STK(3)

NAME
stk - data stack storage library

SYNOPSIS

#include <stk.h>
Stk_t *stkopen(int flags);
void stkoverflow(Stk_t *stack, void *(overflow)(size_t));
Stk_t *stkinstall(Stk_t *stack, char *(overflow)(size_t));
int stkclose(Stk_t *stack);
unsigned int stklink(Stk_t *stack)
void *stkalloc(Stk_t *stack, unsigned size);
char *stkcopy(Stk_t *stack, const char *string);
void *stkset(Stk_t *stack, void *address, unsigned offset);
void *stkseek(Stk_t *stack, unsigned offset);
int stktell(Stk_t *stack);
char *stkptr(Stk_t *stack, unsigned offset);
void *stkfreeze(Stk_t *stack, unsigned extra);
int stkon(Stk *stack, char* addr)

DESCRIPTION

stk is a package of routines designed to provide efficient stack oriented dynamic storage. A stack abstraction consists of an ordered list of contiguous memory regions, called stack frames, that can hold objects of arbitrary size. A stack is represented by the type Stk_t defined in the header <stk.h>. The type Stk_t is compatible with the type Sfio_t defined by the sfio(3) library. There is a preset initial stack which can be referenced by the constant stkstd (stack standard).

Objects of variable size can be added to a stack, and programs can reference these objects directly with pointers. In addition, the last object on the stack (referred to here as the current object) can be built incrementally. The current object has an associated offset that determines its current size. While the current object is being built incrementally, its location might change so that it is necessary to reference the object with relative offsets ranging from zero to the current offset of the object.

The sfio(3) output functions can be used to build the current object incrementally. An object that is built incrementally on the stack will always occupy contiguous memory within a stack frame, but until stkfreeze() is called, the location in memory for the object can change. There is a current offset associated with the current object that determines where subsequent operations apply. Initially, this offset is zero, and the offset changes as a result of the operations you specify.

The stkopen() function creates a new stack. The flags argument is an options bitmask. If the STK_SMALL bit is set, the stack allocates memory in small blocks, optimizing memory usage at the expense of performance. If the STK_NULL bit is set, a stack overflow will cause stack operations to return a null pointer instead of throwing an exception. If the STK_NULL bit is not set and the stkstd stack has a custom overflow function set (see stkoverflow() below), then the new stack inherits the pointer to that overflow function. If successful, stkopen() returns a pointer to a stack whose reference count is 1. Otherwise, stkopen() returns a null pointer.

The stkoverflow() function specifies a function that will be called whenever malloc(3) fails while trying to grow the stack. The overflow function will be called with the size that was passed to malloc(3). The overflow function can call exit(3), call longjmp(3) or return. If the overflow function returns, it must return a pointer to a memory region of the given size. If overflow is NULL, then the stack overflow action is reset to the default. If the stack was opened with the STK_NULL option, the default is to return a null pointer, otherwise the default is to write an error to standard error and to call exit(2) with a non-zero exit value.

The stkinstall() function is deprecated and provided for backward compatibility. stkinstall() makes stkstd refer to the specified stack pointer and returns a pointer to the previous stkstd stack. If the overflow argument is not null and the stack was not opened with the STK_NULL option, then overflow sets the overflow action (see overflow() above). When stack is a null pointer, the stkstd stack is not changed but the overflow function for the stkstd stack can be changed and a pointer to the stkstd stack is returned. (Current usage is simply passing the pointer to the desired stack to the stk functions, and using stkoverflow() to set the stack overflow function).

The stkclose() function decrements the reference count and frees the memory associated with the specified stack when the reference count is zero. The effect of subsequent references to objects on the stack is undefined.

The stklink() function increases the reference count for the given stack and returns the increased count.

The stkalloc() function returns an aligned pointer to space on the active stack that can be used to hold any object of the given size. stkalloc() is similar to malloc(3) except that individual items returned by stkalloc() can not be freed. stkalloc() causes the offset of the current object to be set to zero.

The stkcopy() function copies the given string onto the stack and returns a pointer to the string on the stack. stkcopy() causes the offset of the current object to be set to zero.

The stkset() function finds the frame containing the given address, frees all frames that were created after the one containing the given address, and sets the current object to the given address. The top of the current object is set to offset bytes from current object. If address is null, the stack is reset to the beginning. If it is non-null, but is not the address of an object on the stack, the program aborts and dumps core.

The stkseek() function is used set the offset for the current object. The offset argument to stkseek() specifies the new offset for the current object. The frame will be extended or moved if offset causes the new current offset to extend beyond the current frame. stkseek() returns a pointer to the beginning of the current object.

The stktell() function gives the offset of the current object.

The stkptr() function converts the given offset for the current object into a memory address on the stack. This address is only valid until another stack operation is given. The result is not defined if offset exceeds the size of the current object. This function returns a char pointer instead of void pointer to make it easy to dereference the return value directly and read an individual byte on the stack.

The stkfreeze() function terminates the current object on the stack and returns a pointer to the beginning of this object. If extra is non-zero, extra bytes are added to the stack before the current object is terminated. The first added byte will contain zero and the contents of the remaining bytes are undefined.

The stkon() function returns non-zero if the address given by addr is on the stack stack and 0 otherwise.

HISTORY
The stk interface was derived from similar routines in the KornShell code that is used for building parse trees and carrying out expansