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Apache 2.0 线程安全问题

When using any of the threaded mpms in Apache 2.0 it is important that every function called from Apache be thread safe. When linking in 3rd party extensions it can be difficult to determine whether the resulting server will be thread safe. Casual testing generally won't tell you this either as thread safety problems can lead to subtle race conditons that may only show up in certain conditions under heavy load.


APR, have a look at the apr_atomic_* functions and the apr_thread_mutex_* functions.



This is a common global variable that holds the error number of the last error that occurred. If one thread calls a low-level function that sets errno and then another thread checks it, we are bleeding error numbers from one thread into another. To solve this, make sure your module or library defines _REENTRANT or is compiled with -D_REENTRANT. This will make errno a per-thread variable and should hopefully be transparent to the code. It does this by doing something like this:

#define errno (*(__errno_location()))

which means that Accessing errno will call __errno_location() which is provided by the libc. Setting _REENTRANT also forces redefinition of some other functions to their *_r equivalents and sometimes changes the common getc/putc macros into safer function calls. Check your libc documentation for specifics. Instead of, or in addition to _REENTRANT the symbols that may affect this are _POSIX_C_SOURCE, _THREAD_SAFE, _SVID_SOURCE, and _BSD_SOURCE.


APR you can use apr_strtok().

crypt() is another function that tends to not be reentrant, so if you run across calls to that function in a library, watch out. On some systems it is reentrant though, so it is not always a problem. If your system has crypt_r() chances are you should be using that, or if possible simply avoid the whole mess by using md5 instead.


PHP, for example, try this:

% ldd libphp4.so
libsablot.so.0 => /usr/local/lib/libsablot.so.0 (0x401f6000)
libexpat.so.0 => /usr/lib/libexpat.so.0 (0x402da000)
libsnmp.so.0 => /usr/lib/libsnmp.so.0 (0x402f9000)
libpdf.so.1 => /usr/local/lib/libpdf.so.1 (0x40353000)
libz.so.1 => /usr/lib/libz.so.1 (0x403e2000)
libpng.so.2 => /usr/lib/libpng.so.2 (0x403f0000)
libmysqlclient.so.11 => /usr/lib/libmysqlclient.so.11 (0x40411000)
libming.so => /usr/lib/libming.so (0x40449000)
libm.so.6 => /lib/libm.so.6 (0x40487000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x404a8000)
libjpeg.so.62 => /usr/lib/libjpeg.so.62 (0x404e7000)
libcrypt.so.1 => /lib/libcrypt.so.1 (0x40505000)
libssl.so.2 => /lib/libssl.so.2 (0x40532000)
libcrypto.so.2 => /lib/libcrypto.so.2 (0x40560000)
libresolv.so.2 => /lib/libresolv.so.2 (0x40624000)
libdl.so.2 => /lib/libdl.so.2 (0x40634000)
libnsl.so.1 => /lib/libnsl.so.1 (0x40637000)
libc.so.6 => /lib/libc.so.6 (0x4064b000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)

In addition to these libraries you will need to have a look at any libraries linked statically into the module. You can use nm(1) to look for individual symbols in the module.


dev@Httpd.apache.org if you have additions or corrections to this list.

Library Version Thread Safe? Notes
ASpell/PSpell ?
Berkeley DB 3.x, 4.x Yes Be careful about sharing a connection across threads.
bzip2 Yes Both low-level and high-level APIs are thread-safe. However, high-level API requires thread-safe access to errno.
cdb ?
C-Client Perhaps c-client uses strtok()gethostbyname() which are not thread-safe on most C library implementations. c-client's static data is meant to be shared across threads. If strtok()gethostbyname() are thread-safe on your OS, c-client may be thread-safe.
cpdflib ?
libcrypt ?
Expat Yes Need a separate parser instance per thread
FreeTDS ?
FreeType ?
GD 1.8.x ?
GD 2.0.x ?
gdbm No Errors returned via a static gdbm_error variable
ImageMagick 5.2.2 Yes ImageMagick docs claim it is thread safe since version 5.2.2 (see Change log).
Imlib2 ?
libjpeg v6b ?
libmysqlclient Yes Use mysqlclient_r library variant to ensure thread-safety. For more information, please read http://www.mysql.com/doc/en/Threaded_clients.HTML.
Ming 0.2a ?
Net-SNMP 5.0.x ?
OpenLDAP 2.1.x Yes Use ldap_r library variant to ensure thread-safety.
OpenSSL 0.9.6g Yes Requires proper usage of CRYPTO_num_locks, CRYPTO_set_locking_callback, CRYPTO_set_id_callback
liboci8 (oracle 8+) 8.x,9.x ?
pdflib 5.0.x Yes PDFLib docs claim it is thread safe; changes.txt indicates it has been partially thread-safe since V1.91: http://www.pdflib.com/products/pdflib/index.html.
libpng 1.0.x ?
libpng 1.2.x ?
libpq (PostgreSQL) 7.x Yes Don't share connections across threads and watch out for crypt() calls
Sablotron 0.95 ?
zlib 1.1.4 Yes Relies upon thread-safe zalloc and zfree functions Default is to use libc's calloc/free which are thread-safe.