Is the main function a must in a program?

It depends on the environment your program is written for. If its a hosted environment, then main function is a must for any standard C/C++ program. Hosted environments are those where there is full support of standard libraries and the main function is a must . Freestanding environment are those where the program may have non-standard startup/exit mechanisms and need not have full support for standard libraries, and so the main function is not required in freestanding environments. Usually only a limited number of I/O libraries will be supported and no memory management functions will be supported in freestanding environments. Examples of freestanding implementations are embedded systems∞ and the operating system kernel∞.

References

Freestanding and hosted environments - C book article∞