216 lines
10 KiB
Plaintext
216 lines
10 KiB
Plaintext
uClibc and Glibc are not the same -- there are a number of differences which
|
|
may or may not cause you problems. This document attempts to list these
|
|
differences and, when completed, will contain a full list of all relevant
|
|
differences.
|
|
|
|
|
|
1) uClibc is smaller than glibc. We attempt to maintain a glibc compatible
|
|
interface, allowing applications that compile with glibc to easily compile with
|
|
uClibc. However, we do not include _everything_ that glibc includes, and
|
|
therefore some applications may not compile. If this happens to you, please
|
|
report the failure to the uclibc mailing list, with detailed error messages.
|
|
|
|
2) uClibc is much more configurable then glibc. This means that a developer
|
|
may have compiled uClibc in such a way that significant amounts of
|
|
functionality have been omitted.
|
|
|
|
3) uClibc does not even attempt to ensure binary compatibility across releases.
|
|
When a new version of uClibc is released, you may or may not need to recompile
|
|
all your binaries.
|
|
|
|
4) malloc(0) in glibc returns a valid pointer to something(!?!?) while in
|
|
uClibc calling malloc(0) returns a NULL. The behavior of malloc(0) is listed
|
|
as implementation-defined by SuSv3, so both libraries are equally correct.
|
|
This difference also applies to realloc(NULL, 0). I personally feel glibc's
|
|
behavior is not particularly safe. To enable glibc behavior, one has to
|
|
explicitly enable the MALLOC_GLIBC_COMPAT option.
|
|
|
|
4.1) glibc's malloc() implementation has behavior that is tunable via the
|
|
MALLOC_CHECK_ environment variable. This is primarily used to provide extra
|
|
malloc debugging features. These extended malloc debugging features are not
|
|
available within uClibc. There are many good malloc debugging libraries
|
|
available for Linux (dmalloc, electric fence, valgrind, etc) that work much
|
|
better than the glibc extended malloc debugging. So our omitting this
|
|
functionality from uClibc is not a great loss.
|
|
|
|
5) uClibc does not provide a database library (libdb).
|
|
|
|
6) uClibc does not support NSS (/lib/libnss_*), which allows glibc to easily
|
|
support various methods of authentication and DNS resolution. uClibc only
|
|
supports flat password files and shadow password files for storing
|
|
authentication information. If you need something more complex than this,
|
|
you can compile and install pam.
|
|
|
|
7) uClibc's libresolv is only a stub. Some, but not all of the functionality
|
|
provided by glibc's libresolv is provided internal to uClibc. Other functions
|
|
are not at all implemented.
|
|
|
|
8) libnsl provides support for Network Information Service (NIS) which was
|
|
originally called "Yellow Pages" or "YP", which is an extension of RPC invented
|
|
by Sun to share Unix password files over the network. I personally think NIS
|
|
is an evil abomination and should not be used. These days, using ldap is much
|
|
more effective mechanism for doing the same thing. uClibc provides a stub
|
|
libnsl, but has no actual support for Network Information Service (NIS).
|
|
We therefore, also do not provide any of the headers files provided by glibc
|
|
under /usr/include/rpcsvc.
|
|
|
|
9) uClibc's locale support is not 100% complete yet. We are working on it.
|
|
|
|
10) uClibc's math library only supports long double as inlines, and even
|
|
then the long double support is quite limited. Also, very few of the
|
|
float math functions are implemented. Stick with double and you should
|
|
be just fine.
|
|
|
|
11) uClibc's libcrypt does not support the reentrant crypt_r, setkey_r and
|
|
encrypt_r, since these are not required by SuSv3.
|
|
|
|
12) uClibc directly uses kernel types to define most opaque data types.
|
|
|
|
13) uClibc directly uses the linux kernel's arch specific 'stuct stat'.
|
|
|
|
14) uClibc's librt library currently lacks all aio routines, all clock
|
|
routines, and all shm routines (only the timer routines and the mq
|
|
routines are implemented).
|
|
|
|
<other things as we notice them>
|
|
|
|
|
|
|
|
****************************** Manuel's Notes ******************************
|
|
|
|
Some general comments...
|
|
|
|
The intended target for all my uClibc code is ANSI/ISO C99 and SUSv3
|
|
compliance. While some glibc extensions are present, many will eventually
|
|
be configurable. Also, even when present, the glibc-like extensions may
|
|
differ slightly or be more restrictive than the native glibc counterparts.
|
|
They are primarily meant to be porting _aides_ and not necessarily
|
|
drop-in replacements.
|
|
|
|
Now for some details...
|
|
|
|
time functions
|
|
--------------
|
|
1) Leap seconds are not supported.
|
|
2) /etc/timezone and the whole zoneinfo directory tree are not supported.
|
|
To set the timezone, set the TZ environment variable as specified in
|
|
http://www.opengroup.org/onlinepubs/007904975/basedefs/xbd_chap08.html
|
|
or you may also create an /etc/TZ file of a single line, ending with a
|
|
newline, containing the TZ setting. For example
|
|
echo CST6CDT > /etc/TZ
|
|
3) Currently, locale specific eras and alternate digits are not supported.
|
|
They are on my TODO list.
|
|
|
|
wide char support
|
|
-----------------
|
|
1) The only multibyte encoding currently supported is UTF-8. The various
|
|
ISO-8859-* encodings are (optionally) supported. The internal
|
|
representation of wchar's is assumed to be 31 bit unicode values in
|
|
native endian representation. Also, the underlying char encoding is
|
|
assumed to match ASCII in the range 0-0x7f.
