What is meant by the term hook in programming

Question

I recently heard the term  hook  while talking to some people about a program I was writing   I m unsure exactly what this term implies although I inferred from the conversation that a hook is a type of function   I searched for a definition but was unable to find a good answer   Would someone be able to give me an idea of what this term generally means and perhaps a small example to illustrate the definition

User · Answer

A hook is functionality provided by software for users of that software to have their own code called under certain circumstances. That code can augment or replace the current code.

In the olden days when computers were truly personal and viruses were less prevalent (I'm talking the '80's), it was as simple as patching the operating system software itself to call your code. I remember writing an extension to the Applesoft BASIC language on the Apple II which simply hooked my code into the BASIC interpreter by injecting a call to my code before any of the line was processed.

Some computers had pre-designed hooks, one example being the I/O stream on the Apple II. It used such a hook to inject the whole disk sub-system (Apple II ROMs were originally built in the days where cassettes were the primary storage medium for PCs). You controlled the disks by printing the ASCII code 4 (CTRL-D) followed by the command you wanted to execute then a CR, and it was intercepted by the disk sub-system, which had hooked itself into the Apple ROM print routines.

So for example, the lines:

PRINT CHR(4);"CATALOG"
PRINT CHR(4);"IN#6"

would list the disk contents then re-initialize the machine. This allowed such tricks as protecting your BASIC programs by setting the first line as:

123 REM XIN#6

then using POKE to insert the CTRL-D character in where the X was. Then, anyone trying to list your source would send the re-initialize sequence through the output routines where the disk sub-system would detect it.

That's often the sort of trickery we had to resort to, to get the behavior we wanted.

Nowadays, with the operating system more secure, it provides facilities for hooks itself, since you're no longer supposed to modify the operating system "in-flight" or on the disk.

They've been around for a long time. Mainframes had them (called exits) and a great deal of mainframe software uses those facilities even now. For example, the free source code control system that comes with z/OS (called SCLM) allows you to entirely replace the security subsystem by simply placing your own code in the exit.

User · Answer

In the Drupal content management system   hook  has a relatively specific meaning  When an internal event occurs  like content creation or user login  for example   modules can respond to the event by implementing a special  hook  function  This is done via naming convention --  your-plugin-name  user login   for the User Login event  for example   Because of this convention  the underlying events are referred to as  hooks  and appear with names like  hook user login  and  hook user authenticate    in Drupal s API documentation

User · Answer

Essentially it s a place in code that allows you to tap in to a module to either provide different behavior or to react when something happens

User · Answer

Essentially it s a place in code that allows you to tap in to a module to either provide different behavior or to react when something happens

User · Answer

Hook denotes a place in the code where you dispatch an event of certain type  and if this event was registered before with a proper function to call back  then it would be handled by this registered function  otherwise nothing happens

User · Answer

hooks can be executed when some condition is encountered  e g  some variable changes or some action is called or some event happens  hooks can enter in the process and change things or react upon changes

User · Answer

Hooking in programming is a technique employing so-called hooks to make a chain of procedures as an event handler

User · Answer

In a generic sense  a  hook  is something that will let you  a programmer  view and or interact with and or change something that s already going on in a system program   For example  the Drupal CMS provides developers with hooks that let them take additional action after a  content node  is created   If a developer doesn t implement a hook  the node is created per normal   If a developer implements a hook  they can have some additional code run whenever a node is created   This code could do anything  including rolling back and or altering the original action  It could also do something unrelated to the node creation entirely    A callback could be thought of as a specific kind of hook   By implementing callback functionality into a system  that system is letting you call some additional code after an action has completed  However  hooking  as a generic term  is not limited to callbacks   Another example   Sometimes Web Developers will refer to class names and or IDs on elements as hooks   That s because by placing the ID class name on an element  they can then use Javascript to modify that element  or  hook in  to the page document   this is stretching the meaning  but it is commonly used and worth mentioning

User · Answer

Hooking in programming is a technique employing so-called hooks to make a chain of procedures as an event handler

User · Answer

Oftentimes hooking refers to Win32 message hooking or the Linux OSX equivalents  but more generically hooking is simply notifying another object window program etc that you want to be notified when a specified action happens  For instance  Having all windows on the system notify you as they are about to close   As a general rule  hooking is somewhat hazardous since doing it without understanding how it affects the system can lead to instability or at the very leas unexpected behaviour  It can also be VERY useful in certain circumstances  thought  For instance  FRAPS uses it to determine which windows it should show it s FPS counter on

