Experimental Physics and
Industrial Control System

Hello Andrew,

I tried the gcc packed attribute as below (and individually on structure member):

typedef struct __attribute__ ((packed)){
     char           ch;      
     unsigned char  uCh;    
     short          sInt;   
     unsigned short usInt; 
     long           lInt ;   
     unsigned long  ulInt;   
     float          flt;    
} testdata_ts; /* 18 bytes */ 

On solaris-sparc, and I've found that it does pack the structure (sizeof(testdata) reports size of 18), but when trying to access an unaligned structure member, or point to one, the value is incorrect. Even a simple

printf("testdata.lInt = %lu", testdata.lInt)

does not print the right value.  A pointer to an unaligned structure member results in a segment violation.

Is there further attributes (or compiler switch) needed in order to access packed structure members?

Diane Fairley



-----Original Message-----
From: Andrew Johnson [mailto:[email protected]] 
Sent: Wednesday, September 01, 2004 10:23 AM
To: Dalesio, Leo
Cc: [email protected]
Subject: Re: Compiler switches on RISC byte alignment

Dalesio, Leo wrote:
> Is there a compiler switch on the GNU compiler to turn off the RISC byte alignment?

Define the specific structure you want with the gcc-specific syntax __attribute__((packed).  You do that like this:

struct foo {
   char a;
   short b __attribute__((packed));
   int c __attribute__((packed));
};

I think you can also put the attribute on the structure itself (after the closing {}) to pack all fields together.

Variable attributes in gcc are described here:
http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html

HTH,

- Andrew
--
Dear God, I didn't think orange went with purple until I saw the sunset you made last night.  That was really cool. - Caro

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Re: Compiler switches on RISC byte alignment Lawrence T. Hoff

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