Inline functions in C

Question

How do you do  inline functions  in C   I don t think I understand the concept  Are they like anonymous methods  Like lambda functions   Note  The answers almost entirely deal with the ability to inline functions  i e   a manual or compiler optimization that replaces a function call site with the body of the callee   If you are interested in anonymous  a k a  lambda  functions  see  jalf s answer or What is this   39 Lambda  39  everyone keeps speaking of

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No  there is no such construct in C   but the  NET JIT compiler could decide to do inline function calls on JIT time  But i actually don t know if it is really doing such optimizations   I think it should  -

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You re mixing up two separate concepts  Function inlining is a compiler optimization which has no impact on the semantics  A function behaves the same whether it s inlined or not   On the other hand  lambda functions are purely a semantic concept  There is no requirement on how they should be implemented or executed  as long as they follow the behavior set out in the language spec  They can be inlined if the JIT compiler feels like it  or not if it doesn t   There is no inline keyword in C   because it s an optimization that can usually be left to the compiler  especially in JIT ed languages  The JIT compiler has access to runtime statistics which enables it to decide what to inline much more efficiently than you can when writing the code  A function will be inlined if the compiler decides to  and there s nothing you can do about it either way

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Do you mean inline functions in the C   sense   In which the contents of a normal function are automatically copied inline into the callsite   The end effect being that no function call actually happens when calling a function   Example   inline int Add int left  int right    return left   right      If so then no  there is no C  equivalent to this     Or Do you mean functions that are declared within another function   If so then yes  C  supports this via anonymous methods or lambda expressions     Example   static void Example       Func lt int int int gt  add    x y    gt  x   y    var result   add 4 6       10

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Do you mean inline functions in the C   sense   In which the contents of a normal function are automatically copied inline into the callsite   The end effect being that no function call actually happens when calling a function   Example   inline int Add int left  int right    return left   right      If so then no  there is no C  equivalent to this     Or Do you mean functions that are declared within another function   If so then yes  C  supports this via anonymous methods or lambda expressions     Example   static void Example       Func lt int int int gt  add    x y    gt  x   y    var result   add 4 6       10

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Inline methods are simply a compiler optimization where the code of a function is rolled into the caller   There s no mechanism by which to do this in C   and they re to be used sparingly in languages where they are supported -- if you don t know why they should be used somewhere  they shouldn t be   Edit  To clarify  there are two major reasons they need to be used sparingly    It s easy to make massive binaries by using inline in cases where it s not necessary The compiler tends to know better than you do when something should  from a performance standpoint  be inlined   It s best to leave things alone and let the compiler do its work  then profile and figure out if inline is the best solution for you   Of course  some things just make sense to be inlined  mathematical operators particularly   but letting the compiler handle it is typically the best practice

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Update  Per konrad kruczynski s answer  the following is true for versions of  NET up to and including 4 0   You can use the MethodImplAttribute class to prevent a method from being inlined      MethodImpl MethodImplOptions NoInlining   void SomeMethod                       but there is no way to do the opposite and force it to be inlined

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The statement  its best to leave these things alone and let the compiler do the work       Cody Brocious   is complete rubish   I have been programming high performance game code for 20 years  and I have yet to come across a compiler that is  smart enough  to know which code should be inlined  functions  or not    It would be useful to have a  inline  statement in c   truth is that the compiler just doesnt have all the information it needs to determine which function should be always inlined or not without the  inline  hint   Sure if the function is small  accessor  then it might be automatically inlined  but what if it is a few lines of code    Nonesense  the compiler has no way of knowing  you cant just leave that up to the compiler for optimized code  beyond algorithims

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Inline methods are simply a compiler optimization where the code of a function is rolled into the caller   There s no mechanism by which to do this in C   and they re to be used sparingly in languages where they are supported -- if you don t know why they should be used somewhere  they shouldn t be   Edit  To clarify  there are two major reasons they need to be used sparingly    It s easy to make massive binaries by using inline in cases where it s not necessary The compiler tends to know better than you do when something should  from a performance standpoint  be inlined   It s best to leave things alone and let the compiler do its work  then profile and figure out if inline is the best solution for you   Of course  some things just make sense to be inlined  mathematical operators particularly   but letting the compiler handle it is typically the best practice

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Finally in  NET 4 5  the CLR allows one to hint suggest1 method inlining using MethodImplOptions AggressiveInlining value  It is also available in the Mono s trunk  committed today       The full attribute usage is in mscorlib dll     so should not need to include extra references using System Runtime CompilerServices          MethodImpl MethodImplOptions AggressiveInlining   void MyMethod        1  Previously  force  was used here  Since there were a few downvotes  I ll try to clarify the term  As in the comments and the documentation  The method should be inlined if possible  Especially considering Mono  which is open   there are some mono-specific technical limitations considering inlining or more general one  like virtual functions   Overall  yes  this is a hint to compiler  but I guess that is what was asked for

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I know this question is about C   However  you can write inline functions in  NET with F   see  Use of  inline  in F

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The statement  its best to leave these things alone and let the compiler do the work       Cody Brocious   is complete rubish   I have been programming high performance game code for 20 years  and I have yet to come across a compiler that is  smart enough  to know which code should be inlined  functions  or not    It would be useful to have a  inline  statement in c   truth is that the compiler just doesnt have all the information it needs to determine which function should be always inlined or not without the  inline  hint   Sure if the function is small  accessor  then it might be automatically inlined  but what if it is a few lines of code    Nonesense  the compiler has no way of knowing  you cant just leave that up to the compiler for optimized code  beyond algorithims

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Inline methods are simply a compiler optimization where the code of a function is rolled into the caller   There s no mechanism by which to do this in C   and they re to be used sparingly in languages where they are supported -- if you don t know why they should be used somewhere  they shouldn t be   Edit  To clarify  there are two major reasons they need to be used sparingly    It s easy to make massive binaries by using inline in cases where it s not necessary The compiler tends to know better than you do when something should  from a performance standpoint  be inlined   It s best to leave things alone and let the compiler do its work  then profile and figure out if inline is the best solution for you   Of course  some things just make sense to be inlined  mathematical operators particularly   but letting the compiler handle it is typically the best practice

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I know this question is about C   However  you can write inline functions in  NET with F   see  Use of  inline  in F

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Yes Exactly  the only distinction is the fact it returns a value   Simplification  not using expressions    List lt T gt  ForEach Takes an action  it doesn t expect a return result   So an Action lt T gt  delegate would suffice   say   List lt T gt  ForEach param   gt  Console WriteLine param      is the same as saying   List lt T gt  ForEach delegate T param    Console WriteLine param         the difference is that the param type and delegate decleration are inferred by usage and the braces aren t required on a simple inline method   Where as  List lt T gt  Where Takes a function  expecting a result   So an Function lt T  bool gt  would be expected   List lt T gt  Where param   gt  param Value    SomeExpectedComparison     which is the same as   List lt T gt  Where delegate T param    return param Value    SomeExpectedComparison        You can also declare these methods inline and asign them to variables IE   Action myAction        gt  Console WriteLine  I m doing something Nifty      myAction      or  Function lt object  string gt  myFunction   theObject   gt  theObject ToString     string myString   myFunction someObject     I hope this helps

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Update  Per konrad kruczynski s answer  the following is true for versions of  NET up to and including 4 0   You can use the MethodImplAttribute class to prevent a method from being inlined      MethodImpl MethodImplOptions NoInlining   void SomeMethod                       but there is no way to do the opposite and force it to be inlined

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Lambda expressions are inline functions  I think  that C  doesn t have a extra attribute like inline or something like that

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You re mixing up two separate concepts  Function inlining is a compiler optimization which has no impact on the semantics  A function behaves the same whether it s inlined or not   On the other hand  lambda functions are purely a semantic concept  There is no requirement on how they should be implemented or executed  as long as they follow the behavior set out in the language spec  They can be inlined if the JIT compiler feels like it  or not if it doesn t   There is no inline keyword in C   because it s an optimization that can usually be left to the compiler  especially in JIT ed languages  The JIT compiler has access to runtime statistics which enables it to decide what to inline much more efficiently than you can when writing the code  A function will be inlined if the compiler decides to  and there s nothing you can do about it either way

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Cody has it right  but I want to provide an example of what an inline function is   Let s say you have this code   private void OutputItem string x        Console WriteLine x          maybe encapsulate additional logic to decide         whether to also write the message to Trace or a log file    public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        let s pretend IEnumerable lt T gt  ToList   doesn t exist for the moment     IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y           OutputItem y             return result      The compilerJust-In-Time optimizer could choose to alter the code to avoid repeatedly placing a call to OutputItem   on the stack  so that it would be as if you had written the code like this instead   public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y               full OutputItem   implementation is placed here         Console WriteLine y                 return result      In this case  we would say the OutputItem   function was inlined   Note that it might do this even if the OutputItem   is called from other places as well   Edited to show a scenario more-likely to be inlined

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Lambda expressions are inline functions  I think  that C  doesn t have a extra attribute like inline or something like that

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You re mixing up two separate concepts  Function inlining is a compiler optimization which has no impact on the semantics  A function behaves the same whether it s inlined or not   On the other hand  lambda functions are purely a semantic concept  There is no requirement on how they should be implemented or executed  as long as they follow the behavior set out in the language spec  They can be inlined if the JIT compiler feels like it  or not if it doesn t   There is no inline keyword in C   because it s an optimization that can usually be left to the compiler  especially in JIT ed languages  The JIT compiler has access to runtime statistics which enables it to decide what to inline much more efficiently than you can when writing the code  A function will be inlined if the compiler decides to  and there s nothing you can do about it either way

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C  does not support inline methods  or functions  in the way dynamic languages like python do  However anonymous methods and lambdas can be used for similar purposes including when you need to access a variable in the containing method like in the example below   static void Main string   args        int a   1       Action inline        gt  a        inline          here a   2

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No  there is no such construct in C   but the  NET JIT compiler could decide to do inline function calls on JIT time  But i actually don t know if it is really doing such optimizations   I think it should  -

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Cody has it right  but I want to provide an example of what an inline function is   Let s say you have this code   private void OutputItem string x        Console WriteLine x          maybe encapsulate additional logic to decide         whether to also write the message to Trace or a log file    public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        let s pretend IEnumerable lt T gt  ToList   doesn t exist for the moment     IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y           OutputItem y             return result      The compilerJust-In-Time optimizer could choose to alter the code to avoid repeatedly placing a call to OutputItem   on the stack  so that it would be as if you had written the code like this instead   public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y               full OutputItem   implementation is placed here         Console WriteLine y                 return result      In this case  we would say the OutputItem   function was inlined   Note that it might do this even if the OutputItem   is called from other places as well   Edited to show a scenario more-likely to be inlined

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Cody has it right  but I want to provide an example of what an inline function is   Let s say you have this code   private void OutputItem string x        Console WriteLine x          maybe encapsulate additional logic to decide         whether to also write the message to Trace or a log file    public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        let s pretend IEnumerable lt T gt  ToList   doesn t exist for the moment     IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y           OutputItem y             return result      The compilerJust-In-Time optimizer could choose to alter the code to avoid repeatedly placing a call to OutputItem   on the stack  so that it would be as if you had written the code like this instead   public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y               full OutputItem   implementation is placed here         Console WriteLine y                 return result      In this case  we would say the OutputItem   function was inlined   Note that it might do this even if the OutputItem   is called from other places as well   Edited to show a scenario more-likely to be inlined

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Cody has it right  but I want to provide an example of what an inline function is   Let s say you have this code   private void OutputItem string x        Console WriteLine x          maybe encapsulate additional logic to decide         whether to also write the message to Trace or a log file    public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        let s pretend IEnumerable lt T gt  ToList   doesn t exist for the moment     IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y           OutputItem y             return result      The compilerJust-In-Time optimizer could choose to alter the code to avoid repeatedly placing a call to OutputItem   on the stack  so that it would be as if you had written the code like this instead   public IList lt string gt  BuildListAndOutput IEnumerable lt string gt  x        IList lt string gt  result   new List lt string gt          foreach string y in x                result Add y               full OutputItem   implementation is placed here         Console WriteLine y                 return result      In this case  we would say the OutputItem   function was inlined   Note that it might do this even if the OutputItem   is called from other places as well   Edited to show a scenario more-likely to be inlined

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Do you mean inline functions in the C   sense   In which the contents of a normal function are automatically copied inline into the callsite   The end effect being that no function call actually happens when calling a function   Example   inline int Add int left  int right    return left   right      If so then no  there is no C  equivalent to this     Or Do you mean functions that are declared within another function   If so then yes  C  supports this via anonymous methods or lambda expressions     Example   static void Example       Func lt int int int gt  add    x y    gt  x   y    var result   add 4 6       10

