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# Obtain sizeof struct element using offsetof()?

 P: n/a Is there a way to obtain the size of a struct element based only upon its offset within the struct? I seem unable to figure out a way to do this (short of comparing every element's offset with ). What I would like to do is create an API something like this: #include struct MemMap { unsigned char apple; // 8 bits on my platform unsigned char butter; unsigned short pear; // 16 bits on my platform unsigned long peach; // 32 bits on my platform }; static struct MemMap memMap; /* is the offset of the MemMap struct element ** that this function is to write with the appropriately ** sized value pointed to by . */ int writeMemMapReg(size_t offset, void *pVal) { int failures = 0; size_t size = /* sizeof element at */; unsigned char *p8Bits; unsigned short *p16Bits; unsigned long *p32Bits; switch (size) { case 1: p8Bits = (unsigned char *) pVal; /* somehow write *p8Bits to memMap struct at ** . */ break; case 2: p16Bits = (unsigned short *) pVal; /* somehow write *p16Bits to memMap struct at ** . */ break; case 4: p32Bits = (unsigned long *) pVal; /* somehow write *p32Bits to memMap struct at ** . */ break; default: /* Error */ failures = !0; break; } return failures; } Any help much appreciated. -- - Mark -> -- Nov 14 '05 #1
10 Replies

 P: n/a Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] No. Consider struct x { char cx; }; struct y { char cy[100]; }; Here, `offsetof(struct x, cx) == offsetof(struct y, cy)', yet the sizes of the `cx' and `cy' elements are different. No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs. -- Er*********@sun.com Nov 14 '05 #2

 P: n/a Eric Sosman wrote in news:cj**********@news1brm.Central.Sun.COM: Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] No. Consider struct x { char cx; }; struct y { char cy[100]; }; Here, `offsetof(struct x, cx) == offsetof(struct y, cy)', yet the sizes of the `cx' and `cy' elements are different. No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs. Of course. Thanks Eric. I guess I'll have to do some macro magic with some built in assumptions about the struct if I want to simplify the caller's interface. Regards. -- - Mark -> -- Nov 14 '05 #3

 P: n/a In "Mark A. Odell" writes: Is there a way to obtain the size of a struct element based only upon itsoffset within the struct? Even if you also knew the offset of the next member that would be impossible, due to the padding between members. I seem unable to figure out a way to do this(short of comparing every element's offset with ). Nothing wrong with this approach. What I would like to do is create an API something like this: I can't imagine any practical need for that. You can also pass the type of the value, encoded using an ad hoc convention: the caller *must* know it. The callee now has all the information needed to know how to access the pointed data. Dan -- Dan Pop DESY Zeuthen, RZ group Email: Da*****@ifh.de Currently looking for a job in the European Union Nov 14 '05 #4

 P: n/a Eric Sosman wrote: Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] struct x { char cx; }; struct y { char cy[100]; }; [ . . . ] No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs. That said, you can certainly calculate the size of each member of any struct using only offsetof and sizeof: sizeof(struct x) - offsetof(struct x, cx) == 1 sizeof(struct x) - offsetof(struct x, cy) == 100 It doesn't seem like you could gain much by doing this, though, unless you're creating some kind of metaprogramming tool. -- Derrick Coetzee I grant this newsgroup posting into the public domain. I disclaim all express or implied warranty and all liability. I am not a professional. Nov 14 '05 #5

 P: n/a "Mark A. Odell" wrote in message news:Xn********************************@130.133.1. 4... Is there a way to obtain the size of a struct element based only upon its offset within the struct? I seem unable to figure out a way to do this (short of comparing every element's offset with ). What I would like to do is create an API something like this: #include typedef unsigned char Apple_t; typedef unsigned char Butter_t; typedef unsigned short Pear_t; typedef unsigned long Peach_t; typedef struct mem_map { Apple_t apple; Butter_t butter; Pear_t pear; Peach_t peach; } MemMap; typedef enum mem_map_offsets { MemMapApple = offsetof(MemMap,apple), MemMapButter = offsetof(MemMap,butter), MemMapPear = offsetof(MemMap,pear), MemMapPeach = offsetof(MemMap,peach) } MemMapOffsets; static MemMap memMap; /* is the offset of the MemMap struct element ** that this function is to write with the appropriately ** sized value pointed to by . */ int writeMemMapReg(MemMapOffsets offset, void *pVal) { int failures = 0; switch(offset) { case MemMapApple: memMap.apple = *(Apple_t *) pVal; break; case MemMapButter: memMap.butter = *(Butter_t *) pVal; break; case MemMapPear: memMap.pear = *(Pear_t *) pVal; break; case MemMapPeach: memMap.peach = *(Peach_t *) pVal; break; default: failures = 1; } return failures; } Any help much appreciated. -- - Mark -> -- Nov 14 '05 #6

