How to declare a structure in a header that is to be used by multiple files in c

Question

If I have a source c file with a struct   struct a        int i      struct b           int j             How can this struct be used in another file  i e  func c    Should I create a new header file  declare the struct there and include that header in func c   Or should I define the whole struct in a header file and include that in both source c and func c   How can the struct be declared extern in both files   Should I typedef it  If so  how

User · Answer

For a structure definition that is to be used across more than one source file, you should definitely put it in a header file. Then include that header file in any source file that needs the structure.

The extern declaration is not used for structure definitions, but is instead used for variable declarations (that is, some data value with a structure type that you have defined). If you want to use the same variable across more than one source file, declare it as extern in a header file like:

extern struct a myAValue;

Then, in one source file, define the actual variable:

struct a myAValue;

If you forget to do this or accidentally define it in two source files, the linker will let you know about this.

User · Answer

if this structure is to be used by some other file func c how to do it  When a type is used in a file  i e  func c file   it must be visible  The very worst way to do it is copy paste it in each source file needed it  The right way is putting it in an header file  and include this header file whenever needed  shall we open a new header file and declare the structure there and include that header in the func c  This is the solution I like more  because it makes the code highly modular  I would code your struct as   ifndef SOME HEADER GUARD WITH UNIQUE NAME  define SOME HEADER GUARD WITH UNIQUE NAME  struct a        int i      struct b               int j             endif  I would put functions using this structure in the same header  the function that are  quot semantically quot  part of its  quot interface quot    And usually  I could name the file after the structure name  and use that name again to choose the header guards defines  If you need to declare a function using a pointer to the struct  you won t need the full struct definition  A simple forward declaration like  struct a    Will be enough  and it decreases coupling  or can we define the total structure in header file and include that in both source c and func c  This is another way  easier somewhat  but less modular  Some code needing only your structure to work would still have to include all types  In C    this could lead to interesting complication  but this is out of topic  no C   tag   so I won t elaborate  then how to declare that structure as extern in both the files    I fail to see the point  perhaps  but Greg Hewgill has a very good answer in his post How to declare a structure in a header that is to be used by multiple files in c   shall we typedef it then how   If you are using C    don t  If you are using C  you should   The reason being that C struct managing can be a pain  You have to declare the struct keyword everywhere it is used  struct MyStruct      Forward declaration     struct MyStruct         etc          void doSomething struct MyStruct   p     parameter         struct MyStruct a      variable          etc       While a typedef will enable you to write it without the struct keyword  struct MyStructTag      Forward declaration     typedef struct MyStructTag         etc       MyStruct    void doSomething MyStruct   p     parameter         MyStruct a      variable          etc       It is important you still keep a name for the struct  Writing  typedef struct         etc       MyStruct    will just create an anonymous struct with a typedef-ed name  and you won t be able to forward-declare it  So keep to the following format  typedef struct MyStructTag         etc       MyStruct    Thus  you ll be able to use MyStruct everywhere you want to avoid adding the struct keyword  and still use MyStructTag when a typedef won t work  i e  forward declaration  Edit  Corrected wrong assumption about C99 struct declaration  as rightfully remarked by Jonathan Leffler  Edit 2018-06-01  Craig Barnes reminds us in his comment that you don t need to keep separate names for the struct  quot tag quot  name and its  quot typedef quot  name  like I did above for the sake of clarity  Indeed  the code above could well be written as  typedef struct MyStruct         etc       MyStruct    IIRC  this is actually what C   does with its simpler struct declaration  behind the scenes  to keep it compatible with C     C   explicit declaration by the user struct MyStruct         etc            C   standard then implicitly adds the following line typedef MyStruct MyStruct   Back to C  I ve seen both usages  separate names and same names   and none has drawbacks I know of  so using the same name makes reading simpler if you don t use C separate  quot namespaces quot  for structs and other symbols

User · Answer

For a structure definition that is to be used across more than one source file, you should definitely put it in a header file. Then include that header file in any source file that needs the structure.

The extern declaration is not used for structure definitions, but is instead used for variable declarations (that is, some data value with a structure type that you have defined). If you want to use the same variable across more than one source file, declare it as extern in a header file like:

extern struct a myAValue;

Then, in one source file, define the actual variable:

struct a myAValue;

If you forget to do this or accidentally define it in two source files, the linker will let you know about this.

User · Answer

a h    ifndef A H  define A H  struct a        int i      struct b           int j             endif   there you go  now you just need to include a h to the files where you want to use this structure

User · Answer

a h    ifndef A H  define A H  struct a        int i      struct b           int j             endif   there you go  now you just need to include a h to the files where you want to use this structure

User · Answer

if this structure is to be used by some other file func c how to do it  When a type is used in a file  i e  func c file   it must be visible  The very worst way to do it is copy paste it in each source file needed it  The right way is putting it in an header file  and include this header file whenever needed  shall we open a new header file and declare the structure there and include that header in the func c  This is the solution I like more  because it makes the code highly modular  I would code your struct as   ifndef SOME HEADER GUARD WITH UNIQUE NAME  define SOME HEADER GUARD WITH UNIQUE NAME  struct a        int i      struct b               int j             endif  I would put functions using this structure in the same header  the function that are  quot semantically quot  part of its  quot interface quot    And usually  I could name the file after the structure name  and use that name again to choose the header guards defines  If you need to declare a function using a pointer to the struct  you won t need the full struct definition  A simple forward declaration like  struct a    Will be enough  and it decreases coupling  or can we define the total structure in header file and include that in both source c and func c  This is another way  easier somewhat  but less modular  Some code needing only your structure to work would still have to include all types  In C    this could lead to interesting complication  but this is out of topic  no C   tag   so I won t elaborate  then how to declare that structure as extern in both the files    I fail to see the point  perhaps  but Greg Hewgill has a very good answer in his post How to declare a structure in a header that is to be used by multiple files in c   shall we typedef it then how   If you are using C    don t  If you are using C  you should   The reason being that C struct managing can be a pain  You have to declare the struct keyword everywhere it is used  struct MyStruct      Forward declaration     struct MyStruct         etc          void doSomething struct MyStruct   p     parameter         struct MyStruct a      variable          etc       While a typedef will enable you to write it without the struct keyword  struct MyStructTag      Forward declaration     typedef struct MyStructTag         etc       MyStruct    void doSomething MyStruct   p     parameter         MyStruct a      variable          etc       It is important you still keep a name for the struct  Writing  typedef struct         etc       MyStruct    will just create an anonymous struct with a typedef-ed name  and you won t be able to forward-declare it  So keep to the following format  typedef struct MyStructTag         etc       MyStruct    Thus  you ll be able to use MyStruct everywhere you want to avoid adding the struct keyword  and still use MyStructTag when a typedef won t work  i e  forward declaration  Edit  Corrected wrong assumption about C99 struct declaration  as rightfully remarked by Jonathan Leffler  Edit 2018-06-01  Craig Barnes reminds us in his comment that you don t need to keep separate names for the struct  quot tag quot  name and its  quot typedef quot  name  like I did above for the sake of clarity  Indeed  the code above could well be written as  typedef struct MyStruct         etc       MyStruct    IIRC  this is actually what C   does with its simpler struct declaration  behind the scenes  to keep it compatible with C     C   explicit declaration by the user struct MyStruct         etc            C   standard then implicitly adds the following line typedef MyStruct MyStruct   Back to C  I ve seen both usages  separate names and same names   and none has drawbacks I know of  so using the same name makes reading simpler if you don t use C separate  quot namespaces quot  for structs and other symbols

User · Answer

if this structure is to be used by some other file func c how to do it  When a type is used in a file  i e  func c file   it must be visible  The very worst way to do it is copy paste it in each source file needed it  The right way is putting it in an header file  and include this header file whenever needed  shall we open a new header file and declare the structure there and include that header in the func c  This is the solution I like more  because it makes the code highly modular  I would code your struct as   ifndef SOME HEADER GUARD WITH UNIQUE NAME  define SOME HEADER GUARD WITH UNIQUE NAME  struct a        int i      struct b               int j             endif  I would put functions using this structure in the same header  the function that are  quot semantically quot  part of its  quot interface quot    And usually  I could name the file after the structure name  and use that name again to choose the header guards defines  If you need to declare a function using a pointer to the struct  you won t need the full struct definition  A simple forward declaration like  struct a    Will be enough  and it decreases coupling  or can we define the total structure in header file and include that in both source c and func c  This is another way  easier somewhat  but less modular  Some code needing only your structure to work would still have to include all types  In C    this could lead to interesting complication  but this is out of topic  no C   tag   so I won t elaborate  then how to declare that structure as extern in both the files    I fail to see the point  perhaps  but Greg Hewgill has a very good answer in his post How to declare a structure in a header that is to be used by multiple files in c   shall we typedef it then how   If you are using C    don t  If you are using C  you should   The reason being that C struct managing can be a pain  You have to declare the struct keyword everywhere it is used  struct MyStruct      Forward declaration     struct MyStruct         etc          void doSomething struct MyStruct   p     parameter         struct MyStruct a      variable          etc       While a typedef will enable you to write it without the struct keyword  struct MyStructTag      Forward declaration     typedef struct MyStructTag         etc       MyStruct    void doSomething MyStruct   p     parameter         MyStruct a      variable          etc       It is important you still keep a name for the struct  Writing  typedef struct         etc       MyStruct    will just create an anonymous struct with a typedef-ed name  and you won t be able to forward-declare it  So keep to the following format  typedef struct MyStructTag         etc       MyStruct    Thus  you ll be able to use MyStruct everywhere you want to avoid adding the struct keyword  and still use MyStructTag when a typedef won t work  i e  forward declaration  Edit  Corrected wrong assumption about C99 struct declaration  as rightfully remarked by Jonathan Leffler  Edit 2018-06-01  Craig Barnes reminds us in his comment that you don t need to keep separate names for the struct  quot tag quot  name and its  quot typedef quot  name  like I did above for the sake of clarity  Indeed  the code above could well be written as  typedef struct MyStruct         etc       MyStruct    IIRC  this is actually what C   does with its simpler struct declaration  behind the scenes  to keep it compatible with C     C   explicit declaration by the user struct MyStruct         etc            C   standard then implicitly adds the following line typedef MyStruct MyStruct   Back to C  I ve seen both usages  separate names and same names   and none has drawbacks I know of  so using the same name makes reading simpler if you don t use C separate  quot namespaces quot  for structs and other symbols

