Archive for the 'Code' Category

How to create a list that holds different object types using `void*` in C

Sunday, April 5th, 2015

I remember being in school back around 1998 and not knowing enough about C to do this. After coding in other languages, then going back to C++ and understanding at a lower level how references and pointers work, this was pretty easy to figure out.

In this exercise I store elements of different types in a forward linked list.
In order to know what to dereference as I iterate through the list’s elements, I’ve put a “.type” field, which has an int value representing the type of the object stored.

The “.value” is simply a void*, which lets me store a pointer of any kind, and it works pretty well.

Here’s the code for your enjoyment, I hope this is useful to C apprentices.

The example shows how you can store native types like int, or more complex char* or even a struct person* (which is the more useful probably to your purposes)

It’s a good exercise to see the uses of the “address of” operator “&”, which is used to initialize pointers (the ‘&’ can also be used differently to create references, which I call in my mind as ‘aliases’, but this is not shown in this example)

I also play with a not so popular syntax to access a pointer’s sub-fields:

(*myPointer).structField == myPointer->structField

to teach you that the -> is a short hand for dereferencing a pointer and accessing one of its fields.

// An exercise to play with a struct that stores anything using a void* field.                                                                                                              

#include <stdio.h>

#define TRUE 1

int TYPE_INT = 0;
int TYPE_STRING = 1;
int TYPE_PERSON = 3;

struct node {
  struct node* next;
  int type;
  void* value;

struct person {
  char* name;
  int age;

int main(int args, char **argv) {

  struct person aPerson; = "Angel";
  aPerson.age = 35;

  // Define a linked list of objects.                                                                                                                                                       
  // We use that .type field to know what we're dealing                                                                                                                                     
  // with on every iteration. On .value we store our values.                                                                                                                                
  struct node nodes[] = {
    { .next = &nodes[1], .type = TYPE_INT    , .value=1                   },
    { .next = &nodes[2], .type = TYPE_STRING , .value="anyfing, anyfing!" },
    { .next = &nodes[3], .type = TYPE_PERSON , .value=&aPerson            },
    { .next = NULL     , .type = TYPE_BOOLEAN, .value=TRUE                }

  // We iterate through the list                                                                                                                                                            
  for ( struct node *currentNode = &nodes[0]; currentNode;  currentNode = currentNode->next) {
    int currentType = (*currentNode).type;
    if (currentType == TYPE_INT) {
      printf("%s: %dn", "- INTEGER", (*currentNode).value); // just playing with syntax, same as currentNode->value                                                                        
    } else if (currentType == TYPE_STRING) {
      printf("%s: %sn", "- STRING", currentNode->value);
    } else if (currentType == TYPE_BOOLEAN) {
      printf("%s: %dn", "- BOOLEAN (true:1, false:0)", currentNode->value);
    } else if (currentType == TYPE_PERSON) {
        // since we're using void*, we end up with a pointer to struct person, which we *dereference                                                                                        
        // into a struct in the stack.                                                                                                                                                      
        struct person currentPerson = *(struct person*) currentNode->value;
        printf("%s: %s (%d)n","- TYPE_PERSON",, currentPerson.age);

    return 0;

The output is this:

- STRING: anyfing, anyfing!
- TYPE_PERSON: Angel (35)
- BOOLEAN (true:1, false:0): 1

How to make a “foreach” function in JavaScript

Saturday, April 4th, 2015

I thought this would be a simple exercise in case of having to interview someone for a JavaScript position.

“How would you make your own ‘foreach’ in JavaScript”

I came up with the following solution:

// collection: A list of objects.
// onElementIterationCallback: The function to be called on every element iterated
//    taking the following parameters:    foo(collection : [T], currentIndex : int)
function foreach(collection, onElementIterationCallback) {
    for (var i in collection) {
        if (collection.hasOwnProperty(i)) {
            onElementIterationCallback(collection[i], i);

This is how you’d use it:

var sumOfAges = 0;
var people = [ {name:"Angel", age:35},
               {name:"Paulina", age:33},
               {name:"Nicole", age:16}]

foreach(people, function (person, currentOffset) {
   console.log("("+ currentOffset + ") iterating on " + 
      + ", age: " + person.age);
   sumOfAges += person.age; 


The expected output would be:

(0) iterating on Angel, age: 35
(1) iterating on Paulina, age: 33
(2) iterating on Nicole, age: 16

Hope you enjoyed, just a simple exercise of lists, creativity and callbacks.

