The Stack Segment Under MS-DOS & UNIX [C]

 In DOS, the stack size has to be specified as part of building the executable, and it cannot be grown at runtime. If you guess wrongly, and your stack gets bigger than the space you've allocated, you and your program both lose, and if you've turned checking on, you'll get the STACK OVERFLOW! message. This can also appear at compiletime, when you've exceeded the limits of a segment. Turbo C will tell you Segment overflowed maximum size <lsegname> if too much data or code had to be combined into a single segment. The limit is 64Kbytes, due to the 80x86 architecture.
The method of specifying stack size varies with the compiler you're using. With Microsoft compilers, the programmer can specify the stack size as a linker parameter. 
The 
STACK:nnnn

parameter tells the Microsoft linker to allow nnnn bytes for the stack.
The Borland compilers use a variable with a special name:

unsigned int _stklen = 0x4000; /* 16K stack */


On UNIX, the stack grows automatically as a process needs more space. The programmer can just assume that the stack is indefinitely large. This is one of the many advantages that UNIX has over operating environments such as MS-DOS. UNIX implementations generally use some form of virtual memory. When you try to access beyond the space currently allocated for the stack, it generates a hardware interrupt known as a page fault. A page fault is processed in one of several ways, depending on whether the reference was valid or invalid.

The kernel normally handles a reference to an invalid address by sending the appropriate signal (segmentation fault probably) to the offending process. There's a small "red zone" region just below the top of the stack. A reference to there doesn't pass on a fault; instead, the operating system increases the stack segment size by a good chunk. Details vary among implementations (UNIX), but in effect, additional virtual memory is mapped into the address space following the end of the current stack. The memory mapping hardware ensures you cannot access memory outside that which the OS has allocated to your process.

Posted in: C