Why does C compilation take so long

Question

Compiling a C   file takes a very long time when compared to C  and Java  It takes significantly longer to compile a C   file than it would to run a normal size Python script  I m currently using VC   but it s the same with any compiler  Why is this   The two reasons I could think of were loading header files and running the preprocessor  but that doesn t seem like it should explain why it takes so long

User · Answer

Another reason is the use of the C pre-processor for locating declarations. Even with header guards, .h still have to be parsed over and over, every time they're included. Some compilers support pre-compiled headers that can help with this, but they are not always used.

See also: C++ Frequently Questioned Answers

User · Answer

The slowdown is not necessarily the same with any compiler   I haven t used Delphi or Kylix but back in the MS-DOS days  a Turbo Pascal program would compile almost instantaneously  while the equivalent Turbo C   program would just crawl    The two main differences were a very strong module system and a syntax that allowed single-pass compilation   It s certainly possible that compilation speed just hasn t been a priority for C   compiler developers  but there are also some inherent complications in the C C   syntax that make it more difficult to process   I m not an expert on C  but Walter Bright is  and after building various commercial C C   compilers  he created the D language  One of his changes was to enforce a context-free grammar to make the language easier to parse    Also  you ll notice that generally Makefiles are set up so that every file is compiled separately in C  so if 10 source files all use the same include file  that include file is processed 10 times

User · Answer

Parsing and code generation are actually rather fast   The real problem is opening and closing files   Remember  even with include guards  the compiler still have open the  H file  and read each line  and then ignore it    A friend once  while bored at work   took his company s application and put everything -- all source and header files--  into one big file    Compile time dropped from 3 hours to 7 minutes

User · Answer

There are two issues I can think of that might be affecting the speed at which your programs in C   are compiling   POSSIBLE ISSUE  1 - COMPILING THE HEADER   This may or may not have already been addressed by another answer or comment   Microsoft Visual C    A K A  VC    supports precompiled headers  which I highly recommend  When you create a new project and select the type of program you are making  a setup wizard window should appear on your screen  If you hit the    Next      button at the bottom of it  the window will take you to a page that has several lists of features  make sure that the box next to the    Precompiled header    option is checked   NOTE  This has been my experience with Win32 console applications in C    but this may not be the case with all kinds of programs in C      POSSIBLE ISSUE  2 - THE LOCATION BEING COMPILED TO  This summer  I took a programming course  and we had to store all of our projects on 8GB flash drives  as the computers in the lab we were using got wiped every night at midnight  which would have erased all of our work  If you are compiling to an external storage device for the sake of portability security etc   it can take a very long time  even with the precompiled headers that I mentioned above  for your program to compile  especially if it   s a fairly large program  My advice for you in this case would be to create and compile programs on the hard drive of the computer you   re using  and whenever you want need to stop working on your project s  for whatever reason  transfer them to your external storage device  and then click the    Safely Remove Hardware and Eject Media    icon  which should appear as a small flash drive behind a little green circle with a white check mark on it  to disconnect it   I hope this helps you  let me know if it does

User · Answer

A compiled language is always going to require a bigger initial overhead than an interpreted language  In addition  perhaps you didn t structure your C   code very well  For example    include  BigClass h   class SmallClass      BigClass m bigClass      Compiles a lot slower than   class BigClass   class SmallClass      BigClass  m bigClass

User · Answer

Building C C    what really happens and why does it take so long  A relatively large portion of software development time is not spent on writing  running  debugging or even designing code  but waiting for it to finish compiling  In order to make things fast  we first have to understand what is happening when C C   software is compiled  The steps are roughly as follows    Configuration Build tool startup Dependency checking Compilation Linking   We will now look at each step in more detail focusing on how they can be made faster   Configuration  This is the first step when starting to build  Usually means running a configure script or CMake  Gyp  SCons or some other tool  This can take anything from one second to several minutes for very large Autotools-based configure scripts   This step happens relatively rarely  It only needs to be run when changing configurations or changing the build configuration  Short of changing build systems  there is not much to be done to make this step faster   Build tool startup  This is what happens when you run make or click on the build icon on an IDE  which is usually an alias for make   The build tool binary starts and reads its configuration files as well as the build configuration  which are usually the same thing   Depending on build complexity and size  this can take anywhere from a fraction of a second to several seconds  By itself this would not be so bad  Unfortunately most make-based build systems cause make to be invocated tens to hundreds of times for every single build  Usually this is caused by recursive use of make  which is bad    It should be noted that the reason Make is so slow is not an implementation bug  The syntax of Makefiles has some quirks that make a really fast implementation all but impossible  This problem is even more noticeable when combined with the next step   Dependency checking  Once the build tool has read its configuration  it has to determine what files have changed and which ones need to be recompiled  The configuration files contain a directed acyclic graph describing the build dependencies  This graph is usually built during the configure step  Build tool startup time and the dependency scanner are run on every single build  Their combined runtime determines the lower bound on the edit-compile-debug cycle  For small projects this time is usually a few seconds or so  This is tolerable  There are alternatives to Make  The fastest of them is Ninja  which was built by Google engineers for Chromium  If you are using CMake or Gyp to build  just switch to their Ninja backends  You don   t have to change anything in the build files themselves  just enjoy the speed boost  Ninja is not packaged on most distributions  though  so you might have to install it yourself   Compilation  At this point we finally invoke the compiler  Cutting some corners  here are the approximate steps taken    Merging includes Parsing the code Code generation optimization   Contrary to popular belief  compiling C   is not actually all that slow  The STL is slow and most build tools used to compile C   are slow  However there are faster tools and ways to mitigate the slow parts of the language   Using them takes a bit of elbow grease  but the benefits are undeniable  Faster build times lead to happier developers  more agility and  eventually  better code

