Let us discuss the difference between Linker and loader with the help of a comparison chart. It loads the executable module to the main memory.
Input It takes as input, the object code generated by an assembler. It takes executable module generated by a linker. Function It combines all the object modules of a source code to generate an executable module. It allocates the addresses to an executable module in main memory for execution.
Absolute loading, Relocatable loading and Dynamic Run-time loading. The Assembler generates the object code of a source program and hands it over to the linker. The linker takes this object code and generates the executable code for the program, and hand it over to the Loader. The high-level language, programs have some built-in libraries and header files. The source program may contain some library functions whose definition are stored in the built-in libraries.
The linker links these function to the built-in libraries. In case the built-in libraries are not found it informs to the compiler, and the compiler then generates the error. The steps and responsibilities we describe are generalized so that they apply to most systems. We will supplement the general theory in the following articles with an analysis of real-world implementations. Before we dive into our exploration of how existing systems get to main , we should develop a hypothesis about what generally happens.
Since others have already explored program startup, we can start with a clear idea of what happens before main. The entry function can vary depending on the system, compiler, and standard libraries. For example, OS X only has dynamically linked applications; the loader takes care of setup, and the entry point to the program is actually main. The default entry point can be overridden by clang and GCC linkers using the -e flag, although this is rarely done for most programs.
Compilers typically ship with pre-compiled crt0. Instead, the standards describe the conditions that must be true when the main function is called. For example, early low-level initialization code is commonly found with bare-metal embedded systems, but rarely on host machines with an OS.
Your Linux or OS X program startup code will have multiple scaffolding functions which you will not find in embedded startup code.
This example provides us with a preview of the basic skeleton for program startup. The implementations we will review later in this series are much more complex. At a high level, our runtime setup must accomplish the following:. Initializing global and static memory is broken down into two distinct steps that deserve additional details. This includes global and static variables, but not stack variables.
All uninitialized data that needs to be set to 0 is placed into the. The location of the. The linker places these constructors into the. The constructors are stored in a list by the linker.
The runtime initialization process iterates through the list and calls each constructor. Other implementations implement those tasks in the runtime setup routines.
Assembly files commonly found during this portion of the startup process are crtbegin. Compilers often ship pre-compiled object files for supported architectures. These files are related to calling global constructors and destructors. When the files are not used, equivalent functionality is often implemented in C and invoked during runtime initialization.
The setup process may invoke other functions to set up program scaffolding that the system requires. Abhishak Gagaan. Sonal Sharma. Show More. Views Total views. Actions Shares.
No notes for slide. Introduction to loaders 1. Hence even though there is no modification in the source program it needs to be assembled and executed each time. In which assembler is placed in main memory that results in wastage of memory. Here it will directly placed in memory. Source program Rel.
The length of the object code segment. A list of external symbols could be used by other segments. List of External symbols The segment is using.
Information about address constants. Machine code translation of the source program. Address of the object code could be absolute. The address of object code can be relative. ESD - External symbol dictionary contains information about all symbols that are defined in the program but referenced somewhere.
LD - Local Definition. ER - External Reference. TXT — Text card contains actual object code. RLD — Relocation and linkage directory contains information about locations in the program whose content depend on the address at which program is placed.
END — Indicates the end of the program and specifies starting address for execution
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