User · Answer

a h    ifndef A H  define A H  struct a        int i      struct b           int j             endif   there you go  now you just need to include a h to the files where you want to use this structure

User · Answer

For a structure definition that is to be used across more than one source file, you should definitely put it in a header file. Then include that header file in any source file that needs the structure.

The extern declaration is not used for structure definitions, but is instead used for variable declarations (that is, some data value with a structure type that you have defined). If you want to use the same variable across more than one source file, declare it as extern in a header file like:

extern struct a myAValue;

Then, in one source file, define the actual variable:

struct a myAValue;

If you forget to do this or accidentally define it in two source files, the linker will let you know about this.

User · Answer

if this structure is to be used by some other file func c how to do it  When a type is used in a file  i e  func c file   it must be visible  The very worst way to do it is copy paste it in each source file needed it  The right way is putting it in an header file  and include this header file whenever needed  shall we open a new header file and declare the structure there and include that header in the func c  This is the solution I like more  because it makes the code highly modular  I would code your struct as   ifndef SOME HEADER GUARD WITH UNIQUE NAME  define SOME HEADER GUARD WITH UNIQUE NAME  struct a        int i      struct b               int j             endif  I would put functions using this structure in the same header  the function that are  quot semantically quot  part of its  quot interface quot    And usually  I could name the file after the structure name  and use that name again to choose the header guards defines  If you need to declare a function using a pointer to the struct  you won t need the full struct definition  A simple forward declaration like  struct a    Will be enough  and it decreases coupling  or can we define the total structure in header file and include that in both source c and func c  This is another way  easier somewhat  but less modular  Some code needing only your structure to work would still have to include all types  In C    this could lead to interesting complication  but this is out of topic  no C   tag   so I won t elaborate  then how to declare that structure as extern in both the files    I fail to see the point  perhaps  but Greg Hewgill has a very good answer in his post How to declare a structure in a header that is to be used by multiple files in c   shall we typedef it then how   If you are using C    don t  If you are using C  you should   The reason being that C struct managing can be a pain  You have to declare the struct keyword everywhere it is used  struct MyStruct      Forward declaration     struct MyStruct         etc          void doSomething struct MyStruct   p     parameter         struct MyStruct a      variable          etc       While a typedef will enable you to write it without the struct keyword  struct MyStructTag      Forward declaration     typedef struct MyStructTag         etc       MyStruct    void doSomething MyStruct   p     parameter         MyStruct a      variable          etc       It is important you still keep a name for the struct  Writing  typedef struct         etc       MyStruct    will just create an anonymous struct with a typedef-ed name  and you won t be able to forward-declare it  So keep to the following format  typedef struct MyStructTag         etc       MyStruct    Thus  you ll be able to use MyStruct everywhere you want to avoid adding the struct keyword  and still use MyStructTag when a typedef won t work  i e  forward declaration  Edit  Corrected wrong assumption about C99 struct declaration  as rightfully remarked by Jonathan Leffler  Edit 2018-06-01  Craig Barnes reminds us in his comment that you don t need to keep separate names for the struct  quot tag quot  name and its  quot typedef quot  name  like I did above for the sake of clarity  Indeed  the code above could well be written as  typedef struct MyStruct         etc       MyStruct    IIRC  this is actually what C   does with its simpler struct declaration  behind the scenes  to keep it compatible with C     C   explicit declaration by the user struct MyStruct         etc            C   standard then implicitly adds the following line typedef MyStruct MyStruct   Back to C  I ve seen both usages  separate names and same names   and none has drawbacks I know of  so using the same name makes reading simpler if you don t use C separate  quot namespaces quot  for structs and other symbols

User · Answer

a h    ifndef A H  define A H  struct a        int i      struct b           int j             endif   there you go  now you just need to include a h to the files where you want to use this structure

User · Answer

For a structure definition that is to be used across more than one source file, you should definitely put it in a header file. Then include that header file in any source file that needs the structure.

The extern declaration is not used for structure definitions, but is instead used for variable declarations (that is, some data value with a structure type that you have defined). If you want to use the same variable across more than one source file, declare it as extern in a header file like:

extern struct a myAValue;

Then, in one source file, define the actual variable:

struct a myAValue;

If you forget to do this or accidentally define it in two source files, the linker will let you know about this.

[c] How to declare a structure in a header that is to be used by multiple files in c?

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