Compiling a .CPP using GCC

memcpy and strncmp are declared in <cstring>, atoi and atol in <cstdlib>. Just include these headers to bring in their declarations.

Side notes :

• No semicolon after preprocessor directives, including #include "".
• No using namespace std;, especially not in headers !
  (Why is "using namespace std" considered bad practice?)
• Since you're building a C++ project, don't forget to link with the standard library via -lstdc++, or use g++ which forwards it to the linker for you.

Note that with GCC you don't have to prefix standard C functions with std:: (as they are also declared in the global namespace), but you should anyway for your code to be standard-compliant.

The cplusplus site helps out. If you search for a specific function, on the top you will find the necessary references for the code to compile:

http://www.cplusplus.com/reference/cstdlib/atol/

You can even check in the URL. In this case, you need 'cstdlib'.

Regarding the compiler, there are several available, g++ is a good option.

I also suggest creating a makefile for automating the building process, because it becomes an headache when your codebase grows.

For any library function you use in your code, read the man page for that function to see which header declares it, and #include that header in any source file that uses the function.

For example, man memcpy shows the memcpy(3) man page (the (3) refers to section 3 of the manual; use man 3 memcpy to specify the section).

NAME
memcpy - copy memory area

SYNOPSIS
#include <string.h>

void *memcpy(void *dest, const void *src, size_t n);
...

memcpy is part of the C standard library, and is declared in <string.h>. For C++ you can use <string.h>, but it's probably better to use <cstring>, which puts the function name in the std namespace. In general, each standard C header <foo.h> is duplicated as a C++ header <cfoo>.

I get a lot of errors: memcpy, atoi, atol, strncmp, etc,

memcpy and strncmp are declared in <cstring>. atoi and atol are <cstdlib> (or in <string.h> and <stdlib.h> if you're programming in C rather than in C++).

And be sure to use the g++ command, not the gcc command, to compile C++ source. They both invoke the same compiler (gcc compiles C++ code if the source file name ends in .cpp), but the g++ command adds some options that are needed for C++.