Difference between a Structure and a Union

Question

Is there any good example to give the difference between a struct and a union  Basically I know that struct uses all the memory of its member and union uses the largest members memory space  Is there any other OS level difference

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union  and  struct  are constructs of the C language  Talking of an  OS level  difference between them is inappropriate  since it s the compiler that produces different code if you use one or another keyword

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A union is useful in a couple scenarios  union can be a tool for very low level manipulation like writing device drivers for a kernel   An example of that is dissecting a float number by using union of a struct with bitfields and a float  I save a number in the float  and later I can access particular parts of the float through that struct  The example shows how union is used to have different angles to look at  data    include  lt stdio h gt                                                                                                                                          union foo       struct float guts           unsigned int fraction   23          unsigned int exponent   8          unsigned int sign       1        fg      float f      void print float float f        union foo ff      ff f   f      printf   f   d 0x X 0x X n   f  ff fg sign  ff fg exponent  ff fg fraction       int main        print float 0 15625       return 0      Take a look at single precision description on wikipedia  I used the example and the magic number 0 15625 from there     union can also be used to implement an algebraic data type that has multiple alternatives  I found an example of that in the  Real World Haskell  book by  O Sullivan  Stewart  and Goerzen  Check it out in the The discriminated union section   Cheers

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union  and  struct  are constructs of the C language  Talking of an  OS level  difference between them is inappropriate  since it s the compiler that produces different code if you use one or another keyword

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The uses of union Unions are used frequently when specialized type conversations are needed  To get an idea of the usefulness of union  The c c   standard library defines no function specifically designed to write short integers to a file  Using fwrite   incurs encurs excessive overhead for simple operation  However using a union you can easily create a function which writes binary of a short integer to a file one byte at a time  I assume that short integers are 2 byte long  THE EXAMPLE    include lt stdio h gt  union pw   short int i  char ch 2      int putw short int num  FILE  fp   int main  void    FILE  fp  fp fopen  test tmp    wb     putw 1000  fp      write the value 1000 as an integer   fclose fp   return 0    int putw short int num  FILE  fp    pw word  word i   num  putc word c 0    fp   return putc word c 1    fp           although putw   i called with short integer  it was possble to use putc   and fwrite    But i wanted to show an example to dominstrate how a union can be used

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A union is useful in a couple scenarios  union can be a tool for very low level manipulation like writing device drivers for a kernel   An example of that is dissecting a float number by using union of a struct with bitfields and a float  I save a number in the float  and later I can access particular parts of the float through that struct  The example shows how union is used to have different angles to look at  data    include  lt stdio h gt                                                                                                                                          union foo       struct float guts           unsigned int fraction   23          unsigned int exponent   8          unsigned int sign       1        fg      float f      void print float float f        union foo ff      ff f   f      printf   f   d 0x X 0x X n   f  ff fg sign  ff fg exponent  ff fg fraction       int main        print float 0 15625       return 0      Take a look at single precision description on wikipedia  I used the example and the magic number 0 15625 from there     union can also be used to implement an algebraic data type that has multiple alternatives  I found an example of that in the  Real World Haskell  book by  O Sullivan  Stewart  and Goerzen  Check it out in the The discriminated union section   Cheers

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Here s the short answer  a struct is a record structure  each element in the struct allocates new space   So  a struct like  struct foobarbazquux t       int foo      long bar      double baz       long double quux      allocates at least  sizeof int  sizeof long  sizeof double  sizeof long double   bytes in memory for each instance     At least  because architecture alignment constraints may force the compiler to pad the struct    On the other hand   union foobarbazquux u       int foo      long bar      double baz       long double quux      allocates one chunk of memory and gives it four aliases   So sizeof union foobarbazquux u    max  sizeof int  sizeof long  sizeof double  sizeof long double    again with the possibility of some addition for alignments

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The uses of union Unions are used frequently when specialized type conversations are needed  To get an idea of the usefulness of union  The c c   standard library defines no function specifically designed to write short integers to a file  Using fwrite   incurs encurs excessive overhead for simple operation  However using a union you can easily create a function which writes binary of a short integer to a file one byte at a time  I assume that short integers are 2 byte long  THE EXAMPLE    include lt stdio h gt  union pw   short int i  char ch 2      int putw short int num  FILE  fp   int main  void    FILE  fp  fp fopen  test tmp    wb     putw 1000  fp      write the value 1000 as an integer   fclose fp   return 0    int putw short int num  FILE  fp    pw word  word i   num  putc word c 0    fp   return putc word c 1    fp           although putw   i called with short integer  it was possble to use putc   and fwrite    But i wanted to show an example to dominstrate how a union can be used

