Specifications

Red Hat Enterprise Linux to Oracle Solaris Porting Guide

Chapter 5 Threads and Multiprocessing

POSIX Compliance

The standard POSIX.1c threads extensions (IEEE Std 1003.1c-1995) defines APIs for creating and

manipulating threads. POSIX threads, usually referred to as Pthreads, is a POSIX standard for threads.

POSIX.1 specifies a set of interfaces (functions and header files) for threaded programming. Oracle

Solaris Studio as well as GNU compilers support the POSIX threads programming model.

Linux Threading Model

On RHEL, two threading implementations have been provided over time by the GNU C library

(glibc). Both of these are so-called 1:1 implementations, meaning that each thread maps to a kernel

scheduling entity:

• LinuxThreads. This is the original Pthreads implementation. (Since glibc 2.4, this implementation

is no longer supported.)

• NPTL (Native POSIX Threads Library). This is the modern Pthreads implementation.

By comparison with LinuxThreads, NPTL provides “closer” conformance to the requirements of the

POSIX.1 specification and better performance when large numbers of threads are created. NPTL has

been available since glibc 2.3.2 and requires features that are present in the Linux 2.6 kernel.

Since glibc 2.3.2, the getconf command can be used to determine a RHEL system’s threading

implementation, for example:

bash$ getconf GNU_LIBPTHREAD_VERSION

NPTL 2.12

With older glibc versions, a command such as the following should be sufficient to determine the

default threading implementation:

bash$ $( ldd /bin/ls | grep libc.so | awk '{print $3}' ) | egrep -i 'threads|nptl'

where

nptl refers to the native POSIX threads library by Ulrich Drepper et al.

Selecting the Thread Implementation

On systems with a glibc version that supports both LinuxThreads and NPTL (for example, glibc

2.3.x), the

LD_ASSUME_KERNEL environment variable can be used to override the dynamic linker’s

default choice of threading implementation. This variable tells the dynamic linker to assume that it is

running on top of a particular kernel version. By specifying a kernel version that does not provide the

support required by NPTL, you can force the use of LinuxThreads. The most likely reason for doing

this is to run a (broken) application that depends on some nonconformant behavior in LinuxThreads,

for example: