China DOS Union

How does C read and write DOS variables?
Or, how can a program written in C read/modify DOS variables or assign the return value to a DOS variable?

For example, there is a variable in DOS: vCdrom=D:, but I just want the first character. C can use the left() function to intercept, but there seems to be no such command in DOS.

DOS doesn't have such a function. Write a conversion program in C.

I don't understand what you mean.
But it can be implemented as follows.
For example, you have a program A.exe, and after it finishes, it returns a value, such as maybe 1 or 2.
Execute different programs B.exe or C.exe according to your return value.

It can be implemented through batch processing.

A.exe(Execute A)
if errorlevel 2 goto L_2
if errorlevel 1 goto L_1
goto L_end
:L_2
C.exe
goto L_end
:L_1
B.exe
goto L_end
:L_end

Try the main program return value.

int main()
{
int x=0;
....

retun x;
}