The other day I tried to install the FOS User Bundle but I received the following error:

$ php composer.phar require friendsofsymfony/user-bundle "~2.0@dev"
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
  - Installing friendsofsymfony/user-bundle (dev-master 16b04c4)
    Cloning 16b04c49af05dd3fb381e4abe04f0e5e231ac76d

Installation failed, reverting ./composer.json to its original content.
The following exception is caused by a lack of memory and not having swap configured
Check https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors for details

PHP Fatal error:  Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar:///var/www/composer.phar/vendor/symfony/console/Application.php:954
Stack trace:
#0 [internal function]: Composer\Util\ErrorHandler::handle(2, 'proc_open(): fo...', 'phar:///var/www...', 954, Array)
#1 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(954): proc_open('stty -a | grep ...', Array, NULL, NULL, NULL, Array)
#2 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(754): Symfony\Component\Console\Application->getSttyColumns()
#3 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(715): Symfony\Component\Console\Application->getTerminalDimensions()
#4 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(648): Symfony\Component\Console\Application->getTerminalWidth()
#5 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(130): Symfony\Component\Console\Application->renderException(Object(ErrorException), Object in phar:///var/www/composer.phar/vendor/symfony/console/Application.php on line 954

Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar:///var/www/composer.phar/vendor/symfony/console/Application.php:954
Stack trace:
#0 [internal function]: Composer\Util\ErrorHandler::handle(2, 'proc_open(): fo...', 'phar:///var/www...', 954, Array)
#1 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(954): proc_open('stty -a | grep ...', Array, NULL, NULL, NULL, Array)
#2 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(754): Symfony\Component\Console\Application->getSttyColumns()
#3 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(715): Symfony\Component\Console\Application->getTerminalDimensions()
#4 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(648): Symfony\Component\Console\Application->getTerminalWidth()
#5 phar:///var/www/composer.phar/vendor/symfony/console/Application.php(130): Symfony\Component\Console\Application->renderException(Object(ErrorException), Object in phar:///var/www/composer.phar/vendor/symfony/console/Application.php on line 954

In order to solve this problem I had to setup a swap file:

$ sudo /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 1.54095 s, 697 MB/s
$ sudo /sbin/mkswap /var/swap.1
Setting up swapspace version 1, size = 1048572 KiB
no label, UUID=035dd6b9-2ff5-4597-bc7d-3f89f478e855
$ sudo /sbin/swapon /var/swap.1

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