When g statically links pthread, causing Segmentation fault, why?

In static linking, the linker will stop at the first symbol, even if it is a weak one, and stops looking for strong ones. To force it to look at all symbols (like it would have done for a dynamically linked library), ld supports the --whole-archive option.

The following command will work:

g   -o one one.cpp -Wall -std=c  11 -O3 -static -pthread \
    -Wl,--whole-archive -lpthread -Wl,--no-whole-archive

Here's what's happening:

• -pthread implies linking against pthread (and depending on the platform, it does define extra macros like -D_REENTRANT).
• Even if -pthread implies linking against -lpthread, you still need to specify -lpthread explicitly while statically linking.
• Wl,--whole-archive forces the linker to include every object file in the archive in the link, rather than searching the archive for the required object files.
• Wl,--no-whole-archive turns off the effect of the --whole-archive option for subsequent archive files.

Understanding weak symbols

ELF file format has the concept of weak and strong symbols. By default, symbols in an object file are strong. During linking, a strong symbol can override a weak symbol of the same name.

In the case of glibc and pthreads, they use weak symbols. For example, fputc is required by POSIX to be thread-safe and needs to be synchronized, which is costly. In a single-threaded environment, you do not want to pay these costs. An implementation could therefore implement the synchronization functions as empty stubs, and declare the functions as weak symbols.

Later, if a multi-threading library is linked (e.g., pthread), it becomes obvious that single-thread support is not intended. When linking the multi-threading library, the linker can then replace the stubs by the real synchronization functions (defined as strong symbols and implemented by the threading-library).

Applying this to the example program

The libc.a library contains __pthread_mutex_lock as a weak symbol, and the libpthread.a library contains it as a strong symbol. When linking dynamically, the linker replaces the weak symbol with the strong symbol. However, when linking statically, you need to enforce the same semantic. That's why -Wl,--whole-archive -lpthread -Wl,--no-whole-archive is needed.