User · Answer

Oftentimes hooking refers to Win32 message hooking or the Linux OSX equivalents  but more generically hooking is simply notifying another object window program etc that you want to be notified when a specified action happens  For instance  Having all windows on the system notify you as they are about to close   As a general rule  hooking is somewhat hazardous since doing it without understanding how it affects the system can lead to instability or at the very leas unexpected behaviour  It can also be VERY useful in certain circumstances  thought  For instance  FRAPS uses it to determine which windows it should show it s FPS counter on

User · Answer

In a generic sense  a  hook  is something that will let you  a programmer  view and or interact with and or change something that s already going on in a system program   For example  the Drupal CMS provides developers with hooks that let them take additional action after a  content node  is created   If a developer doesn t implement a hook  the node is created per normal   If a developer implements a hook  they can have some additional code run whenever a node is created   This code could do anything  including rolling back and or altering the original action  It could also do something unrelated to the node creation entirely    A callback could be thought of as a specific kind of hook   By implementing callback functionality into a system  that system is letting you call some additional code after an action has completed  However  hooking  as a generic term  is not limited to callbacks   Another example   Sometimes Web Developers will refer to class names and or IDs on elements as hooks   That s because by placing the ID class name on an element  they can then use Javascript to modify that element  or  hook in  to the page document   this is stretching the meaning  but it is commonly used and worth mentioning

User · Answer

A hook is functionality provided by software for users of that software to have their own code called under certain circumstances. That code can augment or replace the current code.

In the olden days when computers were truly personal and viruses were less prevalent (I'm talking the '80's), it was as simple as patching the operating system software itself to call your code. I remember writing an extension to the Applesoft BASIC language on the Apple II which simply hooked my code into the BASIC interpreter by injecting a call to my code before any of the line was processed.

Some computers had pre-designed hooks, one example being the I/O stream on the Apple II. It used such a hook to inject the whole disk sub-system (Apple II ROMs were originally built in the days where cassettes were the primary storage medium for PCs). You controlled the disks by printing the ASCII code 4 (CTRL-D) followed by the command you wanted to execute then a CR, and it was intercepted by the disk sub-system, which had hooked itself into the Apple ROM print routines.

So for example, the lines:

PRINT CHR(4);"CATALOG"
PRINT CHR(4);"IN#6"

would list the disk contents then re-initialize the machine. This allowed such tricks as protecting your BASIC programs by setting the first line as:

123 REM XIN#6

then using POKE to insert the CTRL-D character in where the X was. Then, anyone trying to list your source would send the re-initialize sequence through the output routines where the disk sub-system would detect it.

That's often the sort of trickery we had to resort to, to get the behavior we wanted.

Nowadays, with the operating system more secure, it provides facilities for hooks itself, since you're no longer supposed to modify the operating system "in-flight" or on the disk.

They've been around for a long time. Mainframes had them (called exits) and a great deal of mainframe software uses those facilities even now. For example, the free source code control system that comes with z/OS (called SCLM) allows you to entirely replace the security subsystem by simply placing your own code in the exit.

User · Answer

Hooks are a category of function that allows base code to call extension code  This can be useful in situations in which a core developer wants to offer extensibility without exposing their code   One usage of hooks is in video game mod development  A game may not allow mod developers to extend base functionality  but hooks can be added by core mod library developers  With these hooks  independent developers can have their custom code called upon any desired event  such as game loading  inventory updates  entity interactions  etc   A common method of implementation is to give a function an empty list of callbacks  then expose the ability to extend the list of callbacks  The base code will always call the function at the same and proper time but  with an empty callback list  the function does nothing  This is by design   A third party  then  has the opportunity to write additional code and add their new callback to the hook s callback list  With nothing more than a reference of available hooks  they have extended functionality at minimal risk to the base system   Hooks don t allow developers to do anything that can t be done with other structures and interfaces  They are a choice to be made with consideration to the task and users  third-party developers    For clarification  a hook allows the extension and may be implemented using callbacks  Callbacks are generally nothing more than a function pointer  the computed address of a function  There appears to be confusion in other answers comments

