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I was trying to check the number of bytes an array of structures contains. 

struct coins {
    int value;
    char name[10];
}
int main() {
    struct coins allCoins[8];
    printf("Total size of the array of structures = %d bytes \n", sizeof(allCoins)); // == 128 bytes (containing 8 structures => 16 bytes / structure)
    printf("Size of a structure within the array of structures = %d bytes \n", sizeof(allCoins[0]));
    printf("Size of the 'value' variable of the structure within the array of structures = %d bytes \n", sizeof(allCoins[0].value));
    printf("Size of the 'name' string array of the structure within the array of structures = %d bytes \n", sizeof(allCoins[0].name));
    return 0;
}

The output is:

Total size of the array of structures = 128 bytes                                                        
Size of a structure within the array of structures = 16 bytes                                                     
Size of the 'value' variable of the structure within the array of structures = 4 bytes
Size of the 'name' string array of the structure within the array of structures = 10 bytes

The array of structures takes 128 bytes, for 8 structures this makes 16 bytes per structure. Each structure however contains an int which is 4 bytes and a string array of 10 characters which should be another 10 bytes. So, totally a structure should contain 10 + 4 = 14 bytes. But it seems it has 16. Where are the other 2 bytes coming from ?

With char name[13]; the output is:

Total size of the array of structures = 160 bytes 
Size of a structure within the array of structures = 20 bytes 
Size of the 'value' variable of the structure within the array of structures = 4 bytes 
Size of the 'name' string array of the structure within the array of structures = 13 bytes

And with char name[17];:

Total size of the array of structures = 192 bytes 
Size of a structure within the array of structures = 24 bytes 
Size of the 'value' variable of the structure within the array of structures = 4 bytes 
Size of the 'name' string array of the structure within the array of structures = 17 bytes

So, it seems that the struct size is a multiple of 4 (16 - 20 - 24) but why ? How is the size calculated or determined and what rules apply ?

Mario Christov
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    There is a concept of "padding" with structures. See [this info](https://stackoverflow.com/questions/4306186/structure-padding-and-packing) – Paul T. Dec 08 '17 at 05:28
  • integer alignment, whis is just another way of saying padding. you could try `#pragma pack` if your target architecture allows unaligned ints. – Jasen Dec 08 '17 at 05:34
  • Structures are usually padded as needed to preserve data type alignments. Presumably an `int` on your machine wants to be aligned on a 4-byte boundary, so in order to maintain alignment in an array or structs, the structure size must be padded to a multiple of 4. – Tom Karzes Dec 08 '17 at 05:35
  • thanks all for the info & link, it was what I wanted to know – Mario Christov Dec 08 '17 at 05:54
  • https://fresh2refresh.com/c-programming/c-structure-padding/ – anil Dec 08 '17 at 07:07

0 Answers0