Use of pragma in C

Question

What are some uses of  pragma in C  with examples

User · Answer

This is a preprocessor directive that can be used to turn on or off certain features.

It is of two types #pragma startup, #pragma exit and #pragma warn.

#pragma startup allows us to specify functions called upon program startup.

#pragma exit allows us to specify functions called upon program exit.

#pragma warn tells the computer to suppress any warning or not.

Many other #pragma styles can be used to control the compiler.

User · Answer

I would generally try to avoid the use of  pragmas if possible  since they re extremely compiler-dependent and non-portable   If you want to use them in a portable fashion  you ll have to surround every pragma with a  if  endif pair   GCC discourages the use of pragmas  and really only supports some of them for compatibility with other compilers  GCC has other ways of doing the same things that other compilers use pragmas for   For example  here s how you d ensure that a structure is packed tightly  i e  no padding between members  in MSVC    pragma pack push  1  struct PackedStructure     char a    int b    short c      pragma pack pop     sizeof PackedStructure     7   Here s how you d do the same thing in GCC   struct PackedStructure   attribute      packed         char a    int b    short c        sizeof PackedStructure    7    The GCC code is more portable  because if you want to compile that with a non-GCC compiler  all you have to do is   define   attribute   x   Whereas if you want to port the MSVC code  you have to surround each pragma with a  if  endif pair   Not pretty

User · Answer

pragma startup is a directive which is used to call a function before the main function and to call another function after the main function  e g    pragma startup func1  pragma exit func2   Here  func1 runs before main and func2 runs afterwards   NOTE  This code works only in Turbo-C compiler  To achieve this functionality in GCC  you can declare func1 and func2 like this   void   attribute    constructor   func1    void   attribute    destructor   func2

User · Answer

To sum it up   pragma tells the compiler to do stuff  Here are a couple of ways I use it     pragma can be used to ignore compiler warnings  For example  to make GCC shut up about implicit function declarations  you can write    pragma GCC diagnostic ignored  -Wimplicit-function-declaration    An older version of libportable does this portably   pragma once  when written at the top of a header file  will cause said header file to be included once  libportable checks for pragma once support

User · Answer

I would generally try to avoid the use of  pragmas if possible  since they re extremely compiler-dependent and non-portable   If you want to use them in a portable fashion  you ll have to surround every pragma with a  if  endif pair   GCC discourages the use of pragmas  and really only supports some of them for compatibility with other compilers  GCC has other ways of doing the same things that other compilers use pragmas for   For example  here s how you d ensure that a structure is packed tightly  i e  no padding between members  in MSVC    pragma pack push  1  struct PackedStructure     char a    int b    short c      pragma pack pop     sizeof PackedStructure     7   Here s how you d do the same thing in GCC   struct PackedStructure   attribute      packed         char a    int b    short c        sizeof PackedStructure    7    The GCC code is more portable  because if you want to compile that with a non-GCC compiler  all you have to do is   define   attribute   x   Whereas if you want to port the MSVC code  you have to surround each pragma with a  if  endif pair   Not pretty

User · Answer

All answers above make nice explanations for  pragma but I wanted to a add small example   I just want to explain a simple OpenMP example that demonstrate some uses of  pragma to doing its work     OpenMp briefly is an implementation for multi-platform shared-memory   parallel programming    then we can say it s machine-specific or operating-system-specific    let s go to the example    include  lt stdio h gt   include  lt omp h gt    compile with   openmp  int main         pragma omp parallel num threads 4             int i   omp get thread num          printf s  Hello from thread  d n   i            the output is  Hello from thread 0 Hello from thread 1 Hello from thread 2 Hello from thread 3  Note that the order of output can vary on different machines    now let me tell you what  pragma did     it tells the OS to run the some block of code on 4 threads  this is just one of many many applications you can do with the little  pragma  sorry for the outside sample OpenMP

User · Answer

pragma is used to do something implementation-specific in C  i e  be pragmatic for the current context rather than ideologically dogmatic   The one I regularly use is  pragma pack 1  where I m trying to squeeze more out of my memory space on embedded solutions  with arrays of structures that would otherwise end up with 8 byte alignment   Pity we don t have a  dogma yet  That would be fun

User · Answer

All answers above make nice explanations for  pragma but I wanted to a add small example   I just want to explain a simple OpenMP example that demonstrate some uses of  pragma to doing its work     OpenMp briefly is an implementation for multi-platform shared-memory   parallel programming    then we can say it s machine-specific or operating-system-specific    let s go to the example    include  lt stdio h gt   include  lt omp h gt    compile with   openmp  int main         pragma omp parallel num threads 4             int i   omp get thread num          printf s  Hello from thread  d n   i            the output is  Hello from thread 0 Hello from thread 1 Hello from thread 2 Hello from thread 3  Note that the order of output can vary on different machines    now let me tell you what  pragma did     it tells the OS to run the some block of code on 4 threads  this is just one of many many applications you can do with the little  pragma  sorry for the outside sample OpenMP

User · Answer

pragma is for compiler directives that are machine-specific or operating-system-specific  i e  it tells the compiler to do something  set some option  take some action  override some default  etc  that may or may not apply to all machines and operating systems   See msdn for more info

User · Answer

Putting  pragma once at the top of your header file will ensure that it is only included once  Note that  pragma once is not standard C99  but supported by most modern compilers   An alternative is to use include guards  e g   ifndef MY FILE  define MY FILE      endif    MY FILE

User · Answer

My best advice is to look at your compiler s documentation  because pragmas are by definition implementation-specific   For instance  in embedded projects I ve used them to locate code and data in different sections  or declare interrupt handlers   i e     pragma code BANK1  pragma data BANK2   pragma INT3 TimerHandler

User · Answer

pragma is used to do something implementation-specific in C  i e  be pragmatic for the current context rather than ideologically dogmatic   The one I regularly use is  pragma pack 1  where I m trying to squeeze more out of my memory space on embedded solutions  with arrays of structures that would otherwise end up with 8 byte alignment   Pity we don t have a  dogma yet  That would be fun

User · Answer

pragma startup is a directive which is used to call a function before the main function and to call another function after the main function  e g    pragma startup func1  pragma exit func2   Here  func1 runs before main and func2 runs afterwards   NOTE  This code works only in Turbo-C compiler  To achieve this functionality in GCC  you can declare func1 and func2 like this   void   attribute    constructor   func1    void   attribute    destructor   func2

User · Answer

what i feel is  pragma is a directive where if you want the code to be location specific  say a situation where you want the program counter to read from the specific address where the ISR is written then you can specify ISR at that location using  pragma vector ADC12 VECTOR and followd by interrupt rotines name and its description

User · Answer

My best advice is to look at your compiler s documentation  because pragmas are by definition implementation-specific   For instance  in embedded projects I ve used them to locate code and data in different sections  or declare interrupt handlers   i e     pragma code BANK1  pragma data BANK2   pragma INT3 TimerHandler

User · Answer

My best advice is to look at your compiler s documentation  because pragmas are by definition implementation-specific   For instance  in embedded projects I ve used them to locate code and data in different sections  or declare interrupt handlers   i e     pragma code BANK1  pragma data BANK2   pragma INT3 TimerHandler

User · Answer

My best advice is to look at your compiler s documentation  because pragmas are by definition implementation-specific   For instance  in embedded projects I ve used them to locate code and data in different sections  or declare interrupt handlers   i e     pragma code BANK1  pragma data BANK2   pragma INT3 TimerHandler

User · Answer

Putting  pragma once at the top of your header file will ensure that it is only included once  Note that  pragma once is not standard C99  but supported by most modern compilers   An alternative is to use include guards  e g   ifndef MY FILE  define MY FILE      endif    MY FILE

User · Answer

pragma is used to do something implementation-specific in C  i e  be pragmatic for the current context rather than ideologically dogmatic   The one I regularly use is  pragma pack 1  where I m trying to squeeze more out of my memory space on embedded solutions  with arrays of structures that would otherwise end up with 8 byte alignment   Pity we don t have a  dogma yet  That would be fun