User · Answer

In a generic sense  a  hook  is something that will let you  a programmer  view and or interact with and or change something that s already going on in a system program   For example  the Drupal CMS provides developers with hooks that let them take additional action after a  content node  is created   If a developer doesn t implement a hook  the node is created per normal   If a developer implements a hook  they can have some additional code run whenever a node is created   This code could do anything  including rolling back and or altering the original action  It could also do something unrelated to the node creation entirely    A callback could be thought of as a specific kind of hook   By implementing callback functionality into a system  that system is letting you call some additional code after an action has completed  However  hooking  as a generic term  is not limited to callbacks   Another example   Sometimes Web Developers will refer to class names and or IDs on elements as hooks   That s because by placing the ID class name on an element  they can then use Javascript to modify that element  or  hook in  to the page document   this is stretching the meaning  but it is commonly used and worth mentioning

User · Answer

In VERY short  you can change the code of an API call such as MessageBox to where it does a different function edited by you  globally will work system wide  locally will work process wide

User · Answer

hooks can be executed when some condition is encountered  e g  some variable changes or some action is called or some event happens  hooks can enter in the process and change things or react upon changes

User · Answer

Essentially it s a place in code that allows you to tap in to a module to either provide different behavior or to react when something happens

User · Answer

Hooks are a category of function that allows base code to call extension code  This can be useful in situations in which a core developer wants to offer extensibility without exposing their code   One usage of hooks is in video game mod development  A game may not allow mod developers to extend base functionality  but hooks can be added by core mod library developers  With these hooks  independent developers can have their custom code called upon any desired event  such as game loading  inventory updates  entity interactions  etc   A common method of implementation is to give a function an empty list of callbacks  then expose the ability to extend the list of callbacks  The base code will always call the function at the same and proper time but  with an empty callback list  the function does nothing  This is by design   A third party  then  has the opportunity to write additional code and add their new callback to the hook s callback list  With nothing more than a reference of available hooks  they have extended functionality at minimal risk to the base system   Hooks don t allow developers to do anything that can t be done with other structures and interfaces  They are a choice to be made with consideration to the task and users  third-party developers    For clarification  a hook allows the extension and may be implemented using callbacks  Callbacks are generally nothing more than a function pointer  the computed address of a function  There appears to be confusion in other answers comments

User · Answer

A hook is functionality provided by software for users of that software to have their own code called under certain circumstances. That code can augment or replace the current code.

In the olden days when computers were truly personal and viruses were less prevalent (I'm talking the '80's), it was as simple as patching the operating system software itself to call your code. I remember writing an extension to the Applesoft BASIC language on the Apple II which simply hooked my code into the BASIC interpreter by injecting a call to my code before any of the line was processed.

Some computers had pre-designed hooks, one example being the I/O stream on the Apple II. It used such a hook to inject the whole disk sub-system (Apple II ROMs were originally built in the days where cassettes were the primary storage medium for PCs). You controlled the disks by printing the ASCII code 4 (CTRL-D) followed by the command you wanted to execute then a CR, and it was intercepted by the disk sub-system, which had hooked itself into the Apple ROM print routines.

So for example, the lines:

PRINT CHR(4);"CATALOG"
PRINT CHR(4);"IN#6"

would list the disk contents then re-initialize the machine. This allowed such tricks as protecting your BASIC programs by setting the first line as:

123 REM XIN#6

then using POKE to insert the CTRL-D character in where the X was. Then, anyone trying to list your source would send the re-initialize sequence through the output routines where the disk sub-system would detect it.

That's often the sort of trickery we had to resort to, to get the behavior we wanted.

Nowadays, with the operating system more secure, it provides facilities for hooks itself, since you're no longer supposed to modify the operating system "in-flight" or on the disk.

They've been around for a long time. Mainframes had them (called exits) and a great deal of mainframe software uses those facilities even now. For example, the free source code control system that comes with z/OS (called SCLM) allows you to entirely replace the security subsystem by simply placing your own code in the exit.

User · Answer

In VERY short  you can change the code of an API call such as MessageBox to where it does a different function edited by you  globally will work system wide  locally will work process wide

User · Answer

A hook is functionality provided by software for users of that software to have their own code called under certain circumstances. That code can augment or replace the current code.

