Operating a global buffer in C

I have a simple program with a couple of functions. I need the program to return a buffer that is being built in the functions and in the main function. I decided to do so with a global buffer. Supposing I know the maximum size of the final string, what is the best way to do so - creating the buffer, appending the strings and returning it? the program looks somewhat like this:

// Declaring the global buffer
int funcB()
{
    // Append string to buffer
}
int funcA()
{
    // Append string to buffer from funcA
    return
}

main()
{
    // Append string to buffer from main
    funcA()
    // Append string to buffer from main
    funcB()
    // Append string to buffer from main

    // Return the buffer
}

each comment is not a comment on the code, it's the action that the program need to do.

Thank you!