• KUV (7/7) Oct 10 2005 struct NodeLink {
• Jarrett Billingsley (14/21) Oct 10 2005 This is because of member alignment, for optimization reasons (it's fast...
• Oskar Linde (18/34) Oct 13 2005 For this reason, it is also generally recommended (when possible) to put...

KUV <KUV_member pathlink.com> writes:

struct NodeLink {
char c;
int node;
}
...
printf("%i\n", NodeLink.sizeof);

This code prints 8, but summary size of fields is 5. Is it normal?

"Jarrett Billingsley" <kb3ctd2 yahoo.com> writes:

"KUV" <KUV_member pathlink.com> wrote in message 
news:dieoqt$15r2$1 digitaldaemon.com...
 struct NodeLink {
 char c;
 int node;
 }
 ...
 printf("%i\n", NodeLink.sizeof);

 This code prints 8, but summary size of fields is 5. Is it normal?

This is because of member alignment, for optimization reasons (it's faster 
for the processor to access memory on 32-bit boundaries).  If you put an 
align(1) in front of the struct, like so:

align(1) struct NodeLink
{
    char c;
    int node;
}

The NodeLink.sizeof will be 5.  Of course, if the layout of the struct 
doesn't have to be so strict, it'd be recommended to use the default 
alignment for performance reasons; but if this is being used to paint a data 
structure over an arbitrary chunk of memory, you'd need the align(1). 

Oskar Linde <olREM OVEnada.kth.se> writes:

Jarrett Billingsley wrote:


 This code prints 8, but summary size of fields is 5. Is it normal?

 
 This is because of member alignment, for optimization reasons (it's faster
 for the processor to access memory on 32-bit boundaries).  If you put an
 align(1) in front of the struct, like so:
 
 align(1) struct NodeLink
 {
     char c;
     int node;
 }
 
 The NodeLink.sizeof will be 5.  Of course, if the layout of the struct
 doesn't have to be so strict, it'd be recommended to use the default
 alignment for performance reasons; but if this is being used to paint a
 data structure over an arbitrary chunk of memory, you'd need the align(1).

For this reason, it is also generally recommended (when possible) to put the
larger fields before the smaller in the struct..

struct a {
  int a;
  short b;
  short c;
}

struct b {
  short b;
  int a;
  short c;
}

void main() {
    printf("%d %d\n",a.sizeof,b.sizeof);
}

should generally print 8 12.

/Oskar