Compiling and Linking

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1 Compiling and Linking ECE2893 Lecture 17 ECE2893 Compiling and Linking Spring / 10

2 The gcc/g++ Compiler 1 The Gnu C and C++ compiler (gcc and g++ respectively) have been under development for decades, and are one of the most successful examples of the community supported open source software development process. 2 The number of supported platforms is extensive, and includes virtually all Unix and Linux based operating systems. 3 It also runs under the Windows operating system using the popular Cygwin environment. 4 It also supports cross compiling, where the compiler runs in one environment but generates executable code for some other environment. This is primarily used to develop and deploy software for embedded systems 5 The compiler consists of several, independent steps. The pre processor, which we will discuss in detail later The compiler The assembler The linker ECE2893 Compiling and Linking Spring / 10

3 The Compiler Steps 1 The pre processor Reads the C/C++ source code file, and processes Compiler Directives. We have already used #include to include source code from other files. There are many others. 2 The Compiler. Reads the output of the pre processor and generates assembly language instructions to perform the various statements in the program. For example, for an arithmetic statement such as i = i + 1;, it will generate the LDM instruction to read the value of variable i into a register, the ADDI instruction to add one to the value, and the STM instruction to store the new value of i in memory. Obviously, more complex statement will generate more complex series of assembly language statements. The compiler also (optionally) performs optimizations. For example, given the statement for (int i = 0; i < 10; ++i) the compiler may choose to not assign a memory location to variable i, but rather just use a register. If the scope of the variable is limited, this is often the case. The output of the compiler is another file containing all the required assembly language instructions. ECE2893 Compiling and Linking Spring / 10

4 The Compiler Steps, Continued 1 The Assembler. Reads the output of the compiler (the assembly language instructions) and produces the object file. This is the equivalent binary-representation of each of the assembly language instructions found in the assembly language file. However, the assembler cannot yet know the exact address of any global variable or any subroutine or function. We will see why momentarily. 2 The Linker The linker combines together all of the various object files and libraries, to produce the final executable binary. ECE2893 Compiling and Linking Spring / 10

5 Computer Conceptual Model (More Realistic) ECE2893 Compiling and Linking Spring / 10

6 Commonly Used gcc/g++ Command Line Switches 1 -c Indicates the compiler should compile, but not link the specified program. 2 -Wall Indicates the compiler should produce warnings for any piece of code the might be incorrect. 3 -o Specifies the name of the file to write the output. If -c is specified, -o can usually be omitted. If omitted, the default file name is formed by replacing the.cc in the source code file name with.o. 4 -g Specifies the compiler should include debugging information in the binary file, to support the later use of gdb. 5 -I Specifies the include file search path. This tells the pre processor a list of subdirectories to search to find any file specicified with a #include directive. 6 -l Specifies the name of a library to use during linking. 7 -L Specifies a library search path, indicating one or more subdirectories to search to find the specified library. 8 There are hundreds of command line switches for gcc and g++. See the gnu.org web page for a complete list. 9 Examples: g++ -g -c gthread.cc Run the pre processor, compiler, and assembler using gthread.cc as the source code file, including debugging information. The output is written to file gthread.o. g++ -g -o pbsort pbsort.o cs1372.o gthread.o -lpthread Runs the linker, combining the three.o files and producing executable file pbsort. Use the library libpthread.a and include debugging information. ECE2893 Compiling and Linking Spring / 10

7 Separate Compilation 1 Any non trivial C/C++ programming project will be decomposed in to a (large) number of individual source code files. We have already seen this in several of the programming assignments. The cs1372.cc file contains the Print and PrintLn functions, as well as the millisecond clock timer we used in some projects. 2 In general, all C/C++ source code files will have a corresponding header file, by convention with the same name as the.cc file but relacing the.cc with.h For example, for cs1372.cc, the header file cs1372.h describes the various functions found in cs1372.cc. 3 In general, the header file will contain one function prototype for every function implemented in the corresponding C/C++ source code file. Recall that a function prototype is just an indication to the compiler that the specified function exists and can be called, but the implementation is defined elsewhere. The prototype contains the return type, function name, and argument list. void RandomizeArray(int* d, int arraysize); The actual parameter names (the d and arraysize above) are often included, but in fact can be omitted on the prototype. void RandomizeArray(int*, int); 4 Each of the source code files is individually compiled, using the -c compiler switch. ECE2893 Compiling and Linking Spring / 10

8 Separate Compilation, continued 1 The header files can also contain global variable declarations. 2 In general, a global variable declared in a header file will have the extern keyword before the declaration, indicating the compiler should not assign a memory location for the variable. Rather, it assumes another object file will declare the variable without the extern keyword. extern int g1; // g1 is defined elsewhere 3 Unless otherwise specified, all functions and global variables defined and implemented in any source code file are fully visible and can be called (or modified) by any other source code file in the project. We have used this many times, for example when calling PrintLn which is declared in cs1372.h and implemented in cs1372.cc. Simply including #include "cs1372.h" in another source code file allows the calling of PrintLn. 4 It is possible for a source code file to implement one or more functions that are private. We might not want to give other source code files the ability to call the functions. This can be done with the static keyword preceeding the function implementation. static int Sub1(int i) { Function Sub1 is not visible outside of the source code file containing the function implementation. ECE2893 Compiling and Linking Spring / 10

9 Linking 1 Question: How does the compiler know the address of any function that is implemented in another source code file. 2 Recall that the compiler produces assembly language code to implement the function call. The assembly language MUST know the exact address in memory of the specified function. 3 Another Question: How does the compiler know the address of all global variables? 4 Clearly, the address of a function or global variable depends on how many object files are combined together and the contents of each one. 5 The answer to both of the above question is it doesn t. 6 Instead, it simply makes a note in the.o file that there is a call to function Sub1 (for example) at a particular instruction in the.o file. 7 The job of the linker is to combine all of the.o files together, and resolving the external references by finding the exact address of each and inserting the right address in the appropriate assembly language instrutions. 8 A library is simply a collection of a (large) number of.o files into a single file for ease of use. An example is libstdc++.a, which contains most of the C++ runtime library. ECE2893 Compiling and Linking Spring / 10

10 Linking Example File1.o int g1 int Sub1() { g1 = 2; Sub2(); } static int Sub3() { // Omitted File2.o int g2 int Sub2() { g2 = 2; Sub3(); g1 = 3; } static int Sub3() { // Omitted Executable int g1 int g2 int Sub1() { g1 = 2; Sub2(); } static int Sub3() { // Omitted int Sub2() { g2 = 2; Sub3(); g1 = 3; } static int Sub3() { // Omitted ECE2893 Compiling and Linking Spring / 10

INTERMEDIATE SOFTWARE DESIGN SPRING 2011 ACCESS SPECIFIER: SOURCE FILE

INTERMEDIATE SOFTWARE DESIGN SPRING 2011 ACCESS SPECIFIER: SOURCE FILE HEADER FILE A header (.h,.hpp,...) file contains Class definitions ( class X {... }; ) Inline function definitions ( inline int get_x() {... } ) Function declarations ( void help(); ) Object declarations