 P: n/a Derrick Coetzee wrote in message news:... Eric Sosman wrote: Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] struct x { char cx; }; struct y { char cy[100]; }; [ . . . ] No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs. That said, you can certainly calculate the size of each member of any struct using only offsetof and sizeof: sizeof(struct x) - offsetof(struct x, cx) == 1 sizeof(struct x) - offsetof(struct x, cy) == 100 It doesn't seem like you could gain much by doing this, though, unless you're creating some kind of metaprogramming tool. This is incorrect. If you have the following: struct foo { char cx; char cy[100]; }; then you can have padding after cy which is included in the size of the actual struct. so sizeof(struct foo) does not give you the size, in bytes, of the summation of all member sizes. Nov 14 '05 #7

 P: n/a "j0mbolar" wrote in message news:2d**************************@posting.google.c om... This is incorrect. If you have the following: struct foo { char cx; char cy[100]; }; then you can have padding after cy which is included in the size of the actual struct. so sizeof(struct foo) does not give you the Practically, there are no chances to have any paddings with this code. Instead, the following: struct foo { int i; char c; }; certainly will have the tail one. Nov 14 '05 #8

 P: n/a In Derrick Coetzee writes: Eric Sosman wrote: Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] struct x { char cx; }; struct y { char cy[100]; }; [ . . . ] No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs.That said, you can certainly calculate the size of each member of anystruct using only offsetof and sizeof: Can you? sizeof(struct x) - offsetof(struct x, cx) == 1sizeof(struct x) - offsetof(struct x, cy) == 100 What about the padding the compiler can insert between any two members or at the end? Dan -- Dan Pop DESY Zeuthen, RZ group Email: Da*****@ifh.de Currently looking for a job in the European Union Nov 14 '05 #9

 P: n/a j0mbolar wrote:That said, you can certainly calculate the size of each member of anystruct using only offsetof and sizeof:sizeof(struct x) - offsetof(struct x, cx) == 1sizeof(struct x) - offsetof(struct x, cy) == 100It doesn't seem like you could gain much by doing this, though, unlessyou're creating some kind of metaprogramming tool. This is incorrect. [ . . . ] you can have padding after cy which is included in the size of the actual struct. I'm sorry, of course you're right. If you have an instance of the struct handy, though, you can always use sizeof directly on the elements: struct x { int a; char b; }; int main() { struct x y; int c = sizeof(y.a); int d = sizeof(y.b); } -- Derrick Coetzee I grant this newsgroup posting into the public domain. I disclaim all express or implied warranty and all liability. I am not a professional. Nov 14 '05 #10

 P: n/a Groovy hepcat Derrick Coetzee was jivin' on Mon, 27 Sep 2004 20:22:52 -0400 in comp.lang.c. Re: Obtain sizeof struct element using offsetof()?'s a cool scene! Dig it! Eric Sosman wrote: Mark A. Odell wrote: Is there a way to obtain the size of a struct element based only upon its offset within the struct? [...] struct x { char cx; }; struct y { char cy[100]; }; [ . . . ] No (legitimate) computation "based only" on the input value zero could produce both 1 and 100 as outputs.That said, you can certainly calculate the size of each member of anystruct using only offsetof and sizeof:sizeof(struct x) - offsetof(struct x, cx) == 1sizeof(struct x) - offsetof(struct x, cy) == 100 Nonsense! Assuming that you are referring to the struct x defined above, it does not have a cy member. And structures may have padding following any member, thus sizeof(struct x) - offsetof(struct x, cx) need not be equal to the size of the structure's only member. And, of course, if the struct has more than one member, then your equations are false anyhow. -- Dig the even newer still, yet more improved, sig! http://alphalink.com.au/~phaywood/ "Ain't I'm a dog?" - Ronny Self, Ain't I'm a Dog, written by G. Sherry & W. Walker. I know it's not "technically correct" English; but since when was rock & roll "technically correct"? Nov 14 '05 #11

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