In the olden days when computers were truly personal and viruses were less prevalent (I'm talking the '80's), it was as simple as patching the operating system software itself to call your code. I remember writing an extension to the Applesoft BASIC language on the Apple II which simply hooked my code into the BASIC interpreter by injecting a call to my code before any of the line was processed.

Some computers had pre-designed hooks, one example being the I/O stream on the Apple II. It used such a hook to inject the whole disk sub-system (Apple II ROMs were originally built in the days where cassettes were the primary storage medium for PCs). You controlled the disks by printing the ASCII code 4 (CTRL-D) followed by the command you wanted to execute then a CR, and it was intercepted by the disk sub-system, which had hooked itself into the Apple ROM print routines.

So for example, the lines:

PRINT CHR(4);"CATALOG"
PRINT CHR(4);"IN#6"

would list the disk contents then re-initialize the machine. This allowed such tricks as protecting your BASIC programs by setting the first line as:

123 REM XIN#6

then using POKE to insert the CTRL-D character in where the X was. Then, anyone trying to list your source would send the re-initialize sequence through the output routines where the disk sub-system would detect it.

That's often the sort of trickery we had to resort to, to get the behavior we wanted.

Nowadays, with the operating system more secure, it provides facilities for hooks itself, since you're no longer supposed to modify the operating system "in-flight" or on the disk.

They've been around for a long time. Mainframes had them (called exits) and a great deal of mainframe software uses those facilities even now. For example, the free source code control system that comes with z/OS (called SCLM) allows you to entirely replace the security subsystem by simply placing your own code in the exit.

User · Answer

Hooking in programming is a technique employing so-called hooks to make a chain of procedures as an event handler

User · Answer

In a generic sense  a  hook  is something that will let you  a programmer  view and or interact with and or change something that s already going on in a system program   For example  the Drupal CMS provides developers with hooks that let them take additional action after a  content node  is created   If a developer doesn t implement a hook  the node is created per normal   If a developer implements a hook  they can have some additional code run whenever a node is created   This code could do anything  including rolling back and or altering the original action  It could also do something unrelated to the node creation entirely    A callback could be thought of as a specific kind of hook   By implementing callback functionality into a system  that system is letting you call some additional code after an action has completed  However  hooking  as a generic term  is not limited to callbacks   Another example   Sometimes Web Developers will refer to class names and or IDs on elements as hooks   That s because by placing the ID class name on an element  they can then use Javascript to modify that element  or  hook in  to the page document   this is stretching the meaning  but it is commonly used and worth mentioning

User · Answer

Hooking in programming is a technique employing so-called hooks to make a chain of procedures as an event handler

User · Answer

In the Drupal content management system   hook  has a relatively specific meaning  When an internal event occurs  like content creation or user login  for example   modules can respond to the event by implementing a special  hook  function  This is done via naming convention --  your-plugin-name  user login   for the User Login event  for example   Because of this convention  the underlying events are referred to as  hooks  and appear with names like  hook user login  and  hook user authenticate    in Drupal s API documentation

User · Answer

hooks can be executed when some condition is encountered  e g  some variable changes or some action is called or some event happens  hooks can enter in the process and change things or react upon changes

User · Answer

In the Drupal content management system   hook  has a relatively specific meaning  When an internal event occurs  like content creation or user login  for example   modules can respond to the event by implementing a special  hook  function  This is done via naming convention --  your-plugin-name  user login   for the User Login event  for example   Because of this convention  the underlying events are referred to as  hooks  and appear with names like  hook user login  and  hook user authenticate    in Drupal s API documentation

User · Answer

Simple said   A hook is a means of executing custom code  function  either before  after  or instead of  existing code   For example  a function may be written to  hook  into the login process in order to execute a Captcha function before continuing on to the normal login process

User · Answer

Simple said   A hook is a means of executing custom code  function  either before  after  or instead of  existing code   For example  a function may be written to  hook  into the login process in order to execute a Captcha function before continuing on to the normal login process

User · Answer

Hook denotes a place in the code where you dispatch an event of certain type  and if this event was registered before with a proper function to call back  then it would be handled by this registered function  otherwise nothing happens

User · Answer

hooks can be executed when some condition is encountered  e g  some variable changes or some action is called or some event happens  hooks can enter in the process and change things or react upon changes