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With a union  you re only supposed to use one of the elements  because they re all stored at the same spot  This makes it useful when you want to store something that could be one of several types  A struct  on the other hand  has a separate memory location for each of its elements and they all can be used at once   To give a concrete example of their use  I was working on a Scheme interpreter a little while ago and I was essentially overlaying the Scheme data types onto the C data types  This involved storing in a struct an enum indicating the type of value and a union to store that value   union foo     int a       can t use both a and b at once   char b    foo   struct bar     int a       can use both a and b simultaneously   char b    bar   union foo x  x a   3     OK x b    c      NO  this affects the value of x a   struct bar y  y a   3     OK y b    c      OK   edit  If you re wondering what setting x b to  c  changes the value of x a to  technically speaking it s undefined  On most modern machines a char is 1 byte and an int is 4 bytes  so giving x b the value  c  also gives the first byte of x a that same value   union foo x  x a   3  x b    c   printf   i   i n   x a  x b     prints  99  99   Why are the two values the same  Because the last 3 bytes of the int 3 are all zero  so it s also read as 99  If we put in a larger number for x a  you ll see that this is not always the case   union foo x  x a   387439  x b    c   printf   i   i n   x a  x b     prints  387427  99   To get a closer look at the actual memory values  let s set and print out the values in hex   union foo x  x a   0xDEADBEEF  x b   0x22  printf   x   x n   x a  x b     prints  deadbe22  22   You can clearly see where the 0x22 overwrote the 0xEF   BUT  In C  the order of bytes in an int are not defined  This program overwrote the 0xEF with 0x22 on my Mac  but there are other platforms where it would overwrite the 0xDE instead because the order of the bytes that make up the int were reversed  Therefore  when writing a program  you should never rely on the behavior of overwriting specific data in a union because it s not portable   For more reading on the ordering of bytes  check out endianness

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Yes  the main difference between struct and union is same as you stated  Struct uses all the memory of its member and union uses the largest members memory space   But all the difference lies by the usage need of the memory  Best usage of the union can be seen in the processes of unix where we make use of signals  like a process can act upon only one signal at a time  So the general declaration will be   union SIGSELECT     SIGNAL 1 signal1    SIGNAL 2 signal2               In this case  process make use of only the highest memory of all signals  but if you use struct in this case  memory usage will be sum of all signals   Makes a lot of difference   To summarize  Union should be selected if you know that you access any one of the member at a time

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A structure allocates the total size of all elements in it   A union only allocates as much memory as its largest member requires

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Here s the short answer  a struct is a record structure  each element in the struct allocates new space   So  a struct like  struct foobarbazquux t       int foo      long bar      double baz       long double quux      allocates at least  sizeof int  sizeof long  sizeof double  sizeof long double   bytes in memory for each instance     At least  because architecture alignment constraints may force the compiler to pad the struct    On the other hand   union foobarbazquux u       int foo      long bar      double baz       long double quux      allocates one chunk of memory and gives it four aliases   So sizeof union foobarbazquux u    max  sizeof int  sizeof long  sizeof double  sizeof long double    again with the possibility of some addition for alignments

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Is there any good example to give the difference between a  struct  and a  union     An imaginary communications protocol  struct packetheader      int sourceaddress     int destaddress     int messagetype     union request          char fourcc 4          int requestnumber             In this imaginary protocol  it has been sepecified that  based on the  message type   the following location in the header will either be a request number  or a four character code  but not both   In short  unions allow for the same storage location to represent more than one data type  where it is guaranteed that you will only want to store one of the types of data at any one time   Unions are largely a low-level detail based in C s heritage as a system programming language  where  overlapping  storage locations are sometimes used in this way   You can sometimes use unions to save memory where you have a data structure where only one of several types will be saved at one time    In general  the OS doesn t care or know about structs and unions -- they are both simply blocks of memory to it   A struct is a block of memory that stores several data objects  where those objects don t overlap   A union is a block of memory that stores several data objects  but has only storage for the largest of these  and thus can only store one of the data objects at any one time

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As you already state in your question  the main difference between union and struct is that union members overlay the memory of each other so that the sizeof of a union is the one   while struct members are laid out one after each other  with optional padding in between   Also an union is large enough to contain all its members  and have an alignment that fits all its members  So let s say int can only be stored at 2 byte addresses and is 2 bytes wide  and long can only be stored at 4 byte addresses and is 4 bytes long  The following union  union test       int a      long b        could have a sizeof of 4  and an alignment requirement of 4  Both an union and a struct can have padding at the end  but not at their beginning  Writing to a struct changes only the value of the member written to  Writing to a member of an union will render the value of all other members invalid  You cannot access them if you haven t written to them before  otherwise the behavior is undefined  GCC provides as an extension that you can actually read from members of an union  even though you haven t written to them most recently  For an Operation System  it doesn t have to matter whether a user program writes to an union or to a structure  This actually is only an issue of the compiler    Another important property of union and struct is  they allow that a pointer to them can point to types of any of its members  So the following is valid   struct test       int a      double b      some test pointer    some test pointer can point to int  or double   If you cast an address of type test to int   it will point to its first member  a  actually  The same is true for an union too  Thus  because an union will always have the right alignment  you can use an union to make pointing to some type valid   union a       int a      double b       That union will actually be able to point to an int  and a double   union a   v    union a  some int pointer   some int pointer   5  v- gt a   10  return  some int pointer        is actually valid  as stated by the C99 standard      An object shall have its stored value accessed only by an lvalue expression that has one of the following types            a type compatible with the effective type of the object           an aggregate or union type that includes one of the aforementioned types among its members      The compiler won t optimize out the v- gt a   10  as it could affect the value of  some int pointer  and the function will return 10 instead of 5

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Non technically speaking means    Assumption  chair   memory block   people   variable  Structure   If there are 3 people they can sit in chair of their size correspondingly      Union   If there are 3 people only one chair will be there to sit   all need to use the same chair when they want to sit    Technically speaking means    The below mentioned program gives a deep dive into structure and union together    struct MAIN STRUCT   UINT64 bufferaddr     union       UINT32 data      struct INNER STRUCT          UINT16 length            UINT8 cso            UINT8 cmd                 flags         data1       Total  MAIN STRUCT size  sizeof UINT64  for bufferaddr   sizeof UNIT32  for union   32 bit for padding depends on processor architecture    128 bits    For structure all the members get the memory block contiguously    Union gets one memory block of  the max size member Here its 32 bit    Inside union one more structure lies INNER STRUCT  its members get a memory block of total size  32 bits 16 8 8    In union either INNER STRUCT 32 bit  member or data 32 bit  can be accessed

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Is there any good example to give the difference between a  struct  and a  union     An imaginary communications protocol  struct packetheader      int sourceaddress     int destaddress     int messagetype     union request          char fourcc 4          int requestnumber             In this imaginary protocol  it has been sepecified that  based on the  message type   the following location in the header will either be a request number  or a four character code  but not both   In short  unions allow for the same storage location to represent more than one data type  where it is guaranteed that you will only want to store one of the types of data at any one time   Unions are largely a low-level detail based in C s heritage as a system programming language  where  overlapping  storage locations are sometimes used in this way   You can sometimes use unions to save memory where you have a data structure where only one of several types will be saved at one time    In general  the OS doesn t care or know about structs and unions -- they are both simply blocks of memory to it   A struct is a block of memory that stores several data objects  where those objects don t overlap   A union is a block of memory that stores several data objects  but has only storage for the largest of these  and thus can only store one of the data objects at any one time

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You have it  that s all  But so  basically  what s the point of unions   You can put in the same location content of different types  You have to know the type of what you have stored in the union  so often you put it in a struct with a type tag       Why is this important  Not really for space gains  Yes  you can gain some bits or do some padding  but that s not the main point anymore   It s for type safety  it enables you to do some kind of  dynamic typing   the compiler knows that your content may have different meanings and the precise meaning of how your interpret it is up to you at run-time  If you have a pointer that can point to different types  you MUST use a union  otherwise you code may be incorrect due to aliasing problems  the compiler says to itself  oh  only this pointer can point to this type  so I can optimize out those accesses      and bad things can happen

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You have it  that s all  But so  basically  what s the point of unions   You can put in the same location content of different types  You have to know the type of what you have stored in the union  so often you put it in a struct with a type tag       Why is this important  Not really for space gains  Yes  you can gain some bits or do some padding  but that s not the main point anymore   It s for type safety  it enables you to do some kind of  dynamic typing   the compiler knows that your content may have different meanings and the precise meaning of how your interpret it is up to you at run-time  If you have a pointer that can point to different types  you MUST use a union  otherwise you code may be incorrect due to aliasing problems  the compiler says to itself  oh  only this pointer can point to this type  so I can optimize out those accesses      and bad things can happen

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With a union  you re only supposed to use one of the elements  because they re all stored at the same spot  This makes it useful when you want to store something that could be one of several types  A struct  on the other hand  has a separate memory location for each of its elements and they all can be used at once   To give a concrete example of their use  I was working on a Scheme interpreter a little while ago and I was essentially overlaying the Scheme data types onto the C data types  This involved storing in a struct an enum indicating the type of value and a union to store that value   union foo     int a       can t use both a and b at once   char b    foo   struct bar     int a       can use both a and b simultaneously   char b    bar   union foo x  x a   3     OK x b    c      NO  this affects the value of x a   struct bar y  y a   3     OK y b    c      OK   edit  If you re wondering what setting x b to  c  changes the value of x a to  technically speaking it s undefined  On most modern machines a char is 1 byte and an int is 4 bytes  so giving x b the value  c  also gives the first byte of x a that same value   union foo x  x a   3  x b    c   printf   i   i n   x a  x b     prints  99  99   Why are the two values the same  Because the last 3 bytes of the int 3 are all zero  so it s also read as 99  If we put in a larger number for x a  you ll see that this is not always the case   union foo x  x a   387439  x b    c   printf   i   i n   x a  x b     prints  387427  99   To get a closer look at the actual memory values  let s set and print out the values in hex   union foo x  x a   0xDEADBEEF  x b   0x22  printf   x   x n   x a  x b     prints  deadbe22  22   You can clearly see where the 0x22 overwrote the 0xEF   BUT  In C  the order of bytes in an int are not defined  This program overwrote the 0xEF with 0x22 on my Mac  but there are other platforms where it would overwrite the 0xDE instead because the order of the bytes that make up the int were reversed  Therefore  when writing a program  you should never rely on the behavior of overwriting specific data in a union because it s not portable   For more reading on the ordering of bytes  check out endianness

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As you already state in your question  the main difference between union and struct is that union members overlay the memory of each other so that the sizeof of a union is the one   while struct members are laid out one after each other  with optional padding in between   Also an union is large enough to contain all its members  and have an alignment that fits all its members  So let s say int can only be stored at 2 byte addresses and is 2 bytes wide  and long can only be stored at 4 byte addresses and is 4 bytes long  The following union  union test       int a      long b        could have a sizeof of 4  and an alignment requirement of 4  Both an union and a struct can have padding at the end  but not at their beginning  Writing to a struct changes only the value of the member written to  Writing to a member of an union will render the value of all other members invalid  You cannot access them if you haven t written to them before  otherwise the behavior is undefined  GCC provides as an extension that you can actually read from members of an union  even though you haven t written to them most recently  For an Operation System  it doesn t have to matter whether a user program writes to an union or to a structure  This actually is only an issue of the compiler    Another important property of union and struct is  they allow that a pointer to them can point to types of any of its members  So the following is valid   struct test       int a      double b      some test pointer    some test pointer can point to int  or double   If you cast an address of type test to int   it will point to its first member  a  actually  The same is true for an union too  Thus  because an union will always have the right alignment  you can use an union to make pointing to some type valid   union a       int a      double b       That union will actually be able to point to an int  and a double   union a   v    union a  some int pointer   some int pointer   5  v- gt a   10  return  some int pointer        is actually valid  as stated by the C99 standard      An object shall have its stored value accessed only by an lvalue expression that has one of the following types            a type compatible with the effective type of the object           an aggregate or union type that includes one of the aforementioned types among its members      The compiler won t optimize out the v- gt a   10  as it could affect the value of  some int pointer  and the function will return 10 instead of 5

