What is the difference between -pthread and -lpthread while compiling

When developing multithreaded programs in a Linux environment, -pthread and -lpthread are common compilation options related to linking with the POSIX threads library (pthread library). However, there are some differences between them:

-pthread Option

Using the -pthread option is the recommended approach to compile and link programs that utilize pthreads. This option not only instructs the compiler and linker to link the program with the pthread library but may also set compiler flags to optimize the generation of multithreaded code.

• Compilation-Time Settings: When used with the compiler, -pthread can enable compiler optimizations and macro definitions for thread safety. For example, it may activate the _REENTRANT macro, which helps ensure the use of thread-safe library versions.
• Linking-Time Settings: During linking, -pthread instructs the linker to add the pthread library, similar to the -lpthread option, but may also include additional system libraries or frameworks to support multithreaded programming.

-lpthread Option

This option solely instructs the linker to link to the pthread library. It does not affect the compiler's behavior or set any compiler-level optimizations or macro definitions.

• Linking-Time Usage: When using -lpthread, it simply directs the linker to include the pthread library during linking. It does not influence the compiler's behavior or introduce any compiler options for multithreaded optimizations.

Practical Example

Suppose you are developing a multithreaded program that employs synchronization mechanisms between threads, such as mutexes. In this scenario, using the -pthread option is preferable over using -lpthread alone, as -pthread not only links to the pthread library but may also enable compiler-level thread-safe optimizations.

bash
gcc -pthread my_thread_program.c -o my_thread_program

In contrast, if you use -lpthread alone:

bash
gcc my_thread_program.c -lpthread -o my_thread_program

While this approach may successfully compile the program, it might not include compiler optimizations for multithreading, potentially resulting in suboptimal performance or reduced security compared to the version using -pthread.

Summary

In practical development, it is recommended to use the -pthread option to ensure your multithreaded program fully leverages all compiler optimizations and correct thread library linking, especially in critical scenarios where performance and thread safety are paramount.