User · Answer

Oftentimes hooking refers to Win32 message hooking or the Linux OSX equivalents  but more generically hooking is simply notifying another object window program etc that you want to be notified when a specified action happens  For instance  Having all windows on the system notify you as they are about to close   As a general rule  hooking is somewhat hazardous since doing it without understanding how it affects the system can lead to instability or at the very leas unexpected behaviour  It can also be VERY useful in certain circumstances  thought  For instance  FRAPS uses it to determine which windows it should show it s FPS counter on

User · Answer

A chain of hooks is a set of functions in which each function calls the next  What is significant about a chain of hooks is that a programmer can add another function to the chain at run time  One way to do this is to look for a known location where the address of the first function in a chain is kept  You then save the value of that function pointer and overwrite the value at the initial address with the address of the function you wish to insert into the hook chain  The function then gets called  does its business and calls the next function in the chain  unless you decide otherwise   Naturally  there are a number of other ways to create a chain of hooks  from writing directly to memory to using the metaprogramming facilities of languages like Ruby or Python    An example of a chain of hooks is the way that an MS Windows application processes messages  Each function in the processing chain either processes a message or sends it to the next function in the chain

User · Answer

Essentially it s a place in code that allows you to tap in to a module to either provide different behavior or to react when something happens

User · Answer

Oftentimes hooking refers to Win32 message hooking or the Linux OSX equivalents  but more generically hooking is simply notifying another object window program etc that you want to be notified when a specified action happens  For instance  Having all windows on the system notify you as they are about to close   As a general rule  hooking is somewhat hazardous since doing it without understanding how it affects the system can lead to instability or at the very leas unexpected behaviour  It can also be VERY useful in certain circumstances  thought  For instance  FRAPS uses it to determine which windows it should show it s FPS counter on

User · Answer

A chain of hooks is a set of functions in which each function calls the next  What is significant about a chain of hooks is that a programmer can add another function to the chain at run time  One way to do this is to look for a known location where the address of the first function in a chain is kept  You then save the value of that function pointer and overwrite the value at the initial address with the address of the function you wish to insert into the hook chain  The function then gets called  does its business and calls the next function in the chain  unless you decide otherwise   Naturally  there are a number of other ways to create a chain of hooks  from writing directly to memory to using the metaprogramming facilities of languages like Ruby or Python    An example of a chain of hooks is the way that an MS Windows application processes messages  Each function in the processing chain either processes a message or sends it to the next function in the chain

User · Answer

In the Drupal content management system   hook  has a relatively specific meaning  When an internal event occurs  like content creation or user login  for example   modules can respond to the event by implementing a special  hook  function  This is done via naming convention --  your-plugin-name  user login   for the User Login event  for example   Because of this convention  the underlying events are referred to as  hooks  and appear with names like  hook user login  and  hook user authenticate    in Drupal s API documentation

User · Answer

A chain of hooks is a set of functions in which each function calls the next  What is significant about a chain of hooks is that a programmer can add another function to the chain at run time  One way to do this is to look for a known location where the address of the first function in a chain is kept  You then save the value of that function pointer and overwrite the value at the initial address with the address of the function you wish to insert into the hook chain  The function then gets called  does its business and calls the next function in the chain  unless you decide otherwise   Naturally  there are a number of other ways to create a chain of hooks  from writing directly to memory to using the metaprogramming facilities of languages like Ruby or Python    An example of a chain of hooks is the way that an MS Windows application processes messages  Each function in the processing chain either processes a message or sends it to the next function in the chain

User · Answer

A chain of hooks is a set of functions in which each function calls the next  What is significant about a chain of hooks is that a programmer can add another function to the chain at run time  One way to do this is to look for a known location where the address of the first function in a chain is kept  You then save the value of that function pointer and overwrite the value at the initial address with the address of the function you wish to insert into the hook chain  The function then gets called  does its business and calls the next function in the chain  unless you decide otherwise   Naturally  there are a number of other ways to create a chain of hooks  from writing directly to memory to using the metaprogramming facilities of languages like Ruby or Python    An example of a chain of hooks is the way that an MS Windows application processes messages  Each function in the processing chain either processes a message or sends it to the next function in the chain

[hook] What is meant by the term "hook" in programming?

Examples related to hook