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A structure allocates the total size of all elements in it   A union only allocates as much memory as its largest member requires

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Is there any good example to give the difference between a  struct  and a  union     An imaginary communications protocol  struct packetheader      int sourceaddress     int destaddress     int messagetype     union request          char fourcc 4          int requestnumber             In this imaginary protocol  it has been sepecified that  based on the  message type   the following location in the header will either be a request number  or a four character code  but not both   In short  unions allow for the same storage location to represent more than one data type  where it is guaranteed that you will only want to store one of the types of data at any one time   Unions are largely a low-level detail based in C s heritage as a system programming language  where  overlapping  storage locations are sometimes used in this way   You can sometimes use unions to save memory where you have a data structure where only one of several types will be saved at one time    In general  the OS doesn t care or know about structs and unions -- they are both simply blocks of memory to it   A struct is a block of memory that stores several data objects  where those objects don t overlap   A union is a block of memory that stores several data objects  but has only storage for the largest of these  and thus can only store one of the data objects at any one time

User · Answer

Yes  the main difference between struct and union is same as you stated  Struct uses all the memory of its member and union uses the largest members memory space   But all the difference lies by the usage need of the memory  Best usage of the union can be seen in the processes of unix where we make use of signals  like a process can act upon only one signal at a time  So the general declaration will be   union SIGSELECT     SIGNAL 1 signal1    SIGNAL 2 signal2               In this case  process make use of only the highest memory of all signals  but if you use struct in this case  memory usage will be sum of all signals   Makes a lot of difference   To summarize  Union should be selected if you know that you access any one of the member at a time

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A structure allocates the total size of all elements in it   A union only allocates as much memory as its largest member requires

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what is the difference between structure and union   Short cut answer is  The deference is in memory allocation  Explanation   In structure  memory space will be created for all members inside structure  In union memory space will be created only for a member which needs largest memory space  Consider the following code   struct s tag      int a      long int b    x   union u tag      int a      long int b    y    Here there are two members inside struct and union  int and long int  Memory space for int is  4 byte and Memory space for long int is  8 in 32 bit operating system   So for struct 4 8 12 bytes will be created while 8 bytes will be created for union  Code example    include lt stdio h gt  struct s tag     int a    long int b    x  union u tag        int a       long int b    y  int main         printf  Memory allocation for structure    d   sizeof x        printf   nMemory allocation for union    d   sizeof y        return 0      Ref http   www codingpractise com home c-programming structure-and-union

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Non technically speaking means    Assumption  chair   memory block   people   variable  Structure   If there are 3 people they can sit in chair of their size correspondingly      Union   If there are 3 people only one chair will be there to sit   all need to use the same chair when they want to sit    Technically speaking means    The below mentioned program gives a deep dive into structure and union together    struct MAIN STRUCT   UINT64 bufferaddr     union       UINT32 data      struct INNER STRUCT          UINT16 length            UINT8 cso            UINT8 cmd                 flags         data1       Total  MAIN STRUCT size  sizeof UINT64  for bufferaddr   sizeof UNIT32  for union   32 bit for padding depends on processor architecture    128 bits    For structure all the members get the memory block contiguously    Union gets one memory block of  the max size member Here its 32 bit    Inside union one more structure lies INNER STRUCT  its members get a memory block of total size  32 bits 16 8 8    In union either INNER STRUCT 32 bit  member or data 32 bit  can be accessed

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union  and  struct  are constructs of the C language  Talking of an  OS level  difference between them is inappropriate  since it s the compiler that produces different code if you use one or another keyword

User · Answer

You have it  that s all  But so  basically  what s the point of unions   You can put in the same location content of different types  You have to know the type of what you have stored in the union  so often you put it in a struct with a type tag       Why is this important  Not really for space gains  Yes  you can gain some bits or do some padding  but that s not the main point anymore   It s for type safety  it enables you to do some kind of  dynamic typing   the compiler knows that your content may have different meanings and the precise meaning of how your interpret it is up to you at run-time  If you have a pointer that can point to different types  you MUST use a union  otherwise you code may be incorrect due to aliasing problems  the compiler says to itself  oh  only this pointer can point to this type  so I can optimize out those accesses      and bad things can happen