# Things to remember when compiling/linking C/C++ software

Wednesday, March 18th, 2015

by Angel Leon. March 17, 2015.

Include Paths

On the compilation phase, you will usually need to specify the different include paths so that the interfaces (.h, .hpp) which define structs, classes, constans, and functions can be found.

With gcc and llvm include paths are passed with -I/path/to/includes, you can pass as many -I as you need.

In Windows, cl.exe takes include paths with the following syntax:
/I"c:\path\to\includes\ you can also pass as many as you need.

Some software uses macro definition variables that should be passed during compile time to decide what code to include.

Compilation flags

These compilation-time variables are passed using -D,

These compilation time flags are by convention usually put into a single variable named CXXFLAGS, which is then passed to the compiler as a parameter for convenience when you’re building your compilation/make script.

Object files

When you compile your .c, or .cpp files, you will end up with object files.
These files usually have .o extensions in Linux, in Windows they might be under .obj extensions.

You can create an .o file for a single or for many source files.

Static Library files

When you have several .o files, you can put them together as a library, a static library. In Linux/Mac these static libraries are simply archive files, or .a files. In windows, static library files exist under the .lib extension.

They are created like this in Linux/Mac:

ar -cvq libctest.a ctest1.o ctest2.o ctest3.o

libctest.a will contain ctest1.o,ctest2.o and ctest2.o

They are created like this in Windows:


When you are creating an executable that needs to make use of a library, if you use these static libraries, the size of your executable will be the sum of all the object files statically linked by the executable. The code is right there along the executable, it’s easier to distribute, but again, the size of the executable can be bigger than it needs to… why? because, sometimes, many of the .o files, or even the entire .a file you’re linking against might be a standard library that many other programs need.

Shared Libraries (Dynamic Libraries)

So shared or dynamic libraries were invented so that different programs or libraries would make external (shared) references to them, since they’re “shared” the symbols defined in them don’t need to be part of your executable or library, your executable contain symbols whose entry points or offset addresses might point to somewhere within themselves, but they will also have symbols whose entry points are expected to exist on shared libraries which need only be loaded once in a single portion of the operating shared memory, thus not just making the size of your executable as small as it needs to be, but you won’t need to load the library for every process/program that needs its symbols.

On Linux shared files exist under the .so (shared object) file extension, on Mac .dylib (dynamic library), and in Windows they’re called .dll (dynamic link libraries)

Another cool thing about dynamic libraries, is that they can be linked during runtime, not just compile time. An example of runtime dynamic libraries are browser plugins.

In Linux .so files are created like this:

[code lang=text]
gcc -Wall -fPIC -c *.c
gcc -shared -Wl,-soname, -o *.o

  • -Wall enables all warnings.
  • -c means don’t run the linker.
  • -fPIC means “Position Independent Code”, a requirement for shared libraries in Linux.
  • -shared makes the object file created shareable by different executables.
  • -Wl passes a comma separated list of arguments to the linker.
  • -soname means “shared object name” to use.

In Mac .dylib files are created like this:

clang -dynamiclib -o libtest.dylib file1.o file2.o -L/some/library/path -lname_of_library_without_lib_prefix

In Windows .dll files are created like this:


Linking to existing libraries

When linking your software you may be faced with a situation on which you want to link against several standard shared libraries.
If all the libraries you need exist in a single folder, you can set the LD_LIBRARY_PATH to that folder. By common standard all shared libraries are prefixed with the word lib. If a library exists in LD_LIBRARY_PATH and you want to link against it, you don’t need to pass the entire path to the library, you simply pass -lname and you will link your executable to the symbols of which should be somewhere inside LD_LIBRARY_PATH.

