Thread: Running 2 functions simultaneously

I'm making a program that requires 2 functions to be run at once. One function checks for user input and writes the input to a file, the other function reads that file and checks to see if it has changed. I cant have one run before the other, they NEED to be run at the same time. All my code is written, except for the part that runs the functions, this is basically what i want in the end:

Code:

int write(void)
{
     code for this function
     start();
}

int read(void)
{
     code for this function
     main();
}

int start(void);
{
     write(); read(); /*WANT TO RUN THESE AT SAME TIME*/
}

int main()
{
     code for this function
     start();
}

This is the format i NEED to have, so any recommendations that involve changing the format of my code TOO much will only be counter-productive in this situation. Thanks guys =)