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You have it  that s all  But so  basically  what s the point of unions   You can put in the same location content of different types  You have to know the type of what you have stored in the union  so often you put it in a struct with a type tag       Why is this important  Not really for space gains  Yes  you can gain some bits or do some padding  but that s not the main point anymore   It s for type safety  it enables you to do some kind of  dynamic typing   the compiler knows that your content may have different meanings and the precise meaning of how your interpret it is up to you at run-time  If you have a pointer that can point to different types  you MUST use a union  otherwise you code may be incorrect due to aliasing problems  the compiler says to itself  oh  only this pointer can point to this type  so I can optimize out those accesses      and bad things can happen

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Unions come handy while writing a byte ordering function which is given below  It s not possible with structs   int main int argc  char   argv        union           short   s          char    c sizeof short          un       un s   0x0102       if  sizeof short     2            if  un c 0     1  amp  amp  un c 1     2              printf  big-endian n            else if  un c 0     2  amp  amp  un c 1     1              printf  little-endian n            else             printf  unknown n          else         printf  sizeof short     d n   sizeof short         exit 0        Program from Unix Network Programming Vol  1 by Stevens

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With a union  you re only supposed to use one of the elements  because they re all stored at the same spot  This makes it useful when you want to store something that could be one of several types  A struct  on the other hand  has a separate memory location for each of its elements and they all can be used at once   To give a concrete example of their use  I was working on a Scheme interpreter a little while ago and I was essentially overlaying the Scheme data types onto the C data types  This involved storing in a struct an enum indicating the type of value and a union to store that value   union foo     int a       can t use both a and b at once   char b    foo   struct bar     int a       can use both a and b simultaneously   char b    bar   union foo x  x a   3     OK x b    c      NO  this affects the value of x a   struct bar y  y a   3     OK y b    c      OK   edit  If you re wondering what setting x b to  c  changes the value of x a to  technically speaking it s undefined  On most modern machines a char is 1 byte and an int is 4 bytes  so giving x b the value  c  also gives the first byte of x a that same value   union foo x  x a   3  x b    c   printf   i   i n   x a  x b     prints  99  99   Why are the two values the same  Because the last 3 bytes of the int 3 are all zero  so it s also read as 99  If we put in a larger number for x a  you ll see that this is not always the case   union foo x  x a   387439  x b    c   printf   i   i n   x a  x b     prints  387427  99   To get a closer look at the actual memory values  let s set and print out the values in hex   union foo x  x a   0xDEADBEEF  x b   0x22  printf   x   x n   x a  x b     prints  deadbe22  22   You can clearly see where the 0x22 overwrote the 0xEF   BUT  In C  the order of bytes in an int are not defined  This program overwrote the 0xEF with 0x22 on my Mac  but there are other platforms where it would overwrite the 0xDE instead because the order of the bytes that make up the int were reversed  Therefore  when writing a program  you should never rely on the behavior of overwriting specific data in a union because it s not portable   For more reading on the ordering of bytes  check out endianness

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what is the difference between structure and union   Short cut answer is  The deference is in memory allocation  Explanation   In structure  memory space will be created for all members inside structure  In union memory space will be created only for a member which needs largest memory space  Consider the following code   struct s tag      int a      long int b    x   union u tag      int a      long int b    y    Here there are two members inside struct and union  int and long int  Memory space for int is  4 byte and Memory space for long int is  8 in 32 bit operating system   So for struct 4 8 12 bytes will be created while 8 bytes will be created for union  Code example    include lt stdio h gt  struct s tag     int a    long int b    x  union u tag        int a       long int b    y  int main         printf  Memory allocation for structure    d   sizeof x        printf   nMemory allocation for union    d   sizeof y        return 0      Ref http   www codingpractise com home c-programming structure-and-union

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A Union is different from a struct as a Union repeats over the others  it redefines the same memory whilst the struct defines one after the other with no overlaps or redefinitions

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structure is collection of different data type where different type of data can reside in it and every one get its own block of memory   we usually used union when we sure that only one of the variable will be used at once and you want fully utilization of present memory because it get only one block of memory which is equal to the biggest type   struct emp       char x   1 byte     float y    4 byte   e    total memory it get   5 byte  union emp       char x   1 byte     float y    4 byte   e    total memory it get  4 byte

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union  and  struct  are constructs of the C language  Talking of an  OS level  difference between them is inappropriate  since it s the compiler that produces different code if you use one or another keyword

