The display format of Bash’s built-in time command

Check the default output format of bash‘s built-in time command:

# time

real    0m0.000s
user    0m0.000s
sys     0m0.000s

You can use -p option to output in POSIX format:

# time -p
real 0.00
user 0.00
sys 0.00

From bash source code, we know the the definitions of these two formats:

#define POSIX_TIMEFORMAT "real %2R\nuser %2U\nsys %2S"
#define BASH_TIMEFORMAT  "\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS"

To decipher the meanings of them, we need to refer bash manual:

TIMEFORMAT

The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The ‘%’ character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.

%%
A literal ‘%’.

%[p][l]R
The elapsed time in seconds.

%[p][l]U
The number of CPU seconds spent in user mode.

%[p][l]S
The number of CPU seconds spent in system mode.

%P
The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.
……

Take POSIX_TIMEFORMAT as an example: %2R denotes using second as time unit, and the precision is two digits after a decimal point; %2U and %2S are similar.

Now you can comprehend the output of time, correct? Try using BASH_TIMEFORMAT as a practice.

 

Downgrade boost on Ubuntu 16.04

In the past 2 days, I was tortured by boost. The default boostversion on Ubuntu 16.04 is 1.58, but I met following compile errors:

......
/usr/include/boost/multi_index/detail/bucket_array.hpp: In static member function ‘static std::size_t boost::multi_index::detail::bucket_array_base<_>::size_index(std::size_t)’:
/usr/include/boost/multi_index/detail/bucket_array.hpp:84:62: error: invalid use of non-lvalue array
     const std::size_t *bound=std::lower_bound(sizes,sizes+sizes_length,n);
                                                              ^~~~~~~~~~~~
/usr/include/boost/multi_index/detail/bucket_array.hpp:85:25: error: invalid use of non-lvalue array
     if(bound==sizes+sizes_length)--bound;
                         ^~~~~~~~~~~~
/usr/include/boost/multi_index/detail/bucket_array.hpp:86:22: error: invalid use of non-lvalue array
     return bound-sizes;
......

I downgraded boost to 1.55; and downloaded and built it:

$ ./bootstrap.sh --prefix=/usr/local  
$ sudo ./b2 -a -q install

This time I found the default gcc-5 could not compile successfully. So I followed this post to install gcc-6, and modified/home/nan/boost_1_55_0/tools/build/v2/user-config.jam file to use gcc-6 to compile boost:

......
# Configure specific gcc version, giving alternative name to use.
using gcc : 6 : g++-6 ;
......

Then my project can be compiled successfully. Check gcc search header file path and library path:

$ echo | gcc-6 -E -Wp,-v -
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/6/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/6/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/6/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
......

$ ldconfig -v 2>/dev/null | grep -v ^$'\t'
/usr/local/cuda-9.0/targets/x86_64-linux/lib:
/usr/lib/x86_64-linux-gnu/libfakeroot:
/usr/local/lib:
/lib/x86_64-linux-gnu:
/usr/lib/x86_64-linux-gnu:
/usr/lib/nvidia-384:
/usr/lib32/nvidia-384:
/lib32:
/usr/lib32:
/lib:
/usr/lib:
/usr/lib/nvidia-384/tls: (hwcap: 0x8000000000000000)
/usr/lib32/nvidia-384/tls: (hwcap: 0x8000000000000000)

You will find the fresh installed boost 1.55 (in /usr/local directory) always be found before default boost 1.58 (header files are in /usr/include/boostand libraries in /usr/lib/x86_64-linux-gnu).

Update keyring first if your Arch Linux is old enough

My Arch Linux is not updated for nearly 3 months. When running pacman -Syu, it prompts following errors:

$ sudo pacman -Syu
......
error: python-dnspython: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python-dnspython-1.16.0-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: python-distlib: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python-distlib-0.2.8-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: python-pytoml: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python-pytoml-0.1.20-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: python2-distlib: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python2-distlib-0.2.8-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: python2-dnspython: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python2-dnspython-1.16.0-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: python2-pytoml: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/python2-pytoml-0.1.20-1-any.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: qbittorrent: signature from "Eli Schwartz <eschwartz@archlinux.org>" is unknown trust
:: File /var/cache/pacman/pkg/qbittorrent-4.1.5-1-x86_64.pkg.tar.xz is corrupted (invalid or corrupted package (PGP signature)).
Do you want to delete it? [Y/n]
error: failed to commit transaction (invalid or corrupted package)
Errors occurred, no packages were upgraded.
......

The solution is updating archlinux-keyring first:

$ sudo pacman -S archlinux-keyring

Then all goes well!

Modifying memory pool helps me find a hidden bug

My project has a CUDA memory pool which uses C++‘s std::queue. Allocating from the head of queue:

ptr = q.front();
q.pop();

While freeing memory insert it into the tail of queue:

q.push(ptr);

I changed the implementation from std::queue to std::deque. Both allocating and freeing all occur in the front of queue:

ptr = q.front();
q.pop_front();
......
q.push_front(ptr);

This modification helps me find a hidden bug which is releasing memory early. In origin code, the memory is inserted at the end of queue. So there is a interval between it is reused by other threads and current thread, and the work can still be done correctly as long as it is not reused by others. But after using std::deque, the memory is immediately used by other threads, which disclose the bug.