12.4 Thread Safety

A hidden problem with threads is that they may call library functions that are not thread-safe, possibly producing spurious results. A function is thread-safe if multiple threads can execute simultaneous active invocations of the function without interference. POSIX specifies that all the required functions, including the functions from the standard C library, be implemented in a thread-safe manner except for the specific functions listed in Table 12.2. Those functions whose traditional interfaces preclude making them thread-safe must have an alternative thread-safe version designated with an _r suffix.

An important example of a function that does not have to be thread-safe is strerror . Although strerror is not guaranteed to be thread-safe, many systems have implemented this function in a thread-safe manner. Unfortunately, because strerror is listed in Table 12.2, you can not assume that it works correctly if multiple threads call it. We use strerror only in the main thread, often to produce error messages for pthread_create and pthread_join . Section 13.7 gives a thread-safe implementation called strerror_r .

Another interaction problem occurs when threads access the same data. The individual copier threads in Program 12.8 work on independent problems and do not interact with each other. In more complicated applications, a thread may not exit after completing its assigned task. Instead, a worker thread may request additional tasks or share information. Chapter 13 explains how to control this type of interaction by using synchronization primitives such as mutex locks and condition variables .

Table 12.2. POSIX functions that are not required to be thread-safe.

asctime	fcvt	getpwnam	nl_langinfo
basename	ftw	getpwuid	ptsname
catgets	gcvt	getservbyname	putc_unlocked
crypt	getc_unlocked	getservbyport	putchar_unlocked
ctime	getchar_unlocked	getservent	putenv
dbm_clearerr	getdate	getutxent	pututxline
dbm_close	getenv	getutxid	rand
dbm_delete	getgrent	getutxline	readdir
dbm_error	getgrgid	gmtime	setenv
dbm_fetch	getgrnam	hcreate	setgrent
dbm_firstkey	gethostbyaddr	hdestroy	setkey
dbm_nextkey	gethostbyname	hsearch	setpwent
dbm_open	gethostent	inet_ntoa	setutxent
dbm_store	getlogin	l64a	strerror
dirname	getnetbyaddr	lgamma	strtok
dlerror	getnetbyname	lgammaf	ttyname
drand48	getnetent	lgammal	unsetenv
ecvt	getopt	localeconv	wcstombs
encrypt	getprotobyname	localtime	wctomb
endgrent	getprotobynumber	lrand48
endpwent	getprotoent	mrand48
endutxent	getpwent	nftw

In traditional UNIX implementations , errno is a global external variable that is set when system functions produce an error. This implementation does not work for multithreading (see Section 2.7), and in most thread implementations errno is a macro that returns thread-specific information. In essence, each thread has a private copy of errno . The main thread does not have direct access to errno for a joined thread, so if needed, this information must be returned through the last parameter of pthread_join .