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What is the size (sizeof(How_Many_Bytes)) of the following structure, given the following machine parameters: 

sizeof(char) == 1; sizeof(int) == 4; sizeof(long) == 8; sizeof(char *) == 8;

Integer values must be aligned. 

typedef struct how_many_bytes { 
 long s; 
 char c, e; 
 int i; 
 char *d; 
} How_Many_Bytes;

I thought it would be 4+1+1+(2+4)+8 = 20 bytes but when I run on my machine I get 24 bytes. I wonder why?

Jonathan Leffler
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ivesingh
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1 Answers1

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Conceptually, what happens is this: 

typedef struct how_many_bytes { 
 long s;          // 8   (NOT 4!)
 char c, e;       // 2
 char pad1, pad2; // 2 note these
 int i;           // 4 
 char *d;         // 8
} How_Many_Bytes; // 24 total

Some types have alignment requirements. Often on 4 or 8 byte boundaries. So what the compiler does is to make the fields align on these boundaries by added unnamed empty fields.

Charlie Burns
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