Category: 技术向

pipe_file=$(mktemp --dry-run)
mkfifo ${pipe_file}
exec {pipe_fd}<>${pipe_file}
rm ${pipe_file}
printf "%-${max_jobs}s" >&${pipe_fd}
for job in ...
  read -n 1 -u ${pipe_fd}
    echo "start ${job}"
    # do stuff for ${job} ...
    echo "finish ${file}"
    printf '%-1s' >&${pipe_fd}

This piece of script uses a FIFO to manage the parallel jobs. Initially, the FIFO is filled with n characters, each representing an available slot for a job. Before a job starts, it reads a character from the FIFO. Once a job finishes, a character is written back to the FIFO. Thus the FIFO is empty if and only if there are n jobs running. New jobs will not be spawned because the FIFO read will block until at least 1 running job finishes.

{varname} style automatic file descriptor allocation requires bash 4.1+.


Make Cosim Great Again

Vivado HLS is a handy(?) high-level synthesis (HLS) tool that supports C++11 features. However, its C-RTL co-simulation (cosim) is broken in the sense that it doesn't fully support C++11. The cosim compiler seems to be stuck at

There are problems in the rt2800usb driver on Linux and according to this post I have to set nohwcrypt=1 for the rt2800usb kernel module.

sudo ifconfig wlan0 down
sudo rmmod -f rt2800usb
sudo modprobe rt2800usb nohwcrypt=1
sudo ifconfig wlan0 up

To make it permenant I have to put the following in /etc/modprobe.d/rt2800usb.conf

options rt2800usb nohwcrypt=1

Damn it… I really should have found this earlier…


If you are working with some software which requires you to source some code and messes up with your PATH and other environment variables like Vivado and Merlin compiler, here is a simple and straightforward method to get rid of the mess (to some extent).

. /space/Xilinx/SDAccel/${VERSION}/
# do something if necessary
if ! test "$0" -ef "$(which $(basename $0))"
    exec $(basename $0) "$@"
    if test -t 2
        echo "Recursive call detected!" 1&>2

Some notes:

  • Start with #!/bin/bash if you or your script use bash-specific syntax!
  • Give this file execution permission with chmod +x!
  • Make sure this file itself has the same name as the target program, or is soft-linked to it!
  • Use . instead of source, which improves compatibility!

On Linux, if you see

mtr: unable to get raw sockets.

The following might help:

sudo chmod 4755 $(which mtr)


The reason this happens is that only privileged processes (or processes with proper capabilities(7) on Linux) can create raw sockets, which is needed to send/receive ICMP packets. By chmod(2), the executable will get the S_ISUID mode, which means it can set process effective user ID when execve(2).

A safer solution is

sudo setcap cap_net_raw+ep $(which mtr)

which only gives mtr the cap_net_raw capability.

Reinstalling the mtr package via the package manager should also solve the problem since it will do whatever appropriate to make it work.


Protected: Renee Go

TL;DR: 注意网站/标识牌上的说明、不懂就问我/查词典/Google、听从工作人员的指示。

According to, std::vector<bool> in C++ is specialized and consumes only 1/8 space as you may have expected. However, as 8-bit bytes are usually the shortest available type in most implementations, thread-safety is not guaranteed. This could have caused significant trouble in my current project. How lucky I am to notice this interesting feature before I have actually encountered trouble.

Besides, std::bitset provides a fixed size bit array whose length is fixed at compile-time.

μtorrent Optimization Guide

Let's assume you have sufficiently high bandwidth. For example, you are in college and your university does not care about torrenting. One frustrating thing when you are torrenting is that your hard disk does not provide zero latency or infinite bandwidth. μtorrent might be complaining "100% Disk IO" all the time and refuses to download or upload for quite a long time. After a few Googling, I have got some useful suggestions. I will first list what to do and then explain why.

OpenWRT’s way

 since 2012. For now I've been able to make it work under multiple environments, with or without native IPv6 support, bridged or NATed. By "work", I mean both IPv4 and IPv6, with a