Advanced Programming in the UNIX® Environment

ptg10805159

442 Thread Control Chapter 12

The value of theprocess-sharedattribute can be eitherPTHREAD_PROCESS_SHARED

(accessible to threads from multiple processes) or PTHREAD_PROCESS_PRIVATE

(accessible to only threads in the process that initialized the barrier).

12.5 Reentrancy

We discussed reentrant functions and signal handlers in Section 10.6. Threads are

similar to signal handlers when it comes to reentrancy.Inboth cases, multiple threads

of control can potentially call the same function at the same time.

basename getchar_unlocked getservent putc_unlocked

catgets getdate getutxent putchar_unlocked

crypt getenv getutxid putenv

dbm_clearerr getgrent getutxline pututxline

dbm_close getgrgid gmtime rand

dbm_delete getgrnam hcreate readdir

dbm_error gethostent hdestroy setenv

dbm_fetch getlogin hsearch setgrent

dbm_firstkey getnetbyaddr inet_ntoa setkey

dbm_nextkey getnetbyname l64a setpwent

dbm_open getnetent lgamma setutxent

dbm_store getopt lgammaf strerror

dirname getprotobyname lgammal strsignal

dlerror getprotobynumber localeconv strtok

drand48 getprotoent localtime system

encrypt getpwent lrand48 ttyname

endgrent getpwnam mrand48 unsetenv

endpwent getpwuid nftw wcstombs

endutxent getservbyname nl_langinfo wctomb

getc_unlocked getservbyport ptsname

Figure 12.9 Functionsnotguaranteed to be thread-safe by POSIX.1

If a function can be safely called by multiple threads at the same time, we say that

the function isthread-safe.All functions defined in the Single UNIX Specification are

guaranteed to be thread-safe, except those listed in Figure12.9. In addition, the

ctermidandtmpnamfunctions arenot guaranteed to be thread-safe if they arepassed

anull pointer.Similarly,there is no guarantee thatwcrtombandwcsrtombsare

thread-safe when they arepassed a null pointer for theirmbstate_targument.

Implementations that support thread-safe functions will define the

_POSIX_THREAD_SAFE_FUNCTIONSsymbol in<unistd.h>.Applications can also

use the_SC_THREAD_SAFE_FUNCTIONSargument withsysconfto check for support

of thread-safe functions at runtime. Prior to Version 4 of the Single UNIX Specification,

all XSI-conforming implementations wererequired to support thread-safe functions.

With SUSv4, however,thread-safe function support is now required for an

implementation to be considered POSIX conforming.

With thread-safe functions, implementations provide alternative, thread-safe

versions of some of the POSIX.1 functions that aren’t thread-safe. Figure12.10 lists the