Tip: You should probably stay away from altering your LD_LIBRARY_PATH, if you do, make sure you keep its original value, and when you’re done restore it, as you might screw the build processes of other software in the system which might depend on what’s on the LD_LIBRARY_PATH.

If you have some other library on another folder outside LD_LIBRARY_PATH you can explictly pass the full path to that library /path/to/that/other/library/, or you can specify the folder that contains it -L/path/to/that/other/library and then the short hand form -lbar. This latter option makes more sense if the second folder contains several other libraries.

Useful tools

Sometimes you may be dealing with issues like undefined symbol errors, and you may want to inspect what symbols (functions) are defined in your library.

On Mac there’s otool, on Linux/Mac there’s nm, on Windows there’s depends.exe (a GUI tool that can be used to see both dependencies and the symbol’s tables. Taking a look at the “Entry Point” column will help you understand clearly the difference between symbols linking to a shared library vs symbols linking statically to the same library)

Useful command options

See shared library dependencies on Mac with otool

[code lang=text]
otool -L libjlibtorrent.dylib
libjlibtorrent.dylib (compatibility version 0.0.0, current version 0.0.0)
/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 120.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)

See shared symbols with nm (Linux/Mac)
With nm, you can see the symbol’s name list.
Familiarize yourself with the meaning of the symbol types:
* T (text section symbol)
* U (undefined – useful for those undefined symbol error),
* I (indirect symbol).
If the symbol is local (non-external) the symbol type is presented in lowercase letters, for example a lowercase u represents an undefined reference to a private external in another module in the same library.

nm‘s documentation says that if you’re working on Mac and you see that the symbol is preceeded by + or - it means it’s an ObjectiveC method, if you’re familiar with ObjectiveC you will know that + is for class methods and - is for instance methods, but in practice it seems to be a bit more explicit and you will often see objc or OBJC prefixed to those methods.

nm is best used along with grep ;)

Find all Undefined symbols

[code lang=text]
nm -u libMacOSXUtilsLeopard.jnilib

What to do when lighttpd won’t start and won’t give out any error output?

Wednesday, February 4th, 2015

So you upgraded your server, or just all of a sudden you try to start lighttpd, it says the server started ok, but you check and there’s no lighttpd process.

You then go after your error log files, and nothing… what the fuck is happening?

try this to attempt to debug.

sudo strace -ff /usr/local/sbin/lighttpd -f /etc/lighttpd/lighttpd.conf

In my case the output showed me lighttpd was having permission issues trying to access a log file…

[pid 28073] close(5) = 0
[pid 28073] munmap(0x7fe884c71000, 4096) = 0
[pid 28073] write(3, "2015-02-04 11:04:23: (log.c.164)"..., 49) = 49
[pid 28073] close(2) = 0
[pid 28073] open("/dev/null", O_RDWR) = 2
[pid 28073] brk(0x2203000) = 0x2203000
[pid 28073] open("/home/bh/access.log", O_WRONLY|O_CREAT|O_APPEND, 0644) = -1 EACCES (Permission denied)
[pid 28073] write(3, "2015-02-04 11:04:23: (log.c.118)"..., 98) = 98
[pid 28073] write(3, "2015-02-04 11:04:23: (server.c.1"..., 83) = 83

2 Java debugging tricks

Tuesday, January 20th, 2015
  1. Embed state in your thread names.

Some times when you see a crash log with a thread dump, you see the stack of all the threads at the moment of the snapshot, but there’s no way to see what the state of the variables and objects at play in the thread were like.

You can use Thread.currentThread.setName(...) and embed variable states on the thread name for a more useful thread dump output.

  1. Set a Default uncaught exception handler and log the error to the thread.

How to resize an EBS (xfs formatted) partition

Saturday, December 20th, 2014

First of all, create a snapshot of your EBS volume. Then out of that snapshot you will be able to create your new volume.