User · Answer

Several reasons  Header files  Every single compilation unit requires hundreds or even thousands of headers to be  1  loaded and  2  compiled  Every one of them typically has to be recompiled for every compilation unit  because the preprocessor ensures that the result of compiling a header might vary between every compilation unit   A macro may be defined in one compilation unit which changes the content of the header    This is probably the main reason  as it requires huge amounts of code to be compiled for every compilation unit  and additionally  every header has to be compiled multiple times  once for every compilation unit that includes it    Linking  Once compiled  all the object files have to be linked together    This is basically a monolithic process that can t very well be parallelized  and has to process your entire project   Parsing  The syntax is extremely complicated to parse  depends heavily on context  and is very hard to disambiguate  This takes a lot of time   Templates  In C   List lt T gt  is the only type that is compiled  no matter how many instantiations of List you have in your program  In C    vector lt int gt  is a completely separate type from vector lt float gt   and each one will have to be compiled separately   Add to this that templates make up a full Turing-complete  sub-language  that the compiler has to interpret  and this can become ridiculously complicated  Even relatively simple template metaprogramming code can define recursive templates that create dozens and dozens of template instantiations  Templates may also result in extremely complex types  with ridiculously long names  adding a lot of extra work to the linker   It has to compare a lot of symbol names  and if these names can grow into many thousand characters  that can become fairly expensive    And of course  they exacerbate the problems with header files  because templates generally have to be defined in headers  which means far more code has to be parsed and compiled for every compilation unit  In plain C code  a header typically only contains forward declarations  but very little actual code  In C    it is not uncommon for almost all the code to reside in header files   Optimization  C   allows for some very dramatic optimizations  C  or Java don t allow classes to be completely eliminated  they have to be there for reflection purposes   but even a simple C   template metaprogram can easily generate dozens or hundreds of classes  all of which are inlined and eliminated again in the optimization phase   Moreover  a C   program must be fully optimized by the compiler  A C  program can rely on the JIT compiler to perform additional optimizations at load-time  C   doesn t get any such  second chances   What the compiler generates is as optimized as it s going to get   Machine  C   is compiled to machine code which may be somewhat more complicated than the bytecode Java or  NET use  especially in the case of x86    This is mentioned out of completeness only because it was mentioned in comments and such  In practice  this step is unlikely to take more than a tiny fraction of the total compilation time    Conclusion  Most of these factors are shared by C code  which actually compiles fairly efficiently  The parsing step is a lot more complicated in C    and can take up significantly more time  but the main offender is probably templates  They re useful  and make C   a far more powerful language  but they also take their toll in terms of compilation speed

User · Answer

A compiled language is always going to require a bigger initial overhead than an interpreted language  In addition  perhaps you didn t structure your C   code very well  For example    include  BigClass h   class SmallClass      BigClass m bigClass      Compiles a lot slower than   class BigClass   class SmallClass      BigClass  m bigClass

User · Answer

C   is compiled into machine code  So you have the pre-processor  the compiler  the optimizer  and finally the assembler  all of which have to run   Java and C  are compiled into byte-code IL  and the Java virtual machine  NET Framework execute  or JIT compile into machine code  prior to execution   Python is an interpreted language that is also compiled into byte-code   I m sure there are other reasons for this as well  but in general  not having to compile to native machine language saves time

User · Answer

C   is compiled into machine code  So you have the pre-processor  the compiler  the optimizer  and finally the assembler  all of which have to run   Java and C  are compiled into byte-code IL  and the Java virtual machine  NET Framework execute  or JIT compile into machine code  prior to execution   Python is an interpreted language that is also compiled into byte-code   I m sure there are other reasons for this as well  but in general  not having to compile to native machine language saves time

User · Answer

Some reasons are   1  C   grammar is more complex than C  or Java and takes more time to parse   2   More important  C   compiler produces machine code and does all optimizations during compilation  C  and Java go just half way and leave these steps to JIT

User · Answer

An easy way to reduce compilation time in larger C   projects is to make a   cpp include file that includes all the cpp files in your project and compile that   This reduces the header explosion problem to once   The advantage of this is that compilation errors will still reference the correct file    For example  assume you have a cpp  b cpp and c cpp   create a file  everything cpp    include  a cpp   include  b cpp   include  c cpp    Then compile the project by just making everything cpp

User · Answer

Some reasons are   1  C   grammar is more complex than C  or Java and takes more time to parse   2   More important  C   compiler produces machine code and does all optimizations during compilation  C  and Java go just half way and leave these steps to JIT

User · Answer

An easy way to reduce compilation time in larger C   projects is to make a   cpp include file that includes all the cpp files in your project and compile that   This reduces the header explosion problem to once   The advantage of this is that compilation errors will still reference the correct file    For example  assume you have a cpp  b cpp and c cpp   create a file  everything cpp    include  a cpp   include  b cpp   include  c cpp    Then compile the project by just making everything cpp

User · Answer

Parsing and code generation are actually rather fast   The real problem is opening and closing files   Remember  even with include guards  the compiler still have open the  H file  and read each line  and then ignore it    A friend once  while bored at work   took his company s application and put everything -- all source and header files--  into one big file    Compile time dropped from 3 hours to 7 minutes

User · Answer

Several reasons  Header files  Every single compilation unit requires hundreds or even thousands of headers to be  1  loaded and  2  compiled  Every one of them typically has to be recompiled for every compilation unit  because the preprocessor ensures that the result of compiling a header might vary between every compilation unit   A macro may be defined in one compilation unit which changes the content of the header    This is probably the main reason  as it requires huge amounts of code to be compiled for every compilation unit  and additionally  every header has to be compiled multiple times  once for every compilation unit that includes it    Linking  Once compiled  all the object files have to be linked together    This is basically a monolithic process that can t very well be parallelized  and has to process your entire project   Parsing  The syntax is extremely complicated to parse  depends heavily on context  and is very hard to disambiguate  This takes a lot of time   Templates  In C   List lt T gt  is the only type that is compiled  no matter how many instantiations of List you have in your program  In C    vector lt int gt  is a completely separate type from vector lt float gt   and each one will have to be compiled separately   Add to this that templates make up a full Turing-complete  sub-language  that the compiler has to interpret  and this can become ridiculously complicated  Even relatively simple template metaprogramming code can define recursive templates that create dozens and dozens of template instantiations  Templates may also result in extremely complex types  with ridiculously long names  adding a lot of extra work to the linker   It has to compare a lot of symbol names  and if these names can grow into many thousand characters  that can become fairly expensive    And of course  they exacerbate the problems with header files  because templates generally have to be defined in headers  which means far more code has to be parsed and compiled for every compilation unit  In plain C code  a header typically only contains forward declarations  but very little actual code  In C    it is not uncommon for almost all the code to reside in header files   Optimization  C   allows for some very dramatic optimizations  C  or Java don t allow classes to be completely eliminated  they have to be there for reflection purposes   but even a simple C   template metaprogram can easily generate dozens or hundreds of classes  all of which are inlined and eliminated again in the optimization phase   Moreover  a C   program must be fully optimized by the compiler  A C  program can rely on the JIT compiler to perform additional optimizations at load-time  C   doesn t get any such  second chances   What the compiler generates is as optimized as it s going to get   Machine  C   is compiled to machine code which may be somewhat more complicated than the bytecode Java or  NET use  especially in the case of x86    This is mentioned out of completeness only because it was mentioned in comments and such  In practice  this step is unlikely to take more than a tiny fraction of the total compilation time    Conclusion  Most of these factors are shared by C code  which actually compiles fairly efficiently  The parsing step is a lot more complicated in C    and can take up significantly more time  but the main offender is probably templates  They re useful  and make C   a far more powerful language  but they also take their toll in terms of compilation speed

User · Answer

C   is compiled into machine code  So you have the pre-processor  the compiler  the optimizer  and finally the assembler  all of which have to run   Java and C  are compiled into byte-code IL  and the Java virtual machine  NET Framework execute  or JIT compile into machine code  prior to execution   Python is an interpreted language that is also compiled into byte-code   I m sure there are other reasons for this as well  but in general  not having to compile to native machine language saves time

User · Answer

The trade off you are getting is that the program runs a wee bit faster  That may be a cold comfort to you during development  but it could matter a great deal once development is complete  and the program is just being run by users

