Services and Modeling for Embedded Software Development
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5.4.  Reentrant System Call Implementations

Reentrancy is achieved by providing a global reentrancy structure, struct _reent for each thread of control, which holds thread specific versions of global data structures, such as errno.

For a fully reentrant system, the BSP should implement the reentrant versions of the system calls, having defined syscall_dir=syscalls and added -DREENTRANT_SYSCALLS_PROVIDED" to newlib_cflags in (see Section 3.3.1).

16 of the system calls have reentrant versions, which take the suffix _r and are passed an additional first argument, which is a pointer to the reentrancy structure, struct reent for the thread of control. Thus _close is replaced by _close_r. The reentrant functions are _close_r, _execve_r, _fcntl_r, _fork_r, _fstat_r, _getpid_r, _link_r, _lseek_r, _open_r, _read_r, _sbrk_r, _stat_r, _times_r, _unlink_r, _wait_r and _write_r.

Two system calls do not need reentrant versions, _kill and _exit, which are provided as with non-reentrant versions.

For many of the reentrant functions, the behavior is almost identical to that of the non-reentrant versions, beyond ensuring the thread specific version of errno in the reentrancy structure is used. Template versions can be found in the libc/reent directory under the newlib directory.

There are two ways in which the end user can be supported with these reentrancy functions. In the first it is up to the user to manage per thread reentrancy data structures and to call the reentrant functions explicitly.

However the more powerful solution is for the system to manage the reentrancy structure itself. The end user can call the standard functions, and they will be mapped to reentrant calls, passing in a reentrancy structure for the thread.

For this approach to be used, -D__DYNAMIC_REENT__ must be added to newlib_cflags and the BSP must define the function __getreent, to return the reentrancy structure for the current thread.

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