nfoTools

T060101d: Hello, What’s Happening Here?

nfoTools	tools>t060101>d	index.html 0.0.5 2023-06-25
	Work in Progress

• T060101d: Hello, What’s Happening Here?
  • 1. Overall Operation
  • 2. Compile and Link

1. Overall Operation

The overall objective is to convert single-file program, hello.c, into a program, hello.exe, that can be run directly on the computer. There is a simple pattern using the command-line interface that is initiated along with the Developer Command Prompt’s initialization of a build tools environment.

The diagrammatic view, below, parallels the observed command-line operation in Getting to “Hello”. This is a general pattern, using the specific example of hello.c for clarity.

Two command-line operations are performed.

• >CL hello.c
  • to compile the file hello.c into the program file hello.exe without error
• >hello
  • to operate hello.exe, providing its output directly in the command-line interface

KEY IDEA hello.exe is repeatedly-usable, while having to be compiled only once. The program can be used by others without building it or even knowing how.

With the simple “>CL hello.c” command, The hello.exe program is automatically produced as a Command-Line Application (CLA). This is the default behavior in the absence of any additional parameters in the command. The resulting program is completely self-contained and structured for operation by command prompt.

EXPERIMENT. From the File Explorer, double-click on the hello.exe entry. A command-line interface will open automatically, the program will run, and the command-line interface will exit. That can happen so rapidly that it might not be observed at all, as if nothing happened.

For now, simply open a command prompt first. Then operate the program with a command. Later, there will be ways to have operation be accomplished more directly and smoothly.

2. Compile and Link

The Build Tools program, CL/exe operates as a precision multi-tool. It has many options and stages of operation. With the simple command-line for “Getting to Hello,” much happens visibly and also under the covers.

An automatic pattern is separation of compiling into two stages: compile and then link. The compile stage converts the source code to a file consisting of (linkable) machine code for that source. The link stage combines modules of such code from many sources, including Standard C libraries, into a single executable, such as hello.exe. The general idea is reflected in the simplified diagram.

• The compile stage accesses the file hello.c and, when there are no errors, derives an “object code” file, hello.obj for subsequent use.
  • #include <stdio.h> causes the file stdio.h to be retrieved and blended into hello.c as if it is part of hello.c.
  • the stdio.h file is called a “header” because of this usage. The file is found among such includable headers in the Software Development Kit (SDK) reachable via the Build Tools Environment. All of the Standard C Language library facilities are accessed via use of headers in this manner.
  • stdio.h does not provide implementations of the Standard C Library in this pattern. Instead, stdio.h provides definitions of a collection of interfaces and some data by which a program can correctly refer to implementations of those facilities in library files supplied elsewhere in the SDK.
  • The VS Build Tools version of stdio.h has some #include operations in its own file, and this nested process is continued until all the implicit definitions are pulled into the hello.c compilation.
• hello.obj is the machine-language code (in “object” file format) that is derived from hello.c, stdio.h, and other settings of CL. hello.obj is decorated with information about how the code can be combined as a module with other code.
  • There is an entry to the hello.obj code named main
  • There are references in the hello.obj to code expected from elsewhere including an entry named fputs (and stdout, in effect).
• The link stage pulls together the modules, along with hello.obj, that are to be stitched together to form the complete hello.exe program.
  • Linking can be viewed as proceding from a default startup code that has a reference to an entry named main to be satisfied elsewhere. That reference is satisfied by hello.obj in this case.
  • The process continues until all of the unsatisfied references in any of the required modules are satisfied. In this manner, the reference to fputs from hello.obj is resolved in the SDK collection of C runtime library modules.
  • The necessary modules are rewritten to form a single file (‘hello.exe`) with all interconnections fixed as part of combining the object codes into a single file that can be loaded into the computer and operated as a unit.

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