User · Answer

Several reasons  Header files  Every single compilation unit requires hundreds or even thousands of headers to be  1  loaded and  2  compiled  Every one of them typically has to be recompiled for every compilation unit  because the preprocessor ensures that the result of compiling a header might vary between every compilation unit   A macro may be defined in one compilation unit which changes the content of the header    This is probably the main reason  as it requires huge amounts of code to be compiled for every compilation unit  and additionally  every header has to be compiled multiple times  once for every compilation unit that includes it    Linking  Once compiled  all the object files have to be linked together    This is basically a monolithic process that can t very well be parallelized  and has to process your entire project   Parsing  The syntax is extremely complicated to parse  depends heavily on context  and is very hard to disambiguate  This takes a lot of time   Templates  In C   List lt T gt  is the only type that is compiled  no matter how many instantiations of List you have in your program  In C    vector lt int gt  is a completely separate type from vector lt float gt   and each one will have to be compiled separately   Add to this that templates make up a full Turing-complete  sub-language  that the compiler has to interpret  and this can become ridiculously complicated  Even relatively simple template metaprogramming code can define recursive templates that create dozens and dozens of template instantiations  Templates may also result in extremely complex types  with ridiculously long names  adding a lot of extra work to the linker   It has to compare a lot of symbol names  and if these names can grow into many thousand characters  that can become fairly expensive    And of course  they exacerbate the problems with header files  because templates generally have to be defined in headers  which means far more code has to be parsed and compiled for every compilation unit  In plain C code  a header typically only contains forward declarations  but very little actual code  In C    it is not uncommon for almost all the code to reside in header files   Optimization  C   allows for some very dramatic optimizations  C  or Java don t allow classes to be completely eliminated  they have to be there for reflection purposes   but even a simple C   template metaprogram can easily generate dozens or hundreds of classes  all of which are inlined and eliminated again in the optimization phase   Moreover  a C   program must be fully optimized by the compiler  A C  program can rely on the JIT compiler to perform additional optimizations at load-time  C   doesn t get any such  second chances   What the compiler generates is as optimized as it s going to get   Machine  C   is compiled to machine code which may be somewhat more complicated than the bytecode Java or  NET use  especially in the case of x86    This is mentioned out of completeness only because it was mentioned in comments and such  In practice  this step is unlikely to take more than a tiny fraction of the total compilation time    Conclusion  Most of these factors are shared by C code  which actually compiles fairly efficiently  The parsing step is a lot more complicated in C    and can take up significantly more time  but the main offender is probably templates  They re useful  and make C   a far more powerful language  but they also take their toll in terms of compilation speed

User · Answer

The trade off you are getting is that the program runs a wee bit faster  That may be a cold comfort to you during development  but it could matter a great deal once development is complete  and the program is just being run by users

User · Answer

The slowdown is not necessarily the same with any compiler   I haven t used Delphi or Kylix but back in the MS-DOS days  a Turbo Pascal program would compile almost instantaneously  while the equivalent Turbo C   program would just crawl    The two main differences were a very strong module system and a syntax that allowed single-pass compilation   It s certainly possible that compilation speed just hasn t been a priority for C   compiler developers  but there are also some inherent complications in the C C   syntax that make it more difficult to process   I m not an expert on C  but Walter Bright is  and after building various commercial C C   compilers  he created the D language  One of his changes was to enforce a context-free grammar to make the language easier to parse    Also  you ll notice that generally Makefiles are set up so that every file is compiled separately in C  so if 10 source files all use the same include file  that include file is processed 10 times

User · Answer

Building C C    what really happens and why does it take so long  A relatively large portion of software development time is not spent on writing  running  debugging or even designing code  but waiting for it to finish compiling  In order to make things fast  we first have to understand what is happening when C C   software is compiled  The steps are roughly as follows    Configuration Build tool startup Dependency checking Compilation Linking   We will now look at each step in more detail focusing on how they can be made faster   Configuration  This is the first step when starting to build  Usually means running a configure script or CMake  Gyp  SCons or some other tool  This can take anything from one second to several minutes for very large Autotools-based configure scripts   This step happens relatively rarely  It only needs to be run when changing configurations or changing the build configuration  Short of changing build systems  there is not much to be done to make this step faster   Build tool startup  This is what happens when you run make or click on the build icon on an IDE  which is usually an alias for make   The build tool binary starts and reads its configuration files as well as the build configuration  which are usually the same thing   Depending on build complexity and size  this can take anywhere from a fraction of a second to several seconds  By itself this would not be so bad  Unfortunately most make-based build systems cause make to be invocated tens to hundreds of times for every single build  Usually this is caused by recursive use of make  which is bad    It should be noted that the reason Make is so slow is not an implementation bug  The syntax of Makefiles has some quirks that make a really fast implementation all but impossible  This problem is even more noticeable when combined with the next step   Dependency checking  Once the build tool has read its configuration  it has to determine what files have changed and which ones need to be recompiled  The configuration files contain a directed acyclic graph describing the build dependencies  This graph is usually built during the configure step  Build tool startup time and the dependency scanner are run on every single build  Their combined runtime determines the lower bound on the edit-compile-debug cycle  For small projects this time is usually a few seconds or so  This is tolerable  There are alternatives to Make  The fastest of them is Ninja  which was built by Google engineers for Chromium  If you are using CMake or Gyp to build  just switch to their Ninja backends  You don   t have to change anything in the build files themselves  just enjoy the speed boost  Ninja is not packaged on most distributions  though  so you might have to install it yourself   Compilation  At this point we finally invoke the compiler  Cutting some corners  here are the approximate steps taken    Merging includes Parsing the code Code generation optimization   Contrary to popular belief  compiling C   is not actually all that slow  The STL is slow and most build tools used to compile C   are slow  However there are faster tools and ways to mitigate the slow parts of the language   Using them takes a bit of elbow grease  but the benefits are undeniable  Faster build times lead to happier developers  more agility and  eventually  better code

User · Answer

The trade off you are getting is that the program runs a wee bit faster  That may be a cold comfort to you during development  but it could matter a great deal once development is complete  and the program is just being run by users

User · Answer

Another reason is the use of the C pre-processor for locating declarations. Even with header guards, .h still have to be parsed over and over, every time they're included. Some compilers support pre-compiled headers that can help with this, but they are not always used.

