modified: components/libc/posix/io/aio/aio.h Align comments within the aiocb structure
modified: components/libc/posix/io/epoll/epoll.c Add comments for all functions and members within structure members
modified: components/libc/posix/signal/posix_signal.c Add comments to the sigqueue function, although it does not have an internal implementation
modified: components/libc/posix/signal/posix_signal.h Added detailed explanation to all members of the rt_signal_value enumeration
为什么提交这份PR (why to submit this PR)
serial_v2驱动框架，rt_serial_tx_enable()函数，在 DMA阻塞发送(RT_DEVICE_FLAG_DMA_TX and RT_SERIAL_TX_BLOCKING) 的时候，使用rt_malloc分配了tx_fifo内存，但是并未对tx_fifo->rb.buffer_ptr指针进行初始化（rt_malloc不会自动初始化内存），因此可能导致buffer_ptr是一个随机指针。但是rt_serail_write()函数需要根据tx_fifo->rb.buffer_ptr是否为RT_NULL来判断是调用_serial_fifo_tx_blocking_nbuf()还是_serial_fifo_tx_blocking_buf()。
由于tx_fifo->rb.buffer_ptr可能是一个随机值（不一定为RT_NULL=0），导致RT_DEVICE_FLAG_DMA_TX and RT_SERIAL_TX_BLOCKING模式下调用了_serial_fifo_tx_blocking_buf()，但是rb没有真正开辟内存空间，导致内存异常
你的解决方案是什么 (what is your solution)
rt_serial_tx_enable()，在DMA阻塞发送分支下，对tx_fifo->rb.buffer_ptr进行初始化，赋值为RT_NULL。
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:24:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
return &__pico_errno;
^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:30:16: warning: returning 'volatile int *' from a function with result type 'int *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
return &__pico_errno;
^~~~~~~~~~~~~
/home/runner/work/rt-thread/rt-thread/components/libc/compilers/picolibc/syscall.c:33:12: warning: incompatible pointer types returning 'rt_err_t *' (aka 'long *') from a function with result type 'int *' [-Wincompatible-pointer-types]
return &tid->error;
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