The fex_merge_flags() function copies into the current environment those exception flags that are set in the environment represented by the object pointed to by envp. The argument envp must point to an object set by a call to feholdexcept(3M) or fegetenv(3M) or equal to the macro FE_DFL_ENV. The fex_merge_flags() function does not raise any exceptions, but only sets its flags.
The fex_merge_flags function does not return a value.
In a multithreaded program, the fex_merge_flags() function affects the floating point environment only for the calling thread.
The fex_merge_flags() function automatically installs and deinstalls SIGFPE handlers and sets and clears the trap enable mode bits in the floating point status register as needed. If a program uses these functions and attempts to install a SIGFPE handler or control the trap enable mode bits independently, the resulting behavior is not defined.