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Annotation of researchv10dc/man/adm/man2/select.2, revision 1.1.1.1

1.1       root        1: .TH SELECT 2
                      2: .CT 2 file_io comm_proc
                      3: .SH NAME
                      4: select \(mi synchronous input/output multiplexing
                      5: .SH SYNOPSIS
                      6: .nf
                      7: .B #include <sys/types.h>
                      8: .PP
                      9: .B int select(nfds, readfds, writefds, milli);
                     10: .B fd_set *readfds, *writefds;
                     11: .fi
                     12: .SH DESCRIPTION
                     13: .I Select
                     14: examines a set of file descriptors
                     15: to see if they will block if read or written.
                     16: .I Readfds
                     17: points to an object of type
                     18: .BR fd_set ,
                     19: which contains a set of descriptors to be checked for reading;
                     20: .I writefds
                     21: similarly for writing.
                     22: Only descriptors
                     23: 0 through
                     24: .IR nfds \-1
                     25: are considered.
                     26: The number of ready descriptors is returned,
                     27: and the
                     28: .B fd_set
                     29: pointed to by
                     30: .I readfds
                     31: .RI ( writefds )
                     32: is overwritten with a set of descriptors
                     33: ready to be read
                     34: (written).
                     35: The call waits until at least one descriptor is ready,
                     36: or until
                     37: .I milli
                     38: milliseconds have passed.
                     39: .PP
                     40: Either
                     41: .I readfds
                     42: or
                     43: .I writefds
                     44: may be 0
                     45: if no descriptors are interesting.
                     46: .PP
                     47: The
                     48: .BR fd_set
                     49: type looks like
                     50: .RS
                     51: .EX
                     52: typedef struct {
                     53:        unsigned int fds_bits[FDWORDS];
                     54: } fd_set;
                     55: .EE
                     56: .RE
                     57: .B FDWORDS
                     58: is sufficient to contain as many file descriptors as the system will allow
                     59: (currently 128).
                     60: If there are
                     61: .I B
                     62: bits in an
                     63: .BR "unsigned int" ,
                     64: file descriptor
                     65: .I n
                     66: is represented by
                     67: .BI "1<<((" n % B )-1)
                     68: in word
                     69: .BI fds_bits[ n / B ]\c
                     70: \&.
                     71: .PP
                     72: These macros should be used
                     73: to manipulate the contents
                     74: of an
                     75: .BR fd_set :
                     76: .TF FD_ISSET(n,\0s)
                     77: .TP
                     78: .PD 0
                     79: .B FD_ZERO(s)
                     80: clear all bits
                     81: in set
                     82: .I s
                     83: .TP
                     84: .B "FD_SET(n, s)
                     85: set bit for file descriptor
                     86: .I n
                     87: in set
                     88: .I s
                     89: .TP
                     90: .B "FD_CLR(n, s)
                     91: clear bit for file descriptor
                     92: .I n
                     93: in s
                     94: .TP
                     95: .B "FD_ISSET(n, s)
                     96: return a value of 1
                     97: if bit for file descriptor
                     98: .I n
                     99: is set in
                    100: .IR s ,
                    101: 0 otherwise
                    102: .PD
                    103: .SH EXAMPLES
                    104: .EX
                    105: int p[2];
                    106: fd_set wfs;
                    107: pipe(p);
                    108: do {
                    109:        FD_SET(p[1], wfs);
                    110:        write(p[1], ".", 1);
                    111:        i++;
                    112: } while(select(p[1]+1, (fd_set*)0, wfs, 0) == 1);
                    113: printf("Pipe capacity = %d\en", i);
                    114: .EE
                    115: .SH "SEE ALSO"
                    116: .IR read (2)
                    117: .SH DIAGNOSTICS
                    118: .BR EBADF ,
                    119: .BR EFAULT ,
                    120: .BR EINTR
                    121: .SH BUGS
                    122: .I Milli
                    123: is rounded up to the nearest second.
                    124: .br
                    125: .I Select
                    126: is intended for use with streams;
                    127: file descriptors referring to ordinary files
                    128: or to non-stream special files
                    129: always appear ready.
                    130: This is a lie for some special files.

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