About Stacks

Typically, thread stacks begin on page boundaries. Any specified size is rounded up to the next page boundary. A page with no access permission is appended to the overflow end of the stack. Most stack overflows result in sending a SIGSEGV signal to the offending thread. Thread stacks allocated by the caller are used without modification.

When a stack is specified, the thread should also be created with PTHREAD_CREATE_JOINABLE. That stack cannot be freed until the pthread_join(3C) call for that thread has returned. The thread's stack cannot be freed until the thread has terminated. The only reliable way to know if such a thread has terminated is through pthread_join(3C).

Allocating Stack Space for Threads

Generally, you do not need to allocate stack space for threads. The system allocates 1 megabyte (for 32 bit systems) or 2 megabytes (for 64 bit systems) of virtual memory for each thread's stack with no swap space reserved. The system uses the MAP_NORESERVE option of mmap() to make the allocations.

Each thread stack created by the system has a red zone. The system creates the red zone by appending a page to the overflow end of a stack to catch stack overflows. This page is invalid and causes a memory fault if accessed. Red zones are appended to all automatically allocated stacks whether the size is specified by the application or the default size is used.

Runtime stack requirements vary for library calls and dynamic linking. You should be absolutely certain that the specified stack satisfies the runtime requirements for library calls and dynamic linking.

Very few occasions exist when specifying a stack, its size, or both, is appropriate. Even an expert has a difficult time knowing whether the right size was specified. Even a program that is compliant with ABI standards cannot determine its stack size statically. The stack size is dependent on the needs of the particular runtime environment in execution.

Building Your Own Stack

When you specify the thread stack size, you must account for the allocations needed by the invoked function and by each subsequent function called. The accounting should include calling sequence needs, local variables, and information structures.

Occasionally, you want a stack that differs a bit from the default stack. An obvious situation is when the thread needs more than the default stack size. A less obvious situation is when the default stack is too large. You might be creating thousands of threads with insufficient virtual memory to handle the gigabytes of stack space required by thousands of default stacks.

The limits on the maximum size of a stack are often obvious, but what about the limits on its minimum size? Sufficient stack space must exist to handle all stack frames that are pushed onto the stack, along with their local variables, and so on.

To get the absolute minimum limit on stack size, call the macro PTHREAD_STACK_MIN. The PTHREAD_STACK_MIN macro returns the amount of required stack space for a thread that executes a NULL procedure. Useful threads need more than the minimum stack size, so be very careful when reducing the stack size.