Starting with the Solaris 10 release, a call to fork() is identical to a call to fork1(). Specifically, only the calling thread is replicated in the child process. The behavior is the same as the POSIX fork().
In previous releases of the Solaris software, the behavior of fork() was dependent on whether the application was linked with the POSIX threads library. When linked with -lthread (Solaris threads) but not linked with -lpthread (POSIX threads), fork() was the same as forkall(). When linked with -lpthread, regardless of whether fork() was also linked with -lthread , fork() was the same as fork1().
Starting with the Solaris 10 release, neither -lthread nor -lpthread is required for multithreaded applications. The -mt option is used to indicate that you are compiling a multithreaded application. The standard C library provides all threading support for both sets of application program interfaces. Applications that require replicate all fork semantics must call forkall().