1、Linking and Loading,CS-502 Fall 2007,1,Linking & Loading,CS-502 Operating SystemsSilbershatz et al barely touch on this topic in 8.1 Tanenbaum does not address it at all,Linking and Loading,CS-502 Fall 2007,2,What happens to your program ,after it is compiled, but before it can be run?,Linking and L
2、oading,CS-502 Fall 2007,3,Executable files,Every OS expects executable files to have a specific format Header info Code locations Data locations Code & data Symbol Table List of names of things defined in your program and where they are located within your program. List of names of things defined el
3、sewhere that are used by your program, and where they are used.,Linking and Loading,CS-502 Fall 2007,4,Example,#include int main () printf (“hello, worldn”),Symbol defined in your program and used elsewhere mainSymbol defined elsewhere and used by your program printf,Linking and Loading,CS-502 Fall
4、2007,5,Example,#include extern int errno;int main () printf (“hello, worldn”),Symbol defined in your program and used elsewhere mainSymbol defined elsewhere and used by your program printf errno,Linking and Loading,CS-502 Fall 2007,6,Two-step operation (in most systems),Linking: Combining a set of p
5、rograms, including library routines, to create a loadable image Resolving symbols defined within the set Listing symbols needing to be resolved by loaderLoading: Copying the loadable image into memory, connecting it with any other programs already loaded, and updating addresses as needed (In Unix) i
6、nterpreting file to initialize the process address space (in all systems) kernel image is special (own format),Linking and Loading,CS-502 Fall 2007,7,From source code to a process,Binding is the act of connecting names to addresses Most compilers produce relocatable object code Addresses relative to
7、 zero The linker combines multiple object files and library modules into a single executable file Addresses also relative to zero The Loader reads the executable file Allocates memory Maps addresses within file to memory addresses Resolves names of dynamic library items,Source (.c, .cc),Object (.o),
8、Executable,In-memory Image,Compiler,Linker,Loader,Other Objects (.o),Dynamic libraries (.dll),Static libraries (.a),Linking and Loading,CS-502 Fall 2007,8,Static Linking and Loading,Linking and Loading,CS-502 Fall 2007,9,Classic Unix,Linker lives inside of cc or gcc command Loader is part of exec sy
9、stem call Executable image contains all object and library modules needed by program Entire image is loaded at onceEvery image contains its own copy of common library routines Every loaded program contain duplicate copy of library routines,Linking and Loading,CS-502 Fall 2007,10,Dynamic Loading,Rout
10、ine is not loaded until it is called Better memory-space utilization; unused routine is never loaded. Useful when large amounts of code are needed to handle infrequently occurring cases. Silbershatz says incorrectly No special support from the operating system is required Must be implemented through
11、 program design,Linking and Loading,CS-502 Fall 2007,11,Program-controlled Dynamic Loading,Requires: A load system call to invoke loader (not in Unix) ability to leave symbols unresolved and resolve at run time (not in Unix) E.g., void myPrintf (*arg) static int loaded = 0;if (!loaded ) load (“print
12、f”);loaded = 1;printf(arg); ,Linking and Loading,CS-502 Fall 2007,12,Linker-assisted Dynamic Loading,Programmer marks modules as “dynamic” to linker For function call to a dynamic function Call is indirect through a link table Each link table entry is initialized with address of small stub of code t
13、o locate and load module. When loaded, loader replaces link table entry with address of loaded function When unloaded, loader restores table entry with stub address Works only for function calls, not static data,Linking and Loading,CS-502 Fall 2007,13,Example Linker-assisted loading (before),Your pr
14、ogramvoid main () printf ();,Link table,Stub void load() load(“IOLib”); ,Linking and Loading,CS-502 Fall 2007,14,Example Linker-assisted loading (after),Your programvoid main () printf ();,Link table,IOLib read() printf() scanf() ,Linking and Loading,CS-502 Fall 2007,15,Shared Libraries,Observation
15、“everyone” links to standard libraries (libc.a, etc.) These consume space in every executable image every process memory at runtime Would it be possible to share the common libraries? Automatically load at runtime?,Linking and Loading,CS-502 Fall 2007,16,Shared libraries (continued),Libraries design
16、ated as “shared” .so, .dll, etc. Supported by corresponding “.a” libraries containing symbol information Linker sets up symbols to be resolved at runtime Loader: Is library already in memory? If yes, map into new process space “map,” an operation to be defined later in course If not, load and then m
17、ap,Linking and Loading,CS-502 Fall 2007,17,Run-time Linking/Loading,Linking and Loading,CS-502 Fall 2007,18,Dynamic Linking,Complete linking postponed until execution time. Stub used to locate the appropriate memory-resident library routine. Stub replaces itself with the address of the routine, and
18、executes the routine. Operating system needs to check if routine is in address space of process Dynamic linking is particularly useful for libraries.,Linking and Loading,CS-502 Fall 2007,19,Dynamic Shared Libraries,Static shared libraries requires address space pre-allocation Dynamic shared librarie
19、s address binding at runtime Code must be position independent At runtime, references are resolved as Library_relative_address + library_base_address,Linking and Loading,CS-502 Fall 2007,20,Overlays (primarily of historical interest),Keep in memory only those instructions and data that are needed at
20、 any given time. Needed when process is larger than amount of memory allocated to it. Can be implemented by user no special support needed from operating system, but programming design of overlay structure is complex Can be done with OS help think about Unix exec system call,Linking and Loading,CS-5
21、02 Fall 2007,21,Linking Summary,Linker key part of OS not in kernel Combines object files and libraries into a “standard” format that the OS loader can interpret Resolves references and does static relocation of addresses Creates information for loader to complete binding process Supports dynamic sh
22、ared libraries,Linking and Loading,CS-502 Fall 2007,22,Loader,An integral part of the OS Resolves addresses and symbols that could not be resolved at link-time May be small or large Small: Classic Unix Large: Linux, Windows XP, etc. May be invoke explicitly or implicitly Explicitly by stub or by program itself Implicitly as part of exec,Linking and Loading,CS-502 Fall 2007,23,Questions?,