However, when you detach the old one from your instance and attach the new one, you will still see the old available space with df

look at my /dev/xvdf/ available space (after mounting the new EBS volume)

ubuntu@ip-10-47-167-74:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       16G  5.9G  9.1G  40% /
udev            7.4G   12K  7.4G   1% /dev
tmpfs           1.5G  176K  1.5G   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            7.4G     0  7.4G   0% /run/shm
/dev/xvdf        20G   19G  1.7G  92% /media/ebs/data  <<< this one

still 20G, I mounted a 80G one!

This drive has been formatted to use an xfs file system. In order to resize it, this is the command I used:

sudo xfs_growfs -d /media/ebs/data

output should be something like this

ubuntu@ip-10-47-167-74:~$ sudo xfs_growfs -d /media/ebs/data
meta-data=/dev/xvdf              isize=256    agcount=4, agsize=1310720 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=5242880, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=2560, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 5242880 to 20971520

now let’s see the df -h output

ubuntu@ip-10-47-167-74:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       16G  5.9G  9.1G  40% /
udev            7.4G   12K  7.4G   1% /dev
tmpfs           1.5G  176K  1.5G   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            7.4G     0  7.4G   0% /run/shm
/dev/xvdf        80G   19G   62G  23% /media/ebs/data

[bash scripting] How to get a file’s name without its extension(s).

Saturday, September 6th, 2014

Say you have an encrypted file and you want to make a shorthand command to decrypt that file, you’ll want the resulting file to be named (without the .gpg), or say you want the name, with no extension?), you can use bash’s magic variable voodo for that.

Screen Shot 2014-09-06 at 4.29.38 PM

A simple version of that script would look something like this:
Screen Shot 2014-09-06 at 4.34.26 PM

GRADLE: How to specify resources from different folders on your sourceSet

Tuesday, July 29th, 2014

Sometimes you need to have resources in your sourceset and these may come from different locations on disk, the official documentation is lacking in real world examples, or at least I just didn’t understand it very well, but from reading forums I finally got it to work.

In the example I specify what files to include/exclude from two different folders.
When the final .jar is created, they’ll keep the package path structure that lives inside the given srcDir folders.

If you just want to add these files (for some reason to the root of your resulting jar, you should make srcDir the full path to where the files live)

sourceSets {
main {
java {
//your java source paths and exclusions go here…

resources {
srcDir ‘components/resources/src/main/resources’
include ‘**/*.properties’
include ‘**/*.png’
include ‘**/*.gif’
include ‘**/*.jpg’
include ‘**/*.html’
include ‘**/*.js’
include ‘**/*.sh’
include ‘**/*.dat’
include ‘**/*.icc’
exclude ‘**/*.DS_Store’

srcDir ‘common/vuze/azureus2/src’
include ‘**/Messages*.properties’
exclude ‘**/*.class’
exclude ‘**/*.java’

GRADLE: How to copy files from another .jar into your resulting output .jar

Tuesday, July 29th, 2014

In our project we like to deliver a single jar as the final product, if you need to copy files that live on an existing jar into the Gradle’s output jar, this example shows you how to do that (and more)

jar {
//this is how you change the name of the output jar

//some exclusion rules to keep your .jar clean
exclude(‘META-INF/*.SF’, ‘META-INF/*.DSA’, ‘META-INF/*.RSA’, ‘META-INF/*.MF’)

//here we grab all the .class files inside messages.jar and we put them in our resulting jar
from (zipTree(‘lib/jars/messages.jar’)) {
include ‘**/*.class’

//how to manipulate the jar’s manifest
manifest {
attributes ‘Main-Class': ‘com.limegroup.gnutella.gui.Main’

GRADLE: How to add a list of local .jar files to the build classpath

Tuesday, July 29th, 2014

Sometimes you don’t want/cant use maven repos, all you have is a bunch of local jars on disk that you want to use as part of your compilation classpath, and the freaking gradle documentation is too vague.

Here is an example:
dependencies {
compile files(‘lib/jars/gettext-commons.jar’,

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