From mboxrd@z Thu Jan 1 00:00:00 1970 From: Giulio Rossato Subject: offsets of fields in a structure Date: Fri, 19 Oct 2007 17:27:30 +0200 Message-ID: <4718CCE2.9060202@infocamere.it> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: Sender: linux-c-programming-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: linux-c-programming@vger.kernel.org Suppose that I define the following structure struct astruct { int field1; float field2; char field3[10]; }; and declare the variable struct astruct r; The amount of memory specified by the structure is not the sum of the storage specified by each of its member types. This vary from one machine and C compiler to another. On most compuers, objects of certain types may not begin anywhere in memory but are constrained to start at certain boundaries. For example, an integer of length 4 bytes may have to start at an address divisible by 4, and a real number of length 8 may have to start at an address divisible by 8. Thus, in my example, if the starting address of r is 200, the integer occupies bytes 200 through 203, but the real number cannot start at byte 204, since that location is not divisible by 8. Thus the real number must start at byte 208. The C compiler associates to each member identifier of a structure an offset that specifies how far beyond the start of the structure the location of that field is. To calculate the location of a member in a structure, the offset of the member identifier is added to the base address of the structure variable. Now the question. I want write a function that receives as parameters a start address and a "description of a structure" and returns the offsets of the fields. The structure is not known at compile time. The offsets should be calculated at runtime and the code should be independent of the machine. How should be written this function?