User · Answer

I would generally try to avoid the use of  pragmas if possible  since they re extremely compiler-dependent and non-portable   If you want to use them in a portable fashion  you ll have to surround every pragma with a  if  endif pair   GCC discourages the use of pragmas  and really only supports some of them for compatibility with other compilers  GCC has other ways of doing the same things that other compilers use pragmas for   For example  here s how you d ensure that a structure is packed tightly  i e  no padding between members  in MSVC    pragma pack push  1  struct PackedStructure     char a    int b    short c      pragma pack pop     sizeof PackedStructure     7   Here s how you d do the same thing in GCC   struct PackedStructure   attribute      packed         char a    int b    short c        sizeof PackedStructure    7    The GCC code is more portable  because if you want to compile that with a non-GCC compiler  all you have to do is   define   attribute   x   Whereas if you want to port the MSVC code  you have to surround each pragma with a  if  endif pair   Not pretty

User · Answer

Putting  pragma once at the top of your header file will ensure that it is only included once  Note that  pragma once is not standard C99  but supported by most modern compilers   An alternative is to use include guards  e g   ifndef MY FILE  define MY FILE      endif    MY FILE

User · Answer

pragma is for compiler directives that are machine-specific or operating-system-specific  i e  it tells the compiler to do something  set some option  take some action  override some default  etc  that may or may not apply to all machines and operating systems   See msdn for more info

User · Answer

This is a preprocessor directive that can be used to turn on or off certain features.

It is of two types #pragma startup, #pragma exit and #pragma warn.

#pragma startup allows us to specify functions called upon program startup.

#pragma exit allows us to specify functions called upon program exit.

#pragma warn tells the computer to suppress any warning or not.

Many other #pragma styles can be used to control the compiler.

User · Answer

pragma is for compiler directives that are machine-specific or operating-system-specific  i e  it tells the compiler to do something  set some option  take some action  override some default  etc  that may or may not apply to all machines and operating systems   See msdn for more info

User · Answer

Putting  pragma once at the top of your header file will ensure that it is only included once  Note that  pragma once is not standard C99  but supported by most modern compilers   An alternative is to use include guards  e g   ifndef MY FILE  define MY FILE      endif    MY FILE

User · Answer

pragma is used to do something implementation-specific in C  i e  be pragmatic for the current context rather than ideologically dogmatic   The one I regularly use is  pragma pack 1  where I m trying to squeeze more out of my memory space on embedded solutions  with arrays of structures that would otherwise end up with 8 byte alignment   Pity we don t have a  dogma yet  That would be fun

User · Answer

pragma is for compiler directives that are machine-specific or operating-system-specific  i e  it tells the compiler to do something  set some option  take some action  override some default  etc  that may or may not apply to all machines and operating systems   See msdn for more info

User · Answer

To sum it up   pragma tells the compiler to do stuff  Here are a couple of ways I use it     pragma can be used to ignore compiler warnings  For example  to make GCC shut up about implicit function declarations  you can write    pragma GCC diagnostic ignored  -Wimplicit-function-declaration    An older version of libportable does this portably   pragma once  when written at the top of a header file  will cause said header file to be included once  libportable checks for pragma once support

User · Answer

what i feel is  pragma is a directive where if you want the code to be location specific  say a situation where you want the program counter to read from the specific address where the ISR is written then you can specify ISR at that location using  pragma vector ADC12 VECTOR and followd by interrupt rotines name and its description

User · Answer

I would generally try to avoid the use of  pragmas if possible  since they re extremely compiler-dependent and non-portable   If you want to use them in a portable fashion  you ll have to surround every pragma with a  if  endif pair   GCC discourages the use of pragmas  and really only supports some of them for compatibility with other compilers  GCC has other ways of doing the same things that other compilers use pragmas for   For example  here s how you d ensure that a structure is packed tightly  i e  no padding between members  in MSVC    pragma pack push  1  struct PackedStructure     char a    int b    short c      pragma pack pop     sizeof PackedStructure     7   Here s how you d do the same thing in GCC   struct PackedStructure   attribute      packed         char a    int b    short c        sizeof PackedStructure    7    The GCC code is more portable  because if you want to compile that with a non-GCC compiler  all you have to do is   define   attribute   x   Whereas if you want to port the MSVC code  you have to surround each pragma with a  if  endif pair   Not pretty

[c] Use of #pragma in C

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