future
Joined: 14 May 2004
Posts: 330
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| The use of 'offsetof' |
 Posted: Mon Mar 12, 2012 8:51 am |
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Hello,
I am trying to retrieve the offset of a struct member.
| Code: | typedef struct __cfg__ {
char params[200];
};
typedef struct __cfg_file__ {
char name[17];
struct __cfg__ table;
};
typedef struct __fram__ {
struct __cfg_file__ config[3];
};
long offset;
offset = offsetof(struct __fram__, config[2].params[100]);
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The compiler does not like the config "2" index. The error message is that a parenthesis is missing there.
What am I doing wrong?
Thank you. |
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RF_Developer
Joined: 07 Feb 2011
Posts: 839
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| Posted: Mon Mar 12, 2012 9:09 am |
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From the CCS manual entry for offsetof(), though this is not specific to CCS, its normal C:
| Quote: |
These functions return an offset into a structure for the indicated field. offsetof returns the offset in bytes and offsetofbit returns the offset in bits.
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config is a field of your structure __fram__ and is all offsetof can tell you about. The error message is telling you that a ) was expected after config: i.e. offsetof(struct _fram__, config) is valid syntax.
config[2] is an element of the field config of __fram__.
config[2].params is a field of the structure that is an element of the field config of __fram__.
So by now you should be getting the message that there's no way config[2].params[100] can be a field of __fram__. To get the offset of that in __fram__ you'd have to do two offsetofs and quite a bit of array index manilpulation.
My question, however, is why? What are you trying to acheive by this? Its very likely there is another way of doing what you want. I for one, have never used offsetof, generally being able to use pointers or references instead.
RF Developer |
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