1 00:00:00,000 --> 00:00:05,592 [MUSIC]. 2 00:00:05,592 --> 00:00:08,383 We've seen multi-dimensional and multi-level arrays. 3 00:00:08,383 --> 00:00:13,489 now let's take a look at how we represent some other structures besides just plain 4 00:00:13,489 --> 00:00:19,091 integers or pointers, in memory. In C, we have the concept of a struct. 5 00:00:19,091 --> 00:00:24,411 a struct is a definition of a composite data type that has sub-elements that are, 6 00:00:24,411 --> 00:00:31,634 are in turn other data types themselves. In this case, we have a small struct here 7 00:00:31,634 --> 00:00:36,010 with three elements inside of it. One is an integer. 8 00:00:36,010 --> 00:00:40,180 One is an array of integers, in this case a three element array. 9 00:00:40,180 --> 00:00:46,787 And the other is a pointer to an integer. So those are three parts to that struct. 10 00:00:46,787 --> 00:00:51,617 In memory, is going to be laid out as a continuous set of bytes for each of those 11 00:00:51,617 --> 00:00:55,421 parts. The first four bytes will of course be 12 00:00:55,421 --> 00:00:59,029 the integer i. the next 12 bytes will be the three 13 00:00:59,029 --> 00:01:03,723 integers of the array a, contiuously allocated. 14 00:01:03,723 --> 00:01:08,779 And then finally, the last 4 bytes will be a pointer to an integer in a 32 bit 15 00:01:08,779 --> 00:01:13,340 architecture. So, total for this struct is a 20 bite 16 00:01:13,340 --> 00:01:17,480 chunk of memory, continuously allocated and we can refer to the members of this 17 00:01:17,480 --> 00:01:23,146 structure, by the names we've given them. but the members can be of different 18 00:01:23,146 --> 00:01:27,612 types, they don't have to be the same. in this case these are either integers or 19 00:01:27,612 --> 00:01:31,385 pointers intergers, but those are very different data types. 20 00:01:31,385 --> 00:01:35,458 OK. So how do we access, a structure member? 21 00:01:35,458 --> 00:01:41,833 well, given, an instance of a struct, we can use the dot operator, just like in 22 00:01:41,833 --> 00:01:46,303 Java. So for example, if we declare one of 23 00:01:46,303 --> 00:01:51,194 these str, rec structs that we just defined, declared here, we can give it 24 00:01:51,194 --> 00:01:56,382 the name r1. Then we can refer to that first integer 25 00:01:56,382 --> 00:02:01,854 in r1 by writing r1 dot i, and that refers to just that first Integer, that 26 00:02:01,854 --> 00:02:09,440 is part of that larger struct, that has a couple of other things in it. 27 00:02:11,670 --> 00:02:16,320 OK, if we have a pointer to a struct, rather than the struct itself, in other 28 00:02:16,320 --> 00:02:21,645 words, we've declared not the struct r1, as we did in this case, but in pointer r 29 00:02:21,645 --> 00:02:28,285 to a struct of type rec. in other words, we, that would be 30 00:02:28,285 --> 00:02:33,251 something like the address of the r1 struct we declared previously. 31 00:02:33,251 --> 00:02:38,555 we're now using, we're going to be using the star and dot operators, because we 32 00:02:38,555 --> 00:02:43,707 first need to de-reference the struct pointer. 33 00:02:43,707 --> 00:02:49,083 get to the starting location of that of that struct in memory, and then 34 00:02:49,083 --> 00:02:55,912 offsetting that by the position of the member within the struct. 35 00:02:55,912 --> 00:02:59,883 OK? So this happens so often in C that we 36 00:02:59,883 --> 00:03:10,110 actually give it a shorthand. we give it the shorthand r right arrow i. 37 00:03:10,110 --> 00:03:13,040 and the arrow is drawn as a - followed by a > sign. 38 00:03:13,040 --> 00:03:19,748 and that's just a shorthand for doing that de-reference first, that's what the 39 00:03:19,748 --> 00:03:24,732 parentheses indicate, and then the dot. OK? 40 00:03:24,732 --> 00:03:27,657 So you'll often see this kind of construct in C because we have pointers 41 00:03:27,657 --> 00:03:30,810 to structs rather than the structs themselves. 