WEBVTT 00:00.410 --> 00:06.050 All right, Now let's dive into the fascinating world of the linking phase, which marks the final stage 00:06.050 --> 00:08.450 of the compilation process. 00:08.480 --> 00:15.500 As the name suggests, this phase brings together all the object files and seamlessly merge them into 00:15.500 --> 00:17.600 a unified binary executable. 00:17.630 --> 00:24.200 In modern systems, this phase may also incorporate an additional optimization path known as link time 00:24.200 --> 00:30.500 optimization or enhancing the overall performance of the resulting executable. 00:30.710 --> 00:37.970 And unsurprisingly, the program that performs the linking phase is called a linker or link editor is 00:37.970 --> 00:43.370 typically separate from the compiler, which usually implements all the preceding phases. 00:43.520 --> 00:51.230 As I've already mentioned, object files are relocatable because they are compiled independently from 00:51.230 --> 00:52.760 each other. 00:52.790 --> 00:55.410 As you can see here, preventing the. 00:56.000 --> 01:03.200 That's because the they are preventing the compiler from assuming that an object will end up at any 01:03.200 --> 01:04.890 particular base address. 01:04.910 --> 01:13.460 So these are relocatable and moreover object files may reference functions or variables in other object 01:13.460 --> 01:17.270 files or in libraries that are external to the program. 01:17.270 --> 01:23.750 So before the linking phase, the addresses at which the reference code and data will be placed are 01:23.750 --> 01:24.740 not yet known. 01:24.740 --> 01:33.140 So the object files only contain relocation symbols that specify how functions and variable references 01:33.140 --> 01:35.190 should eventually be resolved. 01:35.210 --> 01:43.470 So in the context of linking references that rely on a relocation symbol are called symbolic references. 01:43.490 --> 01:50.690 When an object file reference is one of its own functions or variables by absolute address, the reference 01:50.690 --> 01:53.090 will also be symbolic. 01:53.090 --> 01:59.840 So the linker job is to take all the object files belonging to a program and merge them into a single 01:59.840 --> 02:06.390 coherent executable, typically intended to be loaded at a particular memory address. 02:06.420 --> 02:11.670 Now here, the arrangement of all modules in the executable is known. 02:11.700 --> 02:15.720 The linker can also resolve more symbolic references as well. 02:15.840 --> 02:23.160 References to the libraries may or may not be completely resolved depending on the type of library. 02:23.990 --> 02:34.310 And static libraries, which on Linux typically have the extension of dot a as here as a dot a. 02:36.140 --> 02:38.240 And are merged into the binary executable. 02:38.250 --> 02:40.500 So allowing any references to them. 02:40.860 --> 02:42.600 To be resolved entirely. 02:42.600 --> 02:46.260 So there are also dynamic which is also called. 02:46.350 --> 02:53.250 The shared libraries which are shared in memory among all programs that run on a system. 02:53.250 --> 02:58.890 In other words, rather than copying the library into every binary that uses it, the dynamic libraries 02:58.890 --> 03:07.770 are loaded into memory only once, and any binary that wants to use the library needs to use this shared 03:07.770 --> 03:08.220 copy. 03:08.220 --> 03:16.260 So during the linking phase, the addresses at which dynamic libraries will reside are not yet known, 03:16.260 --> 03:18.610 so references to them cannot be resolved. 03:18.630 --> 03:24.780 Instead, the linker leaves symbolic references to these libraries even in the final executable, and 03:24.780 --> 03:31.110 these references are not resolved until the binary is actually loaded into memory to be executed. 03:31.110 --> 03:38.230 So most compilers like GCC automatically call the linker at the end of the compilation process to use 03:38.350 --> 03:47.590 the to produce a complete binary executable so you can simply call GCC without any special switches 03:47.860 --> 03:50.050 and compile your application. 03:50.050 --> 03:53.380 My app dot C and that's it. 03:53.380 --> 03:58.570 And here we will now use the file my app dot a. 03:59.520 --> 04:01.770 Or a dot out. 04:02.100 --> 04:02.730 A dot out. 04:02.730 --> 04:04.740 Because this is our output. 04:05.850 --> 04:09.510 While the final file and that's it. 04:09.510 --> 04:11.160 And here. 04:12.330 --> 04:12.990 The first. 04:12.990 --> 04:16.830 Let's actually understand this firstly. 04:17.010 --> 04:21.470 So by default, the executable is called a dot out. 04:21.480 --> 04:30.060 But you can override this by naming, by passing the or parameter or switch to GCC followed by a name 04:30.060 --> 04:31.080 for the output file. 04:31.080 --> 04:38.460 So the file utility now tells us that we are dealing with an Elf 64 bit. 04:39.620 --> 04:40.310 LSB. 04:41.880 --> 04:45.930 I executable rather than a relocatable file. 04:45.960 --> 04:50.070 As you can see here, we have this relocatable file, as you saw. 04:50.850 --> 04:57.480 Also, you saw in the previous lecture and other important information is that the file is dynamically 04:57.480 --> 04:59.280 linked here. 05:01.220 --> 05:04.040 Dynamically length. 05:07.120 --> 05:15.700 So meaning this dynamically linked here, meaning that it uses some libraries that are not merged into 05:15.700 --> 05:21.460 executable but are instead shared among all programs running on the same system. 05:21.520 --> 05:25.630 And finally, we have this interpreter. 05:25.660 --> 05:26.740 This here. 05:28.530 --> 05:29.280 Here. 05:31.020 --> 05:31.770 So. 05:33.570 --> 05:47.670 Um here the interpreter lib64 Linux and x86 64 .0.2 Here in the file output tells you which dynamic 05:47.670 --> 05:55.050 linker will be used to resolve the final dependencies on dynamic libraries when the executable is loaded 05:55.050 --> 05:57.060 into memory to be executed. 05:57.060 --> 06:06.660 So when you run the binary using the dot a dot out command, we are running here. 06:06.660 --> 06:15.510 You can see that it produces the expected output printing the Hello world to us, which confirms that 06:15.510 --> 06:18.360 you have produced a working binary. 06:18.570 --> 06:24.870 But here we have the node script here or stripped here. 06:26.370 --> 06:32.950 But let's what is bit about this binary not being stripped or not stripped. 06:32.950 --> 06:36.910 You will learn that in next lecture.