From: Gurusamy Sarathy Date: Mon, 11 Oct 1999 23:11:57 +0000 (+0000) Subject: add vec() example from Chaim Frenkel X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=e69129f12231d0e94f3df54c4dc907ba95153319;p=p5sagit%2Fp5-mst-13.2.git add vec() example from Chaim Frenkel p4raw-id: //depot/perl@4348 --- diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod index 2dde78d..9105108 100644 --- a/pod/perlfunc.pod +++ b/pod/perlfunc.pod @@ -5084,11 +5084,12 @@ See also C, C, and C. =item vec EXPR,OFFSET,BITS -Treats the string in EXPR as a vector of unsigned integers, and -returns the value of the bit field specified by OFFSET. BITS -specifies the number of bits that are reserved for each entry in the -bit vector. This must be a power of two from 1 to 32 (or 64, if your -platform supports that). +Treats the string in EXPR as a bit vector made up of elements of +width BITS, and returns the value of the element specified by OFFSET +as an unsigned integer. BITS therefore specifies the number of bits +that are reserved for each element in the bit vector. This must +be a power of two from 1 to 32 (or 64, if your platform supports +that). C may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as in @@ -5106,6 +5107,10 @@ in the same way on big-endian or little-endian machines. my $foo = ''; vec($foo, 0, 32) = 0x5065726C; # 'Perl' + + # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits + print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P') + vec($foo, 2, 16) = 0x5065; # 'PerlPe' vec($foo, 3, 16) = 0x726C; # 'PerlPerl' vec($foo, 8, 8) = 0x50; # 'PerlPerlP' @@ -5125,6 +5130,171 @@ To transform a bit vector into a string or list of 0's and 1's, use these: If you know the exact length in bits, it can be used in place of the C<*>. +Here is an example to illustrate how the bits actually fall in place: + + #!/usr/bin/perl -wl + + print <<'EOT'; + 0 1 2 3 + unpack("V",$_) 01234567890123456789012345678901 + ------------------------------------------------------------------ + EOT + + for $w (0..3) { + $width = 2**$w; + for ($shift=0; $shift < $width; ++$shift) { + for ($off=0; $off < 32/$width; ++$off) { + $str = pack("B*", "0"x32); + $bits = (1<<$shift); + vec($str, $off, $width) = $bits; + $res = unpack("b*",$str); + $val = unpack("V", $str); + write; + } + } + } + + format STDOUT = + vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> + $off, $width, $bits, $val, $res + . + __END__ + +Regardless of the machine architecture on which it is run, the above +example should print the following table: + + 0 1 2 3 + unpack("V",$_) 01234567890123456789012345678901 + ------------------------------------------------------------------ + vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000 + vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000 + vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000 + vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000 + vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000 + vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000 + vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000 + vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000 + vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000 + vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000 + vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000 + vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000 + vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000 + vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000 + vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000 + vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000 + vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000 + vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000 + vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000 + vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000 + vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000 + vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000 + vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000 + vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000 + vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000 + vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000 + vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000 + vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000 + vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000 + vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100 + vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010 + vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001 + vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000 + vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000 + vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000 + vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000 + vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000 + vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000 + vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000 + vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000 + vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000 + vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000 + vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000 + vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000 + vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000 + vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000 + vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000 + vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010 + vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000 + vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000 + vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000 + vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000 + vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000 + vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000 + vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000 + vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000 + vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000 + vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000 + vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000 + vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000 + vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000 + vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000 + vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100 + vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001 + vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000 + vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000 + vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000 + vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000 + vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000 + vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000 + vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000 + vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000 + vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000 + vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000 + vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000 + vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000 + vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000 + vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000 + vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000 + vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100 + vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000 + vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000 + vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000 + vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000 + vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000 + vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000 + vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000 + vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010 + vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000 + vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000 + vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000 + vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000 + vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000 + vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000 + vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000 + vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001 + vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000 + vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000 + vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000 + vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000 + vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000 + vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000 + vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000 + vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000 + vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000 + vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000 + vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000 + vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000 + vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000 + vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000 + vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000 + vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000 + vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000 + vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000 + vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000 + vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000 + vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000 + vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000 + vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000 + vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100 + vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000 + vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000 + vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000 + vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010 + vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000 + vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000 + vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000 + vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001 + =item wait Behaves like the wait(2) system call on your system: it waits for a child