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Unions come handy while writing a byte ordering function which is given below  It s not possible with structs   int main int argc  char   argv        union           short   s          char    c sizeof short          un       un s   0x0102       if  sizeof short     2            if  un c 0     1  amp  amp  un c 1     2              printf  big-endian n            else if  un c 0     2  amp  amp  un c 1     1              printf  little-endian n            else             printf  unknown n          else         printf  sizeof short     d n   sizeof short         exit 0        Program from Unix Network Programming Vol  1 by Stevens

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A structure allocates the total size of all elements in it   A union only allocates as much memory as its largest member requires

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Here s the short answer  a struct is a record structure  each element in the struct allocates new space   So  a struct like  struct foobarbazquux t       int foo      long bar      double baz       long double quux      allocates at least  sizeof int  sizeof long  sizeof double  sizeof long double   bytes in memory for each instance     At least  because architecture alignment constraints may force the compiler to pad the struct    On the other hand   union foobarbazquux u       int foo      long bar      double baz       long double quux      allocates one chunk of memory and gives it four aliases   So sizeof union foobarbazquux u    max  sizeof int  sizeof long  sizeof double  sizeof long double    again with the possibility of some addition for alignments

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A Union is different from a struct as a Union repeats over the others  it redefines the same memory whilst the struct defines one after the other with no overlaps or redefinitions

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Here s the short answer  a struct is a record structure  each element in the struct allocates new space   So  a struct like  struct foobarbazquux t       int foo      long bar      double baz       long double quux      allocates at least  sizeof int  sizeof long  sizeof double  sizeof long double   bytes in memory for each instance     At least  because architecture alignment constraints may force the compiler to pad the struct    On the other hand   union foobarbazquux u       int foo      long bar      double baz       long double quux      allocates one chunk of memory and gives it four aliases   So sizeof union foobarbazquux u    max  sizeof int  sizeof long  sizeof double  sizeof long double    again with the possibility of some addition for alignments

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As you already state in your question  the main difference between union and struct is that union members overlay the memory of each other so that the sizeof of a union is the one   while struct members are laid out one after each other  with optional padding in between   Also an union is large enough to contain all its members  and have an alignment that fits all its members  So let s say int can only be stored at 2 byte addresses and is 2 bytes wide  and long can only be stored at 4 byte addresses and is 4 bytes long  The following union  union test       int a      long b        could have a sizeof of 4  and an alignment requirement of 4  Both an union and a struct can have padding at the end  but not at their beginning  Writing to a struct changes only the value of the member written to  Writing to a member of an union will render the value of all other members invalid  You cannot access them if you haven t written to them before  otherwise the behavior is undefined  GCC provides as an extension that you can actually read from members of an union  even though you haven t written to them most recently  For an Operation System  it doesn t have to matter whether a user program writes to an union or to a structure  This actually is only an issue of the compiler    Another important property of union and struct is  they allow that a pointer to them can point to types of any of its members  So the following is valid   struct test       int a      double b      some test pointer    some test pointer can point to int  or double   If you cast an address of type test to int   it will point to its first member  a  actually  The same is true for an union too  Thus  because an union will always have the right alignment  you can use an union to make pointing to some type valid   union a       int a      double b       That union will actually be able to point to an int  and a double   union a   v    union a  some int pointer   some int pointer   5  v- gt a   10  return  some int pointer        is actually valid  as stated by the C99 standard      An object shall have its stored value accessed only by an lvalue expression that has one of the following types            a type compatible with the effective type of the object           an aggregate or union type that includes one of the aforementioned types among its members      The compiler won t optimize out the v- gt a   10  as it could affect the value of  some int pointer  and the function will return 10 instead of 5