See also: C++ Frequently Questioned Answers

User · Answer

The slowdown is not necessarily the same with any compiler   I haven t used Delphi or Kylix but back in the MS-DOS days  a Turbo Pascal program would compile almost instantaneously  while the equivalent Turbo C   program would just crawl    The two main differences were a very strong module system and a syntax that allowed single-pass compilation   It s certainly possible that compilation speed just hasn t been a priority for C   compiler developers  but there are also some inherent complications in the C C   syntax that make it more difficult to process   I m not an expert on C  but Walter Bright is  and after building various commercial C C   compilers  he created the D language  One of his changes was to enforce a context-free grammar to make the language easier to parse    Also  you ll notice that generally Makefiles are set up so that every file is compiled separately in C  so if 10 source files all use the same include file  that include file is processed 10 times

User · Answer

Most answers are being a bit unclear in mentioning that C  will always run slower due to the cost of performing actions that in C   are performed only once at compile time  this performance cost is also impacted due runtime dependencies  more things to load to be able to run   not to mention that C  programs will always have higher memory footprint  all resulting in performance being more closely related to the capability of hardware available  The same is true to other languages that are interpreted or depend on a VM

User · Answer

A compiled language is always going to require a bigger initial overhead than an interpreted language  In addition  perhaps you didn t structure your C   code very well  For example    include  BigClass h   class SmallClass      BigClass m bigClass      Compiles a lot slower than   class BigClass   class SmallClass      BigClass  m bigClass

User · Answer

The biggest issues are   1  The infinite header reparsing  Already mentioned  Mitigations  like  pragma once  usually only work per compilation unit  not per build    2  The fact that the toolchain is often separated into multiple binaries  make  preprocessor  compiler  assembler  archiver  impdef  linker  and dlltool in extreme cases  that all have to reinitialize and reload all state all the time for each invocation  compiler  assembler  or every couple of files  archiver  linker  and dlltool    See also this discussion on comp compilers  http   compilers iecc com comparch article 03-11-078 specially this one   http   compilers iecc com comparch article 02-07-128  Note that John  the moderator of comp compilers seems to agree  and that this means it should be possible to achieve similar speeds for C too  if one integrates the toolchain fully and implements precompiled headers  Many commercial C compilers do this to some degree   Note that the Unix model of factoring everything out to a separate binary is a kind of the worst case model for Windows  with its slow process creation   It is very noticable when comparing GCC build times between Windows and  nix  especially if the make configure system also calls some programs just to obtain information

User · Answer

The trade off you are getting is that the program runs a wee bit faster  That may be a cold comfort to you during development  but it could matter a great deal once development is complete  and the program is just being run by users

User · Answer

In large object-oriented projects  the significant reason is that C   makes it hard to confine dependencies  Private functions need to be listed in their respective class  public header  which makes dependencies more transitive  contagious  than they need to be     Ugly private dependencies  include  lt map gt   include  lt list gt   include  lt chrono gt   include  lt stdio h gt   include  lt Internal SecretArea h gt   include  lt ThirdParty GodObjectFactory h gt   class ICantHelpButShowMyPrivatePartsSorry   public      int facade int    private      std  map lt int  int gt  implementation detail 1 std  list lt int gt        std  chrono  years implementation detail 2 FILE        Intern  SecretArea implementation detail 3 const GodObjectFactory amp        If this pattern is blissfully repeated into dependency trees of headers  this tends to create a few  quot god headers quot  that indirectly include large portions of all headers in a project  They are as all-knowing as god objects  except that this is not apparent until you draw their inclusion trees  This adds to compilation time in 2 ways   The amount of code they add to each compilation unit   cpp file  that includes them is easily many times more than the cpp files themselves  To put this in perspective  catch2 hpp is 18000 lines  whereas most people  even IDEs  start to struggle editing files larger than 1000-10000 lines  The number of files that have to be recompiled when a header is edited is not contained to the true set of files that depend on it   Yes  there are mitigations  like forward declaration  which has perceived downsides  or the pimpl idiom  which is a nonzero cost abstraction  Even though C   is limitless in what you can do  your peers will be wondering what you have been smoking if you go too far from how it s meant to be  The worst part  If you think about it  the need to declare private functions in their public header is not even necessary  The moral equivalent of member functions can be  and commonly is  mimicked in C  which does not recreate this problem

