How to initialize a struct in accordance with C programming language standards

Question

I want to initialize a struct element  split in declaration and initialization  This is what I have   typedef struct MY TYPE     bool flag    short int value    double stuff    MY TYPE   void function void      MY TYPE a          a     true  15  0 123       Is this the way to declare and initialize a local variable of MY TYPE in accordance with C programming language standards  C89  C90  C99  C11  etc    Or is there anything better or at least working   Update I ended up having a static initialization element where I set every subelement according to my needs

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as Ron Nuni said   typedef struct Item       int a      float b      char  name    Item   int main void        Item item    5  2 2   George        return 0      An important thing to remember  at the moment you initialize even one object variable in the struct  all of its other variables will be initialized to default value   If you don t initialize the values in your struct  i e  if you just declare that variable   all variable members will contain  garbage values   only if the declaration is local   If the declaration is global or static  like in this case   all uninitialized variable members will be initialized automatically to    0 for integers and floating point   0  for char  of course this is just the same as 0  and char is an integer type  NULL for pointers

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void function void      MY TYPE a    a flag   true    a value   15    a stuff   0 123

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You can do it with a compound literal  According to that page  it works in C99  which also counts as ANSI C    MY TYPE a   a    MY TYPE     flag   true   value   123   stuff   0 456        a    MY TYPE     value   234   stuff   1 234   flag   false      The designations in the initializers are optional  you could also write   a    MY TYPE    true   123  0 456        a    MY TYPE    false  234  1 234

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Adding to All of these good answer a summary to how to initialize a structure  union and Array  in C  focused especially on the Designed Initializer  Standard Initialization struct point        double x      double y      double z     p    1 2  1 3      Designed Initializer The Designed Initializer came up since the ISO C99 and is a different and more dynamic way to initialize in C when initializing struct  union or an array  The biggest difference to standard initialization is that you don t have to declare the elements in a fixed order and you can also omit element  From The GNU Guide   Standard C90 requires the elements of an initializer to appear in a fixed order  the same as the order of the elements in the array or structure being initialized    In ISO C99 you can give the elements in random order  specifying the array indices or structure field names they apply to  and GNU C allows this as an extension in C90 mode as well   Examples 1  Array Index Standard Initialization   int a 6      0  0  15  0  29  0     Designed Initialization   int a 6      4    29   2    15       or   int a 6      4 29    2 15       or   int widths        0     9    1   10     99    2   100    3     2  Struct or union  Standard Initialization struct point   int x  y      Designed Initialization  struct point p      y   2   x   3    or  struct point p     y  2  x  3     3  Combine naming elements with ordinary C initialization of successive elements  Standard Initialization int a 6      0  v1  v2  0  v4  0     Designed Initialization int a 6       1    v1  v2   4    v4     4  Others  Labeling the elements of an array initializer int whitespace 256              1     t     1     h     1                             f     1     n     1     r     1     write a series of     fieldname    and     index     designators before an         to specify a nested subobject to initialize struct point ptarray 10       2  y   yv2   2  x   xv2   0  x   xv0      Guides  designated-initializers-c   geeksforgeeks org using-designated-initializers tutorialspoint com   designated-initializers-in-c

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a    MYTYPE   true  15  0 123     would do fine in C99

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In  ANSI  C99  you can use a designated initializer to initialize a structure    MY TYPE a      flag   true   value   123   stuff   0 456      Edit  Other members are initialized as zero   Omitted field members are implicitly initialized the same as objects that have static storage duration    https   gcc gnu org onlinedocs gcc Designated-Inits html

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a    MYTYPE   true  15  0 123     would do fine in C99

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You can do it with a compound literal  According to that page  it works in C99  which also counts as ANSI C    MY TYPE a   a    MY TYPE     flag   true   value   123   stuff   0 456        a    MY TYPE     value   234   stuff   1 234   flag   false      The designations in the initializers are optional  you could also write   a    MY TYPE    true   123  0 456        a    MY TYPE    false  234  1 234

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In  ANSI  C99  you can use a designated initializer to initialize a structure    MY TYPE a      flag   true   value   123   stuff   0 456      Edit  Other members are initialized as zero   Omitted field members are implicitly initialized the same as objects that have static storage duration    https   gcc gnu org onlinedocs gcc Designated-Inits html

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In  ANSI  C99  you can use a designated initializer to initialize a structure    MY TYPE a      flag   true   value   123   stuff   0 456      Edit  Other members are initialized as zero   Omitted field members are implicitly initialized the same as objects that have static storage duration    https   gcc gnu org onlinedocs gcc Designated-Inits html

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You ve almost got it     MY TYPE a     true 15 0 123      Quick search on  struct initialize c  shows me this

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void function void      MY TYPE a    a flag   true    a value   15    a stuff   0 123

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I found another way to initialize structs   The struct   typedef struct test       int num      char  str    test    Initialization   test tt         num  42      str   nice       As per GCC   s documentation  this syntax is obsolete since GCC 2 5

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as Ron Nuni said   typedef struct Item       int a      float b      char  name    Item   int main void        Item item    5  2 2   George        return 0      An important thing to remember  at the moment you initialize even one object variable in the struct  all of its other variables will be initialized to default value   If you don t initialize the values in your struct  i e  if you just declare that variable   all variable members will contain  garbage values   only if the declaration is local   If the declaration is global or static  like in this case   all uninitialized variable members will be initialized automatically to    0 for integers and floating point   0  for char  of course this is just the same as 0  and char is an integer type  NULL for pointers

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Adding to All of these good answer a summary to how to initialize a structure  union and Array  in C  focused especially on the Designed Initializer  Standard Initialization struct point        double x      double y      double z     p    1 2  1 3      Designed Initializer The Designed Initializer came up since the ISO C99 and is a different and more dynamic way to initialize in C when initializing struct  union or an array  The biggest difference to standard initialization is that you don t have to declare the elements in a fixed order and you can also omit element  From The GNU Guide   Standard C90 requires the elements of an initializer to appear in a fixed order  the same as the order of the elements in the array or structure being initialized    In ISO C99 you can give the elements in random order  specifying the array indices or structure field names they apply to  and GNU C allows this as an extension in C90 mode as well   Examples 1  Array Index Standard Initialization   int a 6      0  0  15  0  29  0     Designed Initialization   int a 6      4    29   2    15       or   int a 6      4 29    2 15       or   int widths        0     9    1   10     99    2   100    3     2  Struct or union  Standard Initialization struct point   int x  y      Designed Initialization  struct point p      y   2   x   3    or  struct point p     y  2  x  3     3  Combine naming elements with ordinary C initialization of successive elements  Standard Initialization int a 6      0  v1  v2  0  v4  0     Designed Initialization int a 6       1    v1  v2   4    v4     4  Others  Labeling the elements of an array initializer int whitespace 256              1     t     1     h     1                             f     1     n     1     r     1     write a series of     fieldname    and     index     designators before an         to specify a nested subobject to initialize struct point ptarray 10       2  y   yv2   2  x   xv2   0  x   xv0      Guides  designated-initializers-c   geeksforgeeks org using-designated-initializers tutorialspoint com   designated-initializers-in-c

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You can do it with a compound literal  According to that page  it works in C99  which also counts as ANSI C    MY TYPE a   a    MY TYPE     flag   true   value   123   stuff   0 456        a    MY TYPE     value   234   stuff   1 234   flag   false      The designations in the initializers are optional  you could also write   a    MY TYPE    true   123  0 456        a    MY TYPE    false  234  1 234

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You ve almost got it     MY TYPE a     true 15 0 123      Quick search on  struct initialize c  shows me this

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void function void      MY TYPE a    a flag   true    a value   15    a stuff   0 123

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C programming language standard ISO IEC 9899 1999  commonly known as C99  allows one to use a designated initializer to initialize members of a structure or union as follows   MY TYPE a      stuff   0 456   flag   true   value   123      It is defined in paragraph 7  section 6 7 8 Initialization of ISO IEC 9899 1999 standard as      If a designator has the form     identifier   then the current object  defined below  shall have structure or union type and the identifier shall be the name of a member of that type    Note that paragraph 9 of the same section states that      Except where explicitly stated otherwise  for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization  Unnamed members of structure objects have indeterminate value even after initialization    In GNU GCC implementation however omitted members are initialized as zero or zero-like type-appropriate value  As stated in section 6 27 Designated Initializers of GNU GCC documentation      Omitted field members are implicitly initialized the same as objects that have static storage duration    Microsoft Visual C   compiler should support designated initializers since version 2013 according to official blog post C   Conformance Roadmap  Paragraph Initializing unions and structs of Initializers  article at MSDN Visual Studio documentation suggests that unnamed members initialized to zero-like appropriate values similarly to GNU GCC   ISO IEC 9899 2011 standard  commonly known as C11  which had superseded ISO IEC 9899 1999 retains designated initializers under section 6 7 9 Initialization  It also retains paragraph 9 unchanged   New ISO IEC 9899 2018 standard  commonly known as C18  which had superseded ISO IEC 9899 2011 retains designated initializers under section 6 7 9 Initialization  It also retains paragraph 9 unchanged

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You ve almost got it     MY TYPE a     true 15 0 123      Quick search on  struct initialize c  shows me this

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I didn t like any of these answers so I made my own   I don t know if this is ANSI C or not  it s just GCC 4 2 1 in it s default mode   I never can remember the bracketing so I start with a subset of my data and do battle with compiler error messages until it shuts up   Readability is my first priority          in a header      typedef unsigned char uchar       struct fields         uchar num        uchar lbl 35                  in an actual c file  I have 2 in this case      struct fields labels              0  Package           1  Version           2  Apport           3  Architecture           4  Bugs           5  Description-md5           6  Essential           7  Filename           8  Ghc-Package           9  Gstreamer-Version           10  Homepage           11  Installed-Size           12  MD5sum           13  Maintainer           14  Modaliases           15  Multi-Arch           16  Npp-Description           17  Npp-File           18  Npp-Name           19  Origin            The data may start life as a tab-delimited file that you search-replace to massage into something else   Yes  this is Debian stuff   So one outside pair of     indicating the array   then another pair for each struct inside   With commas between   Putting things in a header isn t strictly necessary  but I ve got about 50 items in my struct so I want them in a separate file  both to keep the mess out of my code and so it s easier to replace

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This can be done in different ways  MY TYPE a     true  1  0 1     MY TYPE a      stuff   0 1   flag   true   value   1      designated initializer  not available in c    MY TYPE a  a    MY TYPE    true   1  0 1     MY TYPE m  true  1  0 1     works in C    not available in C  Also  you can define member while declaring structure   include  lt stdio h gt   struct MY TYPE       int a      int b   m    5 6    int main         printf  quot  d   d n quot  m a m b           return 0

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Structure in C can be declared and initialized like this   typedef struct book       char title 10       char author 10       float price    book   int main         book b1   DS    Ajay   250 0        printf   s  t  s  t  f   b1 title  b1 author  b1 price        return 0

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C programming language standard ISO IEC 9899 1999  commonly known as C99  allows one to use a designated initializer to initialize members of a structure or union as follows   MY TYPE a      stuff   0 456   flag   true   value   123      It is defined in paragraph 7  section 6 7 8 Initialization of ISO IEC 9899 1999 standard as      If a designator has the form     identifier   then the current object  defined below  shall have structure or union type and the identifier shall be the name of a member of that type    Note that paragraph 9 of the same section states that      Except where explicitly stated otherwise  for the purposes of this subclause unnamed members of objects of structure and union type do not participate in initialization  Unnamed members of structure objects have indeterminate value even after initialization    In GNU GCC implementation however omitted members are initialized as zero or zero-like type-appropriate value  As stated in section 6 27 Designated Initializers of GNU GCC documentation      Omitted field members are implicitly initialized the same as objects that have static storage duration    Microsoft Visual C   compiler should support designated initializers since version 2013 according to official blog post C   Conformance Roadmap  Paragraph Initializing unions and structs of Initializers  article at MSDN Visual Studio documentation suggests that unnamed members initialized to zero-like appropriate values similarly to GNU GCC   ISO IEC 9899 2011 standard  commonly known as C11  which had superseded ISO IEC 9899 1999 retains designated initializers under section 6 7 9 Initialization  It also retains paragraph 9 unchanged   New ISO IEC 9899 2018 standard  commonly known as C18  which had superseded ISO IEC 9899 2011 retains designated initializers under section 6 7 9 Initialization  It also retains paragraph 9 unchanged

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This can be done in different ways  MY TYPE a     true  1  0 1     MY TYPE a      stuff   0 1   flag   true   value   1      designated initializer  not available in c    MY TYPE a  a    MY TYPE    true   1  0 1     MY TYPE m  true  1  0 1     works in C    not available in C  Also  you can define member while declaring structure   include  lt stdio h gt   struct MY TYPE       int a      int b   m    5 6    int main         printf  quot  d   d n quot  m a m b           return 0

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Structure in C can be declared and initialized like this   typedef struct book       char title 10       char author 10       float price    book   int main         book b1   DS    Ajay   250 0        printf   s  t  s  t  f   b1 title  b1 author  b1 price        return 0

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I didn t like any of these answers so I made my own   I don t know if this is ANSI C or not  it s just GCC 4 2 1 in it s default mode   I never can remember the bracketing so I start with a subset of my data and do battle with compiler error messages until it shuts up   Readability is my first priority          in a header      typedef unsigned char uchar       struct fields         uchar num        uchar lbl 35                  in an actual c file  I have 2 in this case      struct fields labels              0  Package           1  Version           2  Apport           3  Architecture           4  Bugs           5  Description-md5           6  Essential           7  Filename           8  Ghc-Package           9  Gstreamer-Version           10  Homepage           11  Installed-Size           12  MD5sum           13  Maintainer           14  Modaliases           15  Multi-Arch           16  Npp-Description           17  Npp-File           18  Npp-Name           19  Origin            The data may start life as a tab-delimited file that you search-replace to massage into something else   Yes  this is Debian stuff   So one outside pair of     indicating the array   then another pair for each struct inside   With commas between   Putting things in a header isn t strictly necessary  but I ve got about 50 items in my struct so I want them in a separate file  both to keep the mess out of my code and so it s easier to replace

User · Answer

You can do it with a compound literal  According to that page  it works in C99  which also counts as ANSI C    MY TYPE a   a    MY TYPE     flag   true   value   123   stuff   0 456        a    MY TYPE     value   234   stuff   1 234   flag   false      The designations in the initializers are optional  you could also write   a    MY TYPE    true   123  0 456        a    MY TYPE    false  234  1 234

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You ve almost got it     MY TYPE a     true 15 0 123      Quick search on  struct initialize c  shows me this

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If MS has not updated to C99  MY TYPE a     true 15 0 123

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void function void      MY TYPE a    a flag   true    a value   15    a stuff   0 123

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If MS has not updated to C99  MY TYPE a     true 15 0 123

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I see you ve already received an answer about ANSI C 99  so I ll throw a bone about ANSI C 89  ANSI C 89 allows you to initialize a struct this way   typedef struct Item       int a      float b      char  name    Item   int main void        Item item     5  2 2   George         return 0      An important thing to remember  at the moment you initialize even one object  variable in the struct  all of its other variables will be initialized to default value   If you don t initialize the values in your struct  all variables will contain  garbage values    Good luck

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a    MYTYPE   true  15  0 123     would do fine in C99