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In case your assemblies will be ngen-ed  you might want to take a look at TargetedPatchingOptOut  This will help ngen decide whether to inline methods  MSDN reference  It is still only a declarative hint to optimize though  not an imperative command

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In case your assemblies will be ngen-ed  you might want to take a look at TargetedPatchingOptOut  This will help ngen decide whether to inline methods  MSDN reference  It is still only a declarative hint to optimize though  not an imperative command

User · Answer

No  there is no such construct in C   but the  NET JIT compiler could decide to do inline function calls on JIT time  But i actually don t know if it is really doing such optimizations   I think it should  -

User · Answer

No  there is no such construct in C   but the  NET JIT compiler could decide to do inline function calls on JIT time  But i actually don t know if it is really doing such optimizations   I think it should  -

User · Answer

You re mixing up two separate concepts  Function inlining is a compiler optimization which has no impact on the semantics  A function behaves the same whether it s inlined or not   On the other hand  lambda functions are purely a semantic concept  There is no requirement on how they should be implemented or executed  as long as they follow the behavior set out in the language spec  They can be inlined if the JIT compiler feels like it  or not if it doesn t   There is no inline keyword in C   because it s an optimization that can usually be left to the compiler  especially in JIT ed languages  The JIT compiler has access to runtime statistics which enables it to decide what to inline much more efficiently than you can when writing the code  A function will be inlined if the compiler decides to  and there s nothing you can do about it either way

User · Answer

Inline methods are simply a compiler optimization where the code of a function is rolled into the caller   There s no mechanism by which to do this in C   and they re to be used sparingly in languages where they are supported -- if you don t know why they should be used somewhere  they shouldn t be   Edit  To clarify  there are two major reasons they need to be used sparingly    It s easy to make massive binaries by using inline in cases where it s not necessary The compiler tends to know better than you do when something should  from a performance standpoint  be inlined   It s best to leave things alone and let the compiler do its work  then profile and figure out if inline is the best solution for you   Of course  some things just make sense to be inlined  mathematical operators particularly   but letting the compiler handle it is typically the best practice

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There are occasions where I do wish to force code to be in-lined   For example if I have a complex routine where there are a large number of decisions made within a highly iterative block and those decisions result in similar but slightly differing actions to be carried out  Consider for example  a complex  non DB driven  sort comparer where the sorting algorythm sorts the elements according to a number of different unrelated criteria such as one might do if they were sorting words according to gramatical as well as semantic criteria for a fast language recognition system   I would tend to write helper functions to handle those actions in order to maintain the readability and modularity of the source code   I know that those helper functions should be in-lined because that is the way that the code would be written if it never had to be understood by a human  I would certainly want to ensure in this case that there were no function calling overhead

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Lambda expressions are inline functions  I think  that C  doesn t have a extra attribute like inline or something like that

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Yes Exactly  the only distinction is the fact it returns a value   Simplification  not using expressions    List lt T gt  ForEach Takes an action  it doesn t expect a return result   So an Action lt T gt  delegate would suffice   say   List lt T gt  ForEach param   gt  Console WriteLine param      is the same as saying   List lt T gt  ForEach delegate T param    Console WriteLine param         the difference is that the param type and delegate decleration are inferred by usage and the braces aren t required on a simple inline method   Where as  List lt T gt  Where Takes a function  expecting a result   So an Function lt T  bool gt  would be expected   List lt T gt  Where param   gt  param Value    SomeExpectedComparison     which is the same as   List lt T gt  Where delegate T param    return param Value    SomeExpectedComparison        You can also declare these methods inline and asign them to variables IE   Action myAction        gt  Console WriteLine  I m doing something Nifty      myAction      or  Function lt object  string gt  myFunction   theObject   gt  theObject ToString     string myString   myFunction someObject     I hope this helps