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In large object-oriented projects  the significant reason is that C   makes it hard to confine dependencies  Private functions need to be listed in their respective class  public header  which makes dependencies more transitive  contagious  than they need to be     Ugly private dependencies  include  lt map gt   include  lt list gt   include  lt chrono gt   include  lt stdio h gt   include  lt Internal SecretArea h gt   include  lt ThirdParty GodObjectFactory h gt   class ICantHelpButShowMyPrivatePartsSorry   public      int facade int    private      std  map lt int  int gt  implementation detail 1 std  list lt int gt        std  chrono  years implementation detail 2 FILE        Intern  SecretArea implementation detail 3 const GodObjectFactory amp        If this pattern is blissfully repeated into dependency trees of headers  this tends to create a few  quot god headers quot  that indirectly include large portions of all headers in a project  They are as all-knowing as god objects  except that this is not apparent until you draw their inclusion trees  This adds to compilation time in 2 ways   The amount of code they add to each compilation unit   cpp file  that includes them is easily many times more than the cpp files themselves  To put this in perspective  catch2 hpp is 18000 lines  whereas most people  even IDEs  start to struggle editing files larger than 1000-10000 lines  The number of files that have to be recompiled when a header is edited is not contained to the true set of files that depend on it   Yes  there are mitigations  like forward declaration  which has perceived downsides  or the pimpl idiom  which is a nonzero cost abstraction  Even though C   is limitless in what you can do  your peers will be wondering what you have been smoking if you go too far from how it s meant to be  The worst part  If you think about it  the need to declare private functions in their public header is not even necessary  The moral equivalent of member functions can be  and commonly is  mimicked in C  which does not recreate this problem

User · Answer

Another reason is the use of the C pre-processor for locating declarations. Even with header guards, .h still have to be parsed over and over, every time they're included. Some compilers support pre-compiled headers that can help with this, but they are not always used.

See also: C++ Frequently Questioned Answers

User · Answer

Some reasons are   1  C   grammar is more complex than C  or Java and takes more time to parse   2   More important  C   compiler produces machine code and does all optimizations during compilation  C  and Java go just half way and leave these steps to JIT

User · Answer

The biggest issues are   1  The infinite header reparsing  Already mentioned  Mitigations  like  pragma once  usually only work per compilation unit  not per build    2  The fact that the toolchain is often separated into multiple binaries  make  preprocessor  compiler  assembler  archiver  impdef  linker  and dlltool in extreme cases  that all have to reinitialize and reload all state all the time for each invocation  compiler  assembler  or every couple of files  archiver  linker  and dlltool    See also this discussion on comp compilers  http   compilers iecc com comparch article 03-11-078 specially this one   http   compilers iecc com comparch article 02-07-128  Note that John  the moderator of comp compilers seems to agree  and that this means it should be possible to achieve similar speeds for C too  if one integrates the toolchain fully and implements precompiled headers  Many commercial C compilers do this to some degree   Note that the Unix model of factoring everything out to a separate binary is a kind of the worst case model for Windows  with its slow process creation   It is very noticable when comparing GCC build times between Windows and  nix  especially if the make configure system also calls some programs just to obtain information

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There are two issues I can think of that might be affecting the speed at which your programs in C   are compiling   POSSIBLE ISSUE  1 - COMPILING THE HEADER   This may or may not have already been addressed by another answer or comment   Microsoft Visual C    A K A  VC    supports precompiled headers  which I highly recommend  When you create a new project and select the type of program you are making  a setup wizard window should appear on your screen  If you hit the    Next      button at the bottom of it  the window will take you to a page that has several lists of features  make sure that the box next to the    Precompiled header    option is checked   NOTE  This has been my experience with Win32 console applications in C    but this may not be the case with all kinds of programs in C      POSSIBLE ISSUE  2 - THE LOCATION BEING COMPILED TO  This summer  I took a programming course  and we had to store all of our projects on 8GB flash drives  as the computers in the lab we were using got wiped every night at midnight  which would have erased all of our work  If you are compiling to an external storage device for the sake of portability security etc   it can take a very long time  even with the precompiled headers that I mentioned above  for your program to compile  especially if it   s a fairly large program  My advice for you in this case would be to create and compile programs on the hard drive of the computer you   re using  and whenever you want need to stop working on your project s  for whatever reason  transfer them to your external storage device  and then click the    Safely Remove Hardware and Eject Media    icon  which should appear as a small flash drive behind a little green circle with a white check mark on it  to disconnect it   I hope this helps you  let me know if it does

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The slowdown is not necessarily the same with any compiler   I haven t used Delphi or Kylix but back in the MS-DOS days  a Turbo Pascal program would compile almost instantaneously  while the equivalent Turbo C   program would just crawl    The two main differences were a very strong module system and a syntax that allowed single-pass compilation   It s certainly possible that compilation speed just hasn t been a priority for C   compiler developers  but there are also some inherent complications in the C C   syntax that make it more difficult to process   I m not an expert on C  but Walter Bright is  and after building various commercial C C   compilers  he created the D language  One of his changes was to enforce a context-free grammar to make the language easier to parse    Also  you ll notice that generally Makefiles are set up so that every file is compiled separately in C  so if 10 source files all use the same include file  that include file is processed 10 times

