Resource management (computing): Difference between revisions

Revision as of 23:39, 25 July 2020

Revision as of 23:45, 25 July 2020

Line 37:

The basic approach to resource management is to acquire a resource, do something with it, then release it, yielding code of the form (illustrated with opening a file in Python):

<syntaxhighlight lang=python>

deff work_with_file= open(filename: str):

...

f = open(filename)

f.close()

...

f.close()

</syntaxhighlight>

This is correct if the intervening <code>...</code> code does not contain an early exit (<code>return</code>), the language does not have exceptions, and <code>open</code> is guaranteed to succeed. However, it causes a resource leak if there is a return or exception, and causes an incorrect release of unacquired resource if <code>open</code> can fail.

Line 48 ⟶ 47:

The resource leak can be resolved in languages that support a <code>finally</code> construction (like Python) by placing the body in a <code>try</code> clause, and the release in a <code>finally</code> clause:

<syntaxhighlight lang=python>

deff work_with_file= open(filename: str):

try:

f = open(filename)

try:...

finally:

...

finally:f.close()

f.close()

</syntaxhighlight>

This ensures correct release even if there is a return within the body or an exception thrown. Further, note that the acquisition occurs ''before'' the <code>try</code> clause, ensuring that the <code>finally</code> clause is only executed if the <code>open</code> code succeeds (without throwing an exception), assuming that "no exception" means "success" (as is the case for <code>open</code> in Python). If resource acquisition can fail without throwing an exception, such as by returning a form of <code>null</code>, it must also be checked before release, such as:

<syntaxhighlight lang=python>

deff work_with_file= open(filename: str):

try:

f = open(filename)

try:...

finally:

...

finallyif f:

if f:.close()

f.close()

</syntaxhighlight>

While this ensures correct resource management, it fails to provide adjacency or encapsulation. In many languages there are mechanisms that provide encapsulation, such as the <code>with</code> statement in Python:

<syntaxhighlight lang=python>

defwith work_with_fileopen(filename: str) as f:

...

with open(filename) as f:

...

</syntaxhighlight>