42 00:03:30,810 --> 00:03:38,426 So, here you see a simple function that is used to set the value of that first 43 00:03:38,426 --> 00:03:46,104 element of this type of struct. So it takes a pointer to that struct as 44 00:03:46,104 --> 00:03:50,120 an argument, and a value to put in that first member. 45 00:03:50,120 --> 00:03:54,540 And it says, well, I have that pointer, let's de-reference it, and offset to i, 46 00:03:54,540 --> 00:04:00,960 that first member of the struct, and then give it the value of val. 47 00:04:00,960 --> 00:04:08,148 The way that ends up looking in assembly code, is very straight forward actually. 48 00:04:08,148 --> 00:04:13,575 Because we have the value val in eax, and we're going to have the pointer to the 49 00:04:13,575 --> 00:04:19,083 element in edx, and we're just going to say move that value val to edx at that 50 00:04:19,083 --> 00:04:25,101 address. So the parentheses, again, are the 51 00:04:25,101 --> 00:04:28,996 de-reference operator. In this case, we don't have an offset, 52 00:04:28,996 --> 00:04:33,100 because that was the, i is the very first element of the struct. 53 00:04:33,100 --> 00:04:38,070 So it's at that starting address. If if we had to provide an offset, there 54 00:04:38,070 --> 00:04:45,388 would have been an additional offset here, in front of the parentheses. 55 00:04:45,388 --> 00:04:48,541 OK. So let's take a look at a slightly more 56 00:04:48,541 --> 00:04:52,995 complex case. generating a pointer to one of those 57 00:04:52,995 --> 00:04:56,880 elements of that array inside of the struct. 58 00:04:56,880 --> 00:05:01,905 so in this case what we would like to do is find the particular element that we're 59 00:05:01,905 --> 00:05:08,158 interested in, by providing an index. So we're going to provide a pointer to 60 00:05:08,158 --> 00:05:11,750 the struct, and then an index into the array. 61 00:05:11,750 --> 00:05:19,220 And in this case, what we want to return is a pointer to that integer element. 62 00:05:19,220 --> 00:05:22,568 OK? So to return the pointer what we will do 63 00:05:22,568 --> 00:05:28,828 is first do that r Arrow a. that gets us to the starting address of 64 00:05:28,828 --> 00:05:33,160 the array within memory within that struct. 65 00:05:33,160 --> 00:05:38,080 And then, index that array with the appropriate offset, and then the 66 00:05:38,080 --> 00:05:43,410 ampersand says well I don't want the value there, I want the address of that 67 00:05:43,410 --> 00:05:48,920 value, and return that. Mm-kay. 68 00:05:48,920 --> 00:05:54,060 How we see that in assembly code, is just two simple instructions. 69 00:05:54,060 --> 00:06:00,070 the very first instruction computes the offset into the struct. 70 00:06:00,070 --> 00:06:05,229 So in this case, the offset is is four times the index to get to the right 71 00:06:05,229 --> 00:06:12,640 position within the array. you'll then notice we'll take that value, 72 00:06:12,640 --> 00:06:18,080 which we store in eax, here and add another 4 offset to it, because that 73 00:06:18,080 --> 00:06:23,890 array starts at four bytes into the struct. 74 00:06:23,890 --> 00:06:28,410 then, of course, eax has the offset within the array. 75 00:06:28,410 --> 00:06:34,510 And the edx is the starting address of the struct, that first parameter. 76 00:06:34,510 --> 00:06:39,408 We add all of those together, so we start from r, add on the 4 to get to the a 77 00:06:39,408 --> 00:06:45,930 portion of the struct, the array portion of the struct. 78 00:06:45,930 --> 00:06:50,352 And then we add on the, 4 times the index to get to the right element inside the 79 00:06:50,352 --> 00:06:55,926 array. And since we're returning just the 80 00:06:55,926 --> 00:07:00,278 address, we can stop there after we've computed the address, we don't need to 81 00:07:00,278 --> 00:07:04,216 actually do a memory reference.