|
|
2) In the next iteration of locale support, I plan to add support for
|
|
(at least some) other multibyte encodings.
|
|
|
|
locale support
|
|
--------------
|
|
1) The target for support is SUSv3 locale functionality. While nl_langinfo
|
|
has been extended, similar to glibc, it only returns values for related
|
|
locale entries.
|
|
2) Currently, all SUSv3 libc locale functionality should be implemented
|
|
except for wcsftime and collating item support in regex.
|
|
|
|
stdio
|
|
-----
|
|
1) Conversion of large magnitude floating-point values by printf suffers a loss
|
|
of precision due to the algorithm used.
|
|
2) uClibc's printf is much stricter than glibcs, especially regarding positional
|
|
args. The entire format string is parsed first and an error is returned if
|
|
a problem is detected. In locales other than C, the format string is checked
|
|
to be a valid multibyte sequence as well. Also, currently at most 10 positional
|
|
args are allowed (although this is configurable).
|
|
3) BUFSIZ is configurable, but no attempt is made at automatic tuning of internal
|
|
buffer sizes for stdio streams. In fact, the stdio code in general sacrifices
|
|
sophistication/performace for minimal size.
|
|
4) uClibc allows glibc-like custom printf functions. However, while not
|
|
currently checked, the specifier must be <= 0x7f.
|
|
5) uClibc allows glibc-like custom streams. However, no in-buffer seeking is
|
|
done.
|
|
6) The functions fcloseall() and __fpending() can behave differently than their
|
|
glibc counterparts.
|
|
7) uClibc's setvbuf is more restrictive about when it can be called than glibc's
|
|
is. The standards specify that setvbuf must occur before any other operations
|
|
take place on the stream.
|
|
8) Right now, %m is not handled properly by printf when the format uses positional
|
|
args.
|
|
9) The FILEs created by glibc's fmemopen(), open_memstream(), and fopencookie()
|
|
are not capable of wide orientation. The corresponding uClibc routines do
|
|
not have this limitation.
|
|
10) For scanf, the C99 standard states "The fscanf function returns the value of
|
|
the macro EOF if an input failure occurs before any conversion." But glibc's
|
|
scanf does not respect conversions for which assignment was surpressed, even
|
|
though the standard states that the value is converted but not stored.
|
|
|
|
glibc bugs that Ulrich Drepper has refused to acknowledge or comment on
|
|
( http://sources.redhat.com/ml/libc-alpha/2003-09/ )
|
|
-----------------------------------------------------------------------
|
|
1) The C99 standard says that for printf, a %s conversion makes no special
|
|
provisions for multibyte characters. SUSv3 is even more clear, stating
|
|
that bytes are written and a specified precision is in bytes. Yet glibc
|
|
treats the arg as a multibyte string when a precision is specified and
|
|
not otherwise.
|
|
2) Both C99 and C89 state that the %c conversion for scanf reads the exact
|
|
number of bytes specified by the optional field width (or 1 if not specified).
|
|
uClibc complies with the standard. There is an argument that perhaps the
|
|
specified width should be treated as an upper bound, based on some historical
|
|
use. However, such behavior should be mentioned in the Conformance document.
|
|
3) glibc's scanf is broken regarding some numeric patterns. Some invalid
|
|
strings are accepted as valid ("0x.p", "1e", digit grouped strings).
|
|
In spite of my posting examples clearly illustrating the bugs, they remain
|
|
unacknowledged by the glibc developers.
|
|
4) glibc's scanf seems to require a 'p' exponent for hexadecimal float strings.
|
|
According to the standard, this is optional.
|
|
5) C99 requires that once an EOF is encountered, the stream should be treated
|
|
as if at end-of-file even if more data becomes available. Further reading
|
|
can be attempted by clearing the EOF flag though, via clearerr() or a file
|
|
positioning function. For details concerning the original change, see
|
|
Defect Report #141. glibc is currently non-compliant, and the developers
|
|
did not comment when I asked for their official position on this issue.
|
|
6) glibc's collation routines and/or localedef are broken regarding implicit
|
|
and explicit UNDEFINED rules.
|
|
|
|
More to follow as I think of it...
|
|
|
|
|
|
|
|
|
|
Profiling:
|
|
-------------------------------------------------------------------
|
|
|
|
uClibc no longer supports 'gcc -fprofile-arcs -pg' style profiling, which
|
|
causes your application to generate a 'gmon.out' file that can then be analyzed
|
|
by 'gprof'. Not only does this require explicit extra support in uClibc, it
|
|
requires that you rebuild everything with profiling support. There is both a
|
|
size and performance penalty to profiling your applications this way, as well
|
|
as Heisenberg effects, where the act of measuring changes what is measured.
|
|
|
|
There exist a number of less invasive alternatives that do not require you to
|
|
specially instrument your application, and recompile and relink everything.
|
|
|
|
The OProfile system-wide profiler is an excellent alternative:
|
|
http://oprofile.sourceforge.net/
|
|
|
|
Many people have had good results using the combination of Valgrind
|
|
to generate profiling information and KCachegrind for analysis:
|
|
http://developer.kde.org/~sewardj/
|
|
http://kcachegrind.sourceforge.net/
|
|
|
|
Prospect is another alternative based on OProfile:
|
|
http://prospect.sourceforge.net/
|
|
|
|
And the Linux Trace Toolkit (LTT) is also a fine tool:
|
|
http://www.opersys.com/LTT/
|
|
|
|
FunctionCheck:
|
|
http://www710.univ-lyon1.fr/~yperret/fnccheck/
|
|
|