Picolibc on XWOS

Thread Local Storage

Mon, 01 Jan 0001 00:00:00 +0000

Overview

Thread Local Storage (TLS) refers to variables that are private to a thread.

The C11 standard began supporting thread local storage, introducing the _Thread_local keyword. The C2X standard introduces the thread_local keyword. The gcc and clang compilers also introduce the __thread keyword.

When global variables are defined using the above keywords, each thread gets a copy of this global variable and accesses its own copy.

TLS Models

There are four TLS models in gcc and clang’s implementation:

Dynamic Memory Management

Mon, 01 Jan 0001 00:00:00 +0000

Description

XWOS does not use the dynamic memory management algorithm provided by picolibc. It uses a memory pool algorithm:

Porting Method

Source path: xwmd/libc/picolibcac/mem.c
Rewrite picolibc’s functions:

void * malloc(size_t n);
void * realloc(void * p, size_t n);
void * calloc(size_t elem_nr, size_t elem_sz);
void * memalign(size_t alignment, size_t n);
void * valloc(size_t n);
void * pvalloc(size_t n);
void free(void * p);
int getpagesize(void);

Functions that do not need rewriting:

int posix_memalign(void ** memptr, size_t align, size_t size); // picolibc provides an implementation based on memalign
void * aligned_alloc(size_t alignment, size_t size); // picolibc provides an implementation based on memalign

Functions not yet implemented:
- mallopt
- malloc_usable_size
- malloc_stats
- mallinfo

errno

Mon, 01 Jan 0001 00:00:00 +0000

Porting Method

Set Picolibc configuration newlib-global-errno to true, not using the TLS mechanism to implement errno;
Set Picolibc configuration errno-function to '__errno';
XWOS defines an __errno variable in each thread object structure and re-implements the int * __errno(void) function, returning the address of the current thread object’s __errno:

int * __errno(void)
{
        xwos_thd_d thdd = xwos_cthd_self();
        return &thdd.thd->osthd.libc.error_number;
}

When using the errno macro from <errno.h>, the thread’s own __errno can be obtained, no longer depending on libgloss.