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Displaying posts with tag: c (reset)
C Library Visibility

I was surprised by the recent announcement that MySQL are going to start to conceal the hidden function calls in their C connector. Surprised because although this is great news I had expected them to do this years ago. Working for HP's Advanced Technology Group I realise I take such things for granted. For this blog post I'm going to talk about why it is important and how to do it.

So, when you create a dynamic library in C the default thing that happens is every function call in that library effectively becomes a potential API call. Whether you document every single function or not to make it official API is up to you but I suspect in 99.99% of cases there are private functions you don't want users to mess with. Additionally holding the symbol information for every function so that you can link your application to it takes a …

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Popular Programming Languages

First of all, Happy New Year!

IEEE Spectrum published a ranking of the most popular programming languages. Computational journalist Nick Diakopoulos wrote the article. While it may surprise some, I wasn’t surprised to find SQL in the top ten.

Nick weighted and combined 12 metrics from 10 sources (including IEEE Xplore, Google, and GitHub) to rank the most popular programming languages.

  • Compiled programming languages (Java [#1], C [#2], C++ [#3], C# [#4], Objective-C [#16])
  • Interpreted programming languages (Python [#5], JavaScript [#6], PHP [#7], Ruby [#8], Perl [#11], HTML [#12])
  • Data languages (SQL [#9], MATLAB …
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Why Stored Programs?

Why should you use stored programs? Great question, here’s my little insight into a situation that I heard about in a large organization.

A very large organization is having a technology argument. In someway, like politics, half-truth drives this type of discussion. This company has hundreds of databases and they’re about half SQL Server and Oracle. The argument (half-truth) states that using T-SQL or PL/SQL yields “spaghetti” code!

It seems like an old argument from my perspective. After all, I’ve been working with T-SQL and PL/SQL for a long time. Spaghetti code exists in every language when unskilled programmers solve problems but the point here is one of software architecture, and an attempt to malign stored programming in general. Let’s examine the merit of the argument against stored programs.

First of all, the argument against stored programs is simply not true. SQL DML statements, like the …

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Separate docs for MySQL Connectors

The MySQL documentation section has always had this Topic Guides page containing links to the docs for the various MySQL Connectors -- the official database drivers for various languages and programming technologies. That is the most convenient way to get the information for each Connector in PDF form, rather than downloading the entire Ref Man PDF. For HTML, it was more of a shortcut, because

MySQL LDAP Authentication Plugin (Clear password client plugin)

Based on my last post MySQL LDAP Authentication Plugin, I received feedback from MySql Joro Blog by Oracle.

They told me:

Insted of writing (and having to deply) your own client plugin you probably can reuse the cleartext client side plugin, specially because it’s available in a number of mysql clients already. Check sql-common/client.c on MySQL 5.5+ for details.

This is very useful because you only need to put the plugin in server side, and in the client side you only need to check if the clear password plugin is enabled.

Now, I present the updated code with the only server side plugin, and I reused the cleartext client side plugin from MySql, it’s more short and very focused in LDAP authentication:

Author: Ignacio Ocampo …
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MySQL LDAP Authentication Plugin

As a continuation of previous post, now, I will show how to make a mysql plugin for ldap authentication.

Get the mysql-server source code at (

Installing necessary packages

yum groupinstall 'Development Tools'
yum install cmake ncurses-devel

Download source code, build and start MySQL Server

tar -xzf mysql-5.5.27.tar.gz
cd mysql-5.5.25

# Preconfiguration setup
groupadd mysql
useradd -r -g mysql mysql

# Beginning of source-build specific instructions
cmake .
make install

# Postinstallation setup
chown -R mysql .
chgrp -R mysql .
./scripts/mysql_install_db --user=mysql
chown -R root .
chown -R mysql data

cp support-files/mysql.server …
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LDAP C Client Authentication Example (with OpenLDAP)

I have the goal of authenticate MySQL users with an LDAP server, currently, employees of my company are authenticated in several services (ftp, ssh, svn) through my LDAP server, except MySQL. (As you can imagine, I need to add manually every user in MySQL, a very tedious task).

In this post I only leave the example with LDAP authentication.

Installing necessary packages

yum groupinstall 'Development Tools'
yum install openldap-devel

Source ldapClient.c

#include <stdio.h>
#include <ldap.h>
/* LDAP Server settings */
#define LDAP_SERVER "ldap://"
main( int argc, char **argv )
LDAP        *ld;
int        rc;
char        bind_dn[100];

/* Get username and password */
if( argc != 3 )
perror( "invalid args, required: username password" );
return( 1 );
sprintf( bind_dn, "cn=%s,ou=People,dc=nafiux,dc=com", argv[1] );
printf( "Connecting as %s...\n", …
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Integer overflow

What do you think of this piece of C code?

  void foo(long v) {
    unsigned long u;
    unsigned sign;
    if (v < 0) {
      u = -v;
      sign = 1;
    } else {
      u = v;
      sign = 0;

Seems pretty simple, right? Then what do you think of this output from MySQL:

  mysql> create table t1 (a bigint) as select '-9223372036854775807.5' as a;
  mysql> select * from t1;
  | a                    |
  | -'..--).0-*(+,))+(0( | 

Yes, that is authentic output from older versions of MySQL. Not just the wrong number, the output is complete garbage! This is my all-time favorite MySQL bug#31799. It was caused by code like the above C snippet.

So can you spot what is wrong with the code? Looks pretty simple, does it not? But the title of this post …

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Dennis Ritchie, the creator of C, dies at 70..

When I first got in touch with C it was in the early 1980's. I was a sysadmin at a Swedish telco operator (then THE Swedish Telco operator, Televerket, nowadays called Telia) for a system used for software development for a PABX system called A345 in Sweden, better known as Meridian in the rest of the world (co-developed by Televrket and Nortel). The Meridian system was the biggest of the non-custom built PABXes in those days. The language used to program it was called SL-1 (Switching Language 1) and the development system, like editors (vi / ined), compilers etc was running on a Unix system.

This sure was one of the earliest commercial uses for Unix, the Unix variant was version 6 and was not a BSD or anything like that, this was way before BSD really. Rather, the system was built by Interactive Systems which was the first commercial Unix vendor. This was version Interactive 2.5, based on Unix version 6, mind you. sh and csh only, no …

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New algorithm for calculating 95 percentile

The 95 percentile for query response times is and old concept; Peter and Roland blogged about it in 2008. Since then, MySQL tools have calculated the 95 percentile by collecting all values, either exactly or approximately, and returning all_values[int(number_of_values * 0.95)] (that’s an extreme simplification). But recently I asked myself*: must we save all values? The answer is no. I created a new algorithm** for calculating the 95 percentile that is faster, more accurate, and saves only 100 values.***

Firstly, my basis of comparison is the 95 percentile algo used by …

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