Because it contains definitions of mathematical functions such as cos, sin, tan, power (pow) and so on. Thus if you want to use mathematical functions in your programs you have to include math.h
Use a text-editor.
There is no system header called share.h, but if there were, it would be: #include <share.h>
use grphics.h header file
Create the header file as a .h file in the same way you create a source file, .c and place it in the same directory as the .c files. To incorporate it, use the... #include "my_header_file.h" ... directive. Note that there are double quotes instead of greater/less signs. This tells the compiler to look first in the source directory.
Include the header file math.h and use the function acos(d)
You do not need a header file to add two numbers in C. You do need the header files if you want to use the run-time library, which includes the ability to format and display the result.
Classes and structures can be put in a header file the same way you would use them in a main program; the only difference is that they are placed in a separate file, called a header file. Then, after creating a new file, include that new file with the definition by the use of the preprocessor #include statement.
the use of header files is to add functionality. Header files are basically saying put code in that header file here so you don't have to type that many lines of code.
C programs do not require header files. If you want a C program without header files, you can simply not create them. However, you may or may not be able to include your non-header file source files.
Header files are used to have declarations. It is simple to include a single header file than writing all the needed functions prototypes.
Look for typedef in it, but I don't think you will find any.
list of header files in c and function prototype associated with each file