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To answer this question simply  C   is a much more complex language than other languages available on the market  It has a legacy inclusion model that parses code multiple times  and its templated libraries are not optimized for compilation speed  Grammar and ADL Let s have a look at the grammatical complexity of C   by considering a very simple example  x y   While you   d be likely to say that the above is an expression with multiplication  this is not necessarily the case in C    If x is a type  then the statement is  in fact  a pointer declaration  This means that C   grammar is context-sensitive  Here   s another example  foo lt x gt  a   Again  you might think this is a declaration of the variable  quot a quot  of type foo  but it could also be interpreted as   foo  lt  x   gt  a   which would make it a comparison expression  C   has a feature called Argument Dependent Lookup  ADL   ADL establishes the rules that govern how the compiler looks up a name  Consider the following example  namespace A    struct Aa       void foo Aa arg     namespace B    struct Bb      void foo A  Aa arg  Bb arg2     namespace C     struct Cc       void foo A  Aa arg  B  Bb arg2  C  Cc arg3      foo A  Aa    B  Bb    C  Cc      ADL rules state that we will be looking for the name  quot foo quot  considering all arguments of the function call  In this case  all of the functions named    foo    will be considered for overload resolution  This process might take time  especially if there are lots of function overloads  In a templated context  ADL rules become even more complicated   include This command is something that might significantly influence compilation times  Depending on the type of file you include  the preprocessor might copy only a couple of lines of code  or it might copy thousands  Furthermore  this command cannot be optimized by the compiler  You can copy different pieces of code that can be modified just before inclusion if the header file depends on macros  There are some solutions to these issues  You can use precompiled headers  which are the compiler s internal representation of what was parsed in the header  This can   t be done without the user s effort  however  because precompiled headers assume that headers are not macro dependent  The modules feature provides a language-level solution to this problem  It   s available from the C  20 release onward  Templates The compilation speed for templates is challenging  Each translation unit that uses templates needs to have them included  and the definitions of these templates need to be available  Some instantiations of templates end up in instantiations of other templates  In some extreme cases  template instantiation can consume lots of resources  A library that uses templates and that was not designed for compilation speed can become troublesome  as you can see in a comparison of metaprogramming libraries provided at this link  http   metaben ch   Their differences in compilation speed are significant  If you want to understand why some metaprogramming libraries are better for compilation times than others  check out this video about the Rule of Chiel  Conclusion C   is a slowly compiled language because compilation performance was not the highest priority when the language was initially developed  As a result  C   ended up with features that might be effective during runtime  but are not necessarily effective during compile time  P S     I work at Incredibuild  a software development acceleration company specializing in accelerating C   compilations  you are welcome to try it for free

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A compiled language is always going to require a bigger initial overhead than an interpreted language  In addition  perhaps you didn t structure your C   code very well  For example    include  BigClass h   class SmallClass      BigClass m bigClass      Compiles a lot slower than   class BigClass   class SmallClass      BigClass  m bigClass

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Another reason is the use of the C pre-processor for locating declarations. Even with header guards, .h still have to be parsed over and over, every time they're included. Some compilers support pre-compiled headers that can help with this, but they are not always used.

See also: C++ Frequently Questioned Answers

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Most answers are being a bit unclear in mentioning that C  will always run slower due to the cost of performing actions that in C   are performed only once at compile time  this performance cost is also impacted due runtime dependencies  more things to load to be able to run   not to mention that C  programs will always have higher memory footprint  all resulting in performance being more closely related to the capability of hardware available  The same is true to other languages that are interpreted or depend on a VM

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Parsing and code generation are actually rather fast   The real problem is opening and closing files   Remember  even with include guards  the compiler still have open the  H file  and read each line  and then ignore it    A friend once  while bored at work   took his company s application and put everything -- all source and header files--  into one big file    Compile time dropped from 3 hours to 7 minutes

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Some reasons are   1  C   grammar is more complex than C  or Java and takes more time to parse   2   More important  C   compiler produces machine code and does all optimizations during compilation  C  and Java go just half way and leave these steps to JIT

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C   is compiled into machine code  So you have the pre-processor  the compiler  the optimizer  and finally the assembler  all of which have to run   Java and C  are compiled into byte-code IL  and the Java virtual machine  NET Framework execute  or JIT compile into machine code  prior to execution   Python is an interpreted language that is also compiled into byte-code   I m sure there are other reasons for this as well  but in general  not having to compile to native machine language saves time

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Parsing and code generation are actually rather fast   The real problem is opening and closing files   Remember  even with include guards  the compiler still have open the  H file  and read each line  and then ignore it    A friend once  while bored at work   took his company s application and put everything -- all source and header files--  into one big file    Compile time dropped from 3 hours to 7 minutes

