Multithreaded Programming Guide

MT Interface Safety Levels

The man pages for functions and interfaces indicate how well the function or interface supports threads. The ATTRIBUTES section of each man page lists the MT-Level attribute, which is set to one of the safety level categories listed in Table 7–1. These categories are explained more fully in the attributes(5) man page.

If a man page does not state explicitly that a function is MT-Safe, you must assume that the function is unsafe.

Table 7–1 Interface Safety Levels


Category	Description
Safe	This code can be called from a multithreaded application
Safe with exceptions	See the NOTES sections of the man page for a description of the exceptions.
Unsafe	This interface is not safe to use with multithreaded applications unless the application arranges for only one thread at a time to execute within the library.
MT-Safe	This interface is fully prepared for multithreaded access. The interface is both safe and supports some concurrency.
MT-Safe with exceptions	See the NOTES sections of the man page for a description of the exceptions.
Async-Signal-Safe	This routine can safely be called from a signal handler. A thread that is executing an Async-Signal-Safe routine does not deadlock with itself when interrupted by a signal. See Async-Signal-Safe Functions in Solaris Threads
Fork1–Safe	This interface releases locks it has held whenever Solaris `fork1`(2) or POSIX `fork`(2) is called.

Some functions have purposely not been made safe for the following reasons.

The interface made MT-Safe would have negatively affected the performance of single-threaded applications.
The library has an unsafe interface. For example, a function might return a pointer to a buffer in the stack. You can use re-entrant counterparts for some of these functions. The re-entrant function name is the original function name with “_r” appended.

Note –

The only way to be certain that a function with a name not ending in “_r” is MT-Safe is to check the function's manual page. Use of a function identified as not MT-Safe must be protected by a synchronizing device or by restriction to the initial thread.

Reentrant Functions for Unsafe Interfaces

For most functions with unsafe interfaces, an MT-Safe version of the routine exists. The name of the MT-Safe routine is the name of the Unsafe routine with “_r” appended. For example, the MT-Safe version of asctime() is asctime_r(). The Table 7–2 “_r” routines are supplied in the Solaris environment.

Table 7–2 Reentrant Functions


`asctime_r`(3c)	`gethostbyname_r`(3nsl)	`getservbyname_r`(3socket)
`ctermid_r`(3c)	`gethostent_r`(3nsl)	`getservbyport_r`(3socket)
`ctime_r`(3c)	`getlogin_r`(3c)	`getservent_r`(3socket)
`fgetgrent_r`(3c)	`getnetbyaddr_r`(3socket)	`getspent_r`(3c)
`fgetpwent_r`(3c)	`getnetbyname_r`(3socket)	`getspnam_r`(3c)
`fgetspent_r`(3c)	`getnetent_r`(3socket)	`gmtime_r`(3c)
`gamma_r(3m)`	`getnetgrent_r`(3c)	`lgamma_r`(3m)
`getauclassent_r`(3bsm)	`getprotobyname_r`(3socket)	`localtime_r`(3c)
`getauclassnam_r`(3bsm)	`getprotobynumber_r`(3socket)	`nis_sperror_r`(3nsl)
`getauevent_r`(3bsm)	`getprotoent_r`(3socket)	`rand_r`(3c)
`getauevnam_r`(3bsm)	`getpwent_r`(3c)	`readdir_r`(3c)
`getauevnum_r`(3bsm)	`getpwnam_r`(3c)	`strtok_r`(3c)
`getgrent_r`(3c)	`getpwuid_r`(3c)	`tmpnam_r`(3c)
`getgrgid_r`(3c)	`getrpcbyname_r`(3nsl)	`ttyname_r`(3c)
`getgrnam_r`(3c)	`getrpcbynumber_r`(3nsl)
`gethostbyaddr_r`(3nsl)	`getrpcent_r`(3nsl)