A program

What is a program

A program runs on a system and performs specific tasks. The input is data flow from the network or local database. The program handles the input data with specific algorithms and produces the output.

how to write a program

Take the C program as an example. The C program file is written/read by a C programmer. It is a text file with “.c” suffix. The file will finally be transformed to an executable file which is a binary file. The binary file contains machine code that can be read by the system and runs on cpu.

how to load a program

The C program can be run directly on the command line.

under the hood

The .text and .data sections are loaded in the User Process Memory Space. A PC pointer goes through the .text segment, runs the command it points to.

There’re another two important segments on the memory space: stack and heap. The stack segment contains data of the call stack frames. Each frame contains data of a function on the callback stacks. For example, data of the local variables, function arguments, which are saved in some registers. The stack is constructed/destroyed by the system automatically.

The heap segment contains data that are allocated by the program explicitly. For example, by the momory allocation functions: malloc, realloc and so on.

There may be memory leaks on the heap if the program is not written well. And the stack segement may go beyond its space if the callback stacks are very deep and some stacks contains too much space.

Both the two segments can be injected illegal code by virus.