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Finally in  NET 4 5  the CLR allows one to hint suggest1 method inlining using MethodImplOptions AggressiveInlining value  It is also available in the Mono s trunk  committed today       The full attribute usage is in mscorlib dll     so should not need to include extra references using System Runtime CompilerServices          MethodImpl MethodImplOptions AggressiveInlining   void MyMethod        1  Previously  force  was used here  Since there were a few downvotes  I ll try to clarify the term  As in the comments and the documentation  The method should be inlined if possible  Especially considering Mono  which is open   there are some mono-specific technical limitations considering inlining or more general one  like virtual functions   Overall  yes  this is a hint to compiler  but I guess that is what was asked for

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C  does not support inline methods  or functions  in the way dynamic languages like python do  However anonymous methods and lambdas can be used for similar purposes including when you need to access a variable in the containing method like in the example below   static void Main string   args        int a   1       Action inline        gt  a        inline          here a   2

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There are occasions where I do wish to force code to be in-lined   For example if I have a complex routine where there are a large number of decisions made within a highly iterative block and those decisions result in similar but slightly differing actions to be carried out  Consider for example  a complex  non DB driven  sort comparer where the sorting algorythm sorts the elements according to a number of different unrelated criteria such as one might do if they were sorting words according to gramatical as well as semantic criteria for a fast language recognition system   I would tend to write helper functions to handle those actions in order to maintain the readability and modularity of the source code   I know that those helper functions should be in-lined because that is the way that the code would be written if it never had to be understood by a human  I would certainly want to ensure in this case that there were no function calling overhead

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Yes Exactly  the only distinction is the fact it returns a value   Simplification  not using expressions    List lt T gt  ForEach Takes an action  it doesn t expect a return result   So an Action lt T gt  delegate would suffice   say   List lt T gt  ForEach param   gt  Console WriteLine param      is the same as saying   List lt T gt  ForEach delegate T param    Console WriteLine param         the difference is that the param type and delegate decleration are inferred by usage and the braces aren t required on a simple inline method   Where as  List lt T gt  Where Takes a function  expecting a result   So an Function lt T  bool gt  would be expected   List lt T gt  Where param   gt  param Value    SomeExpectedComparison     which is the same as   List lt T gt  Where delegate T param    return param Value    SomeExpectedComparison        You can also declare these methods inline and asign them to variables IE   Action myAction        gt  Console WriteLine  I m doing something Nifty      myAction      or  Function lt object  string gt  myFunction   theObject   gt  theObject ToString     string myString   myFunction someObject     I hope this helps

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Do you mean inline functions in the C   sense   In which the contents of a normal function are automatically copied inline into the callsite   The end effect being that no function call actually happens when calling a function   Example   inline int Add int left  int right    return left   right      If so then no  there is no C  equivalent to this     Or Do you mean functions that are declared within another function   If so then yes  C  supports this via anonymous methods or lambda expressions     Example   static void Example       Func lt int int int gt  add    x y    gt  x   y    var result   add 4 6       10

User · Answer

Lambda expressions are inline functions  I think  that C  doesn t have a extra attribute like inline or something like that

User · Answer

C  does not support inline methods  or functions  in the way dynamic languages like python do  However anonymous methods and lambdas can be used for similar purposes including when you need to access a variable in the containing method like in the example below   static void Main string   args        int a   1       Action inline        gt  a        inline          here a   2

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Yes Exactly  the only distinction is the fact it returns a value   Simplification  not using expressions    List lt T gt  ForEach Takes an action  it doesn t expect a return result   So an Action lt T gt  delegate would suffice   say   List lt T gt  ForEach param   gt  Console WriteLine param      is the same as saying   List lt T gt  ForEach delegate T param    Console WriteLine param         the difference is that the param type and delegate decleration are inferred by usage and the braces aren t required on a simple inline method   Where as  List lt T gt  Where Takes a function  expecting a result   So an Function lt T  bool gt  would be expected   List lt T gt  Where param   gt  param Value    SomeExpectedComparison     which is the same as   List lt T gt  Where delegate T param    return param Value    SomeExpectedComparison        You can also declare these methods inline and asign them to variables IE   Action myAction        gt  Console WriteLine  I m doing something Nifty      myAction      or  Function lt object  string gt  myFunction   theObject   gt  theObject ToString     string myString   myFunction someObject     I hope this helps

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C  does not support inline methods  or functions  in the way dynamic languages like python do  However anonymous methods and lambdas can be used for similar purposes including when you need to access a variable in the containing method like in the example below   static void Main string   args        int a   1       Action inline        gt  a        inline          here a   2

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