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As you already state in your question  the main difference between union and struct is that union members overlay the memory of each other so that the sizeof of a union is the one   while struct members are laid out one after each other  with optional padding in between   Also an union is large enough to contain all its members  and have an alignment that fits all its members  So let s say int can only be stored at 2 byte addresses and is 2 bytes wide  and long can only be stored at 4 byte addresses and is 4 bytes long  The following union  union test       int a      long b        could have a sizeof of 4  and an alignment requirement of 4  Both an union and a struct can have padding at the end  but not at their beginning  Writing to a struct changes only the value of the member written to  Writing to a member of an union will render the value of all other members invalid  You cannot access them if you haven t written to them before  otherwise the behavior is undefined  GCC provides as an extension that you can actually read from members of an union  even though you haven t written to them most recently  For an Operation System  it doesn t have to matter whether a user program writes to an union or to a structure  This actually is only an issue of the compiler    Another important property of union and struct is  they allow that a pointer to them can point to types of any of its members  So the following is valid   struct test       int a      double b      some test pointer    some test pointer can point to int  or double   If you cast an address of type test to int   it will point to its first member  a  actually  The same is true for an union too  Thus  because an union will always have the right alignment  you can use an union to make pointing to some type valid   union a       int a      double b       That union will actually be able to point to an int  and a double   union a   v    union a  some int pointer   some int pointer   5  v- gt a   10  return  some int pointer        is actually valid  as stated by the C99 standard      An object shall have its stored value accessed only by an lvalue expression that has one of the following types            a type compatible with the effective type of the object           an aggregate or union type that includes one of the aforementioned types among its members      The compiler won t optimize out the v- gt a   10  as it could affect the value of  some int pointer  and the function will return 10 instead of 5

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Is there any good example to give the difference between a  struct  and a  union     An imaginary communications protocol  struct packetheader      int sourceaddress     int destaddress     int messagetype     union request          char fourcc 4          int requestnumber             In this imaginary protocol  it has been sepecified that  based on the  message type   the following location in the header will either be a request number  or a four character code  but not both   In short  unions allow for the same storage location to represent more than one data type  where it is guaranteed that you will only want to store one of the types of data at any one time   Unions are largely a low-level detail based in C s heritage as a system programming language  where  overlapping  storage locations are sometimes used in this way   You can sometimes use unions to save memory where you have a data structure where only one of several types will be saved at one time    In general  the OS doesn t care or know about structs and unions -- they are both simply blocks of memory to it   A struct is a block of memory that stores several data objects  where those objects don t overlap   A union is a block of memory that stores several data objects  but has only storage for the largest of these  and thus can only store one of the data objects at any one time

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With a union  you re only supposed to use one of the elements  because they re all stored at the same spot  This makes it useful when you want to store something that could be one of several types  A struct  on the other hand  has a separate memory location for each of its elements and they all can be used at once   To give a concrete example of their use  I was working on a Scheme interpreter a little while ago and I was essentially overlaying the Scheme data types onto the C data types  This involved storing in a struct an enum indicating the type of value and a union to store that value   union foo     int a       can t use both a and b at once   char b    foo   struct bar     int a       can use both a and b simultaneously   char b    bar   union foo x  x a   3     OK x b    c      NO  this affects the value of x a   struct bar y  y a   3     OK y b    c      OK   edit  If you re wondering what setting x b to  c  changes the value of x a to  technically speaking it s undefined  On most modern machines a char is 1 byte and an int is 4 bytes  so giving x b the value  c  also gives the first byte of x a that same value   union foo x  x a   3  x b    c   printf   i   i n   x a  x b     prints  99  99   Why are the two values the same  Because the last 3 bytes of the int 3 are all zero  so it s also read as 99  If we put in a larger number for x a  you ll see that this is not always the case   union foo x  x a   387439  x b    c   printf   i   i n   x a  x b     prints  387427  99   To get a closer look at the actual memory values  let s set and print out the values in hex   union foo x  x a   0xDEADBEEF  x b   0x22  printf   x   x n   x a  x b     prints  deadbe22  22   You can clearly see where the 0x22 overwrote the 0xEF   BUT  In C  the order of bytes in an int are not defined  This program overwrote the 0xEF with 0x22 on my Mac  but there are other platforms where it would overwrite the 0xDE instead because the order of the bytes that make up the int were reversed  Therefore  when writing a program  you should never rely on the behavior of overwriting specific data in a union because it s not portable   For more reading on the ordering of bytes  check out endianness

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structure is collection of different data type where different type of data can reside in it and every one get its own block of memory   we usually used union when we sure that only one of the variable will be used at once and you want fully utilization of present memory because it get only one block of memory which is equal to the biggest type   struct emp       char x   1 byte     float y    4 byte   e    total memory it get   5 byte  union emp       char x   1 byte     float y    4 byte   e    total memory it get  4 byte

[c] Difference between a Structure and a Union

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