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I have read the Microsoft Visual Studio 2015 Documentation for Initializing Aggregate Types yet  all forms of initializing with       are explained there  but the initializing with dot  named   designator   isn t mentioned there  It does not work also    The C99 standard chapter 6 7 8 Initialization explains the possibility of designators  but in my mind it is not really clear for complex structs  The C99 standard as pdf    In my mind  it may be better to   Use the    0  -initialization for all static data  It is less effort for the machine code  Use macros for initializing  for example  typedef MyStruct t  int x  int a  int b    MyStruct  define INIT MyStruct A B    0  A  B    The macro can be adapted  its argument list can be independent of changed struct content  It is proper if less elements should be initialized  It is also proper for nested struct  3  A simple form is  Initialize in a subroutine   void init MyStruct MyStruct  thiz  int a  int b      thiz- gt a   a  thiz- gt b   b      This routine looks like ObjectOriented in C  Use thiz  not this to compile it with C   too   MyStruct data    0     all is zero  init MyStruct  amp data  3  456

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In  ANSI  C99  you can use a designated initializer to initialize a structure    MY TYPE a      flag   true   value   123   stuff   0 456      Edit  Other members are initialized as zero   Omitted field members are implicitly initialized the same as objects that have static storage duration    https   gcc gnu org onlinedocs gcc Designated-Inits html

User · Answer

I have read the Microsoft Visual Studio 2015 Documentation for Initializing Aggregate Types yet  all forms of initializing with       are explained there  but the initializing with dot  named   designator   isn t mentioned there  It does not work also    The C99 standard chapter 6 7 8 Initialization explains the possibility of designators  but in my mind it is not really clear for complex structs  The C99 standard as pdf    In my mind  it may be better to   Use the    0  -initialization for all static data  It is less effort for the machine code  Use macros for initializing  for example  typedef MyStruct t  int x  int a  int b    MyStruct  define INIT MyStruct A B    0  A  B    The macro can be adapted  its argument list can be independent of changed struct content  It is proper if less elements should be initialized  It is also proper for nested struct  3  A simple form is  Initialize in a subroutine   void init MyStruct MyStruct  thiz  int a  int b      thiz- gt a   a  thiz- gt b   b      This routine looks like ObjectOriented in C  Use thiz  not this to compile it with C   too   MyStruct data    0     all is zero  init MyStruct  amp data  3  456

User · Answer

I found another way to initialize structs   The struct   typedef struct test       int num      char  str    test    Initialization   test tt         num  42      str   nice       As per GCC   s documentation  this syntax is obsolete since GCC 2 5

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I ve been looking for a nice way to initialize my struct  and I ve got to using the below  C99   This lets me initialize either a single structure or an array of structures in the same way as plain types   typedef struct       char  str      size t len      jsmntok t  tok      int tsz    jsmn ts    define jsmn ts default  jsmn ts  NULL  0  NULL  0    This can be used in the code as   jsmn ts mydata   jsmn ts default     initialization of a single struct     jsmn ts myarray 10     jsmn ts default  jsmn ts default      initialization of                                                     first 2 structs in the array

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I ve been looking for a nice way to initialize my struct  and I ve got to using the below  C99   This lets me initialize either a single structure or an array of structures in the same way as plain types   typedef struct       char  str      size t len      jsmntok t  tok      int tsz    jsmn ts    define jsmn ts default  jsmn ts  NULL  0  NULL  0    This can be used in the code as   jsmn ts mydata   jsmn ts default     initialization of a single struct     jsmn ts myarray 10     jsmn ts default  jsmn ts default      initialization of                                                     first 2 structs in the array

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I see you ve already received an answer about ANSI C 99  so I ll throw a bone about ANSI C 89  ANSI C 89 allows you to initialize a struct this way   typedef struct Item       int a      float b      char  name    Item   int main void        Item item     5  2 2   George         return 0      An important thing to remember  at the moment you initialize even one object  variable in the struct  all of its other variables will be initialized to default value   If you don t initialize the values in your struct  all variables will contain  garbage values    Good luck

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a    MYTYPE   true  15  0 123     would do fine in C99

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