to be 2048.
EINVAL: One of the specified file descriptors refers to a STREAM or multiplexer that is linked (directly or indirectly) downstream from a multiplexer.

EXAMPLES

The following call to select() checks if any of 4 terminals are ready for reading. select() times out after 5 seconds if no terminals are ready for reading. Note that the code for opening the terminals or reading from the terminals is not shown in this example. Also, note that this example must be modified if the calling process has more than 32 file descriptors open. Following this first example is an example of select with more than 32 file descriptors.

#define MASK(f) (1 << (f)) 
#define NTTYS 4 

     int tty[NTTYS]; 
     int ttymask[NTTYS]; 
     int readmask = 0; 
     int readfds; 
     int nfound, i; 
     struct timeval timeout; 

        /* First open each terminal for reading and put the 
         * file descriptors into array tty[NTTYS].  The code 
         * for opening the terminals is not shown here. 
         */ 

     for (i=0; i < NTTYS; i++) { 
        ttymask[i] = MASK(tty[i]); 
        readmask |= ttymask[i]; 
     } 

     timeout.tv_sec  = 5; 
     timeout.tv_usec = 0; 
     readfds = readmask; 

     /* select on NTTYS+3 file descriptors if stdin, stdout 
      * and stderr are also open 
      */ 
     if ((nfound = select (NTTYS+3, &readfds, 0, 0, &timeout)) == -1) 
        perror ("select failed"); 
     else if (nfound == 0) 
        printf ("select timed out \n"); 
     else for (i=0; i < NTTYS; i++) 
        if (ttymask[i] & readfds) 
           /* Read from tty[i].  The code for reading 
            * is not shown here. 
            */ 
     else printf ("tty[%d] is not ready for reading \n",i); 

The following example is the same as the previous example, except that it works for more than 32 open files. Definitions for howmany, fd_set, and NFDBITS are in <sys/types.h>.

#include <sys/param.h> 
#include <sys/types.h> 
#include <sys/time.h> 

#define MASK(f) (1 << (f)) 
#define NTTYS NOFILE - 3 
#define NWORDS  howmany(FD_SETSIZE, NFDBITS) 

int tty[NTTYS]; 
int ttymask[NTTYS]; 
struct fd_set readmask, readfds; 
int nfound, i, j, k; 
struct timeval timeout; 

   /* First open each terminal for reading and put the 
    * file descriptors into array tty[NTTYS].  The code 
    * for opening the terminals is not shown here. 
    */ 

   for (k=0; k < NWORDS; k++) 
      readmask.fds_bits[k] = 0; 

   for (i=0, k=0; i < NTTYS && k < NWORDS; k++) 
      for (j=0; j < NFDBITS && i < NTTYS; j++, i++) { 
         ttymask[i] = MASK(tty[i]); 
         readmask.fds_bits[k] |= ttymask[i]; 
      } 

   timeout.tv_sec  = 5; 
   timeout.tv_usec = 0; 
   for (k=0; k < NWORDS; k++) 
      readfds.fds_bits[k] = readmask.fds_bits[k]; 

   /* select on NTTYS+3 file descriptors if stdin, stdout 
    * and stderr are also open 
    */ 
   if ((nfound = select (NTTYS+3, &readfds, 0, 0, &timeout)) == -1) 
      perror ("select failed"); 
   else if (nfound == 0) 
      printf ("select timed out \n"); 
   else for (i=0, k=0; i < NTTYS && k < NWORDS; k++) 
      for (j=0; j < NFDBITS && i < NTTYS; j++, i++) 
         if (ttymask[i] & readfds.fds_bits[k]) 
            /* Read from tty[i].  The code for reading 
             * is not shown here. 
             */ 
         else printf ("tty[%d] is not ready for reading \n",i); 

WARNINGS

The file descriptor masks are always modified on return, even if the call returns as the result of a timeout.

DEPENDENCIES

select() supports the following devices and file types:

pipes
fifo special files (named pipes)
all serial devices
All ITEs (internal terminal emulators) and HP-HIL input devices
lan(7) special files
pty(7) special files
sockets

AUTHOR

select() was developed by HP and the University of California, Berkeley.