[Python-Dev] UUID module
Phillip J. Eby
pje at telecommunity.com
Sat Jun 10 19:32:37 CEST 2006
At 06:39 PM 6/10/2006 +0200, Thomas Heller wrote:
>I don't know if this is the uuidgen you're talking about, but on linux there
>is libuuid:
>
> >>> from ctypes import *
> >>> lib = CDLL("libuuid.so.1")
> >>> uuid = create_string_buffer(16)
> >>> lib.uuid_generate(byref(uuid))
>2131088494
> >>> from binascii import hexlify
> >>> hexlify(buffer(uuid))
>'0c77b6d7e5f940b18e29a749057f6ed4'
> >>>
Nice. :) But no, this one's a uuidgen() system call on FreeBSD>=5.0 and
NetBSD>=2.0; it may be in other BSD variants as well... perhaps OS X?
NAME
uuidgen -- generate universally unique identifiers
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/uuid.h>
int
uuidgen(struct uuid *store, int count);
DESCRIPTION
The uuidgen() system call generates count universally unique identifiers
(UUIDs) and writes them to the buffer pointed to by store. The identi-
fiers are generated according to the syntax and semantics of the DCE ver-
sion 1 variant of universally unique identifiers. See below for a more
in-depth description of the identifiers. When no IEEE 802 address is
available for the node field, a random multi-cast address is generated
for each invocation of the system call. According to the algorithm of
generating time-based UUIDs, this will also force a new random clock
sequence, thereby increasing the likelihood for the identifier to be
unique.
When multiple identifiers are to be generated, the uuidgen() system call
will generate a set of identifiers that is dense in such a way that there
is no identifier that is larger than the smallest identifier in the set
and smaller than the largest identifier in the set and that is not
already in the set.
Universally unique identifiers, also known as globally unique identifiers
(GUIDs), have a binary representation of 128-bits. The grouping and
meaning of these bits is described by the following structure and its
description of the fields that follow it:
struct uuid {
uint32_t time_low;
uint16_t time_mid;
uint16_t time_hi_and_version;
uint8_t clock_seq_hi_and_reserved;
uint8_t clock_seq_low;
uint8_t node[_UUID_NODE_LEN];
};
time_low The least significant 32 bits of a 60-bit
timestamp. This field is stored in the native
byte-order.
time_mid The least significant 16 bits of the most sig-
nificant 28 bits of the 60-bit timestamp.
This field is stored in the native byte-order.
time_hi_and_version The most significant 12 bits of the 60-bit
timestamp multiplexed with a 4-bit version
number. The version number is stored in the
most significant 4 bits of the 16-bit field.
This field is stored in the native byte-order.
clock_seq_hi_and_reserved The most significant 6 bits of a 14-bit
sequence number multiplexed with a 2-bit vari-
ant value. Note that the width of the variant
value is determined by the variant itself.
Identifiers generated by the uuidgen() system
call have variant value 10b. the variant
value is stored in the most significant bits
of the field.
clock_seq_low The least significant 8 bits of a 14-bit
sequence number.
node The 6-byte IEEE 802 (MAC) address of one of
the interfaces of the node. If no such inter-
face exists, a random multi-cast address is
used instead.
The binary representation is sensitive to byte ordering. Any multi-byte
field is to be stored in the local or native byte-order and identifiers
must be converted when transmitted to hosts that do not agree on the
byte-order. The specification does not however document what this means
in concrete terms and is otherwise beyond the scope of this system call.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The uuidgen() system call can fail with:
[EFAULT] The buffer pointed to by store could not be written to
for any or all identifiers.
[EINVAL] The count argument is less than 1 or larger than the
hard upper limit of 2048.
SEE ALSO
uuidgen(1), uuid(3)
STANDARDS
The identifiers are represented and generated in conformance with the DCE
1.1 RPC specification. The uuidgen() system call is itself not part of
the specification.
HISTORY
The uuidgen() system call first appeared in FreeBSD 5.0 and was subse-
quently added to NetBSD 2.0.
time_low The least significant 32 bits of a 60-bit
timestamp. This field is stored in the native
byte-order.
time_mid The least significant 16 bits of the most sig-
nificant 28 bits of the 60-bit timestamp.
This field is stored in the native byte-order.
time_hi_and_version The most significant 12 bits of the 60-bit
timestamp multiplexed with a 4-bit version
number. The version number is stored in the
most significant 4 bits of the 16-bit field.
This field is stored in the native byte-order.
clock_seq_hi_and_reserved The most significant 6 bits of a 14-bit
sequence number multiplexed with a 2-bit vari-
ant value. Note that the width of the variant
value is determined by the variant itself.
Identifiers generated by the uuidgen() system
call have variant value 10b. the variant
value is stored in the most significant bits
of the field.
clock_seq_low The least significant 8 bits of a 14-bit
sequence number.
node The 6-byte IEEE 802 (MAC) address of one of
the interfaces of the node. If no such inter-
face exists, a random multi-cast address is
used instead.
The binary representation is sensitive to byte ordering. Any multi-byte
field is to be stored in the local or native byte-order and identifiers
must be converted when transmitted to hosts that do not agree on the
byte-order. The specification does not however document what this means
in concrete terms and is otherwise beyond the scope of this system call.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The uuidgen() system call can fail with:
[EFAULT] The buffer pointed to by store could not be written to
for any or all identifiers.
[EINVAL] The count argument is less than 1 or larger than the
hard upper limit of 2048.
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