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Several reasons  Header files  Every single compilation unit requires hundreds or even thousands of headers to be  1  loaded and  2  compiled  Every one of them typically has to be recompiled for every compilation unit  because the preprocessor ensures that the result of compiling a header might vary between every compilation unit   A macro may be defined in one compilation unit which changes the content of the header    This is probably the main reason  as it requires huge amounts of code to be compiled for every compilation unit  and additionally  every header has to be compiled multiple times  once for every compilation unit that includes it    Linking  Once compiled  all the object files have to be linked together    This is basically a monolithic process that can t very well be parallelized  and has to process your entire project   Parsing  The syntax is extremely complicated to parse  depends heavily on context  and is very hard to disambiguate  This takes a lot of time   Templates  In C   List lt T gt  is the only type that is compiled  no matter how many instantiations of List you have in your program  In C    vector lt int gt  is a completely separate type from vector lt float gt   and each one will have to be compiled separately   Add to this that templates make up a full Turing-complete  sub-language  that the compiler has to interpret  and this can become ridiculously complicated  Even relatively simple template metaprogramming code can define recursive templates that create dozens and dozens of template instantiations  Templates may also result in extremely complex types  with ridiculously long names  adding a lot of extra work to the linker   It has to compare a lot of symbol names  and if these names can grow into many thousand characters  that can become fairly expensive    And of course  they exacerbate the problems with header files  because templates generally have to be defined in headers  which means far more code has to be parsed and compiled for every compilation unit  In plain C code  a header typically only contains forward declarations  but very little actual code  In C    it is not uncommon for almost all the code to reside in header files   Optimization  C   allows for some very dramatic optimizations  C  or Java don t allow classes to be completely eliminated  they have to be there for reflection purposes   but even a simple C   template metaprogram can easily generate dozens or hundreds of classes  all of which are inlined and eliminated again in the optimization phase   Moreover  a C   program must be fully optimized by the compiler  A C  program can rely on the JIT compiler to perform additional optimizations at load-time  C   doesn t get any such  second chances   What the compiler generates is as optimized as it s going to get   Machine  C   is compiled to machine code which may be somewhat more complicated than the bytecode Java or  NET use  especially in the case of x86    This is mentioned out of completeness only because it was mentioned in comments and such  In practice  this step is unlikely to take more than a tiny fraction of the total compilation time    Conclusion  Most of these factors are shared by C code  which actually compiles fairly efficiently  The parsing step is a lot more complicated in C    and can take up significantly more time  but the main offender is probably templates  They re useful  and make C   a far more powerful language  but they also take their toll in terms of compilation speed

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To answer this question simply  C   is a much more complex language than other languages available on the market  It has a legacy inclusion model that parses code multiple times  and its templated libraries are not optimized for compilation speed  Grammar and ADL Let s have a look at the grammatical complexity of C   by considering a very simple example  x y   While you   d be likely to say that the above is an expression with multiplication  this is not necessarily the case in C    If x is a type  then the statement is  in fact  a pointer declaration  This means that C   grammar is context-sensitive  Here   s another example  foo lt x gt  a   Again  you might think this is a declaration of the variable  quot a quot  of type foo  but it could also be interpreted as   foo  lt  x   gt  a   which would make it a comparison expression  C   has a feature called Argument Dependent Lookup  ADL   ADL establishes the rules that govern how the compiler looks up a name  Consider the following example  namespace A    struct Aa       void foo Aa arg     namespace B    struct Bb      void foo A  Aa arg  Bb arg2     namespace C     struct Cc       void foo A  Aa arg  B  Bb arg2  C  Cc arg3      foo A  Aa    B  Bb    C  Cc      ADL rules state that we will be looking for the name  quot foo quot  considering all arguments of the function call  In this case  all of the functions named    foo    will be considered for overload resolution  This process might take time  especially if there are lots of function overloads  In a templated context  ADL rules become even more complicated   include This command is something that might significantly influence compilation times  Depending on the type of file you include  the preprocessor might copy only a couple of lines of code  or it might copy thousands  Furthermore  this command cannot be optimized by the compiler  You can copy different pieces of code that can be modified just before inclusion if the header file depends on macros  There are some solutions to these issues  You can use precompiled headers  which are the compiler s internal representation of what was parsed in the header  This can   t be done without the user s effort  however  because precompiled headers assume that headers are not macro dependent  The modules feature provides a language-level solution to this problem  It   s available from the C  20 release onward  Templates The compilation speed for templates is challenging  Each translation unit that uses templates needs to have them included  and the definitions of these templates need to be available  Some instantiations of templates end up in instantiations of other templates  In some extreme cases  template instantiation can consume lots of resources  A library that uses templates and that was not designed for compilation speed can become troublesome  as you can see in a comparison of metaprogramming libraries provided at this link  http   metaben ch   Their differences in compilation speed are significant  If you want to understand why some metaprogramming libraries are better for compilation times than others  check out this video about the Rule of Chiel  Conclusion C   is a slowly compiled language because compilation performance was not the highest priority when the language was initially developed  As a result  C   ended up with features that might be effective during runtime  but are not necessarily effective during compile time  P S     I work at Incredibuild  a software development acceleration company specializing in accelerating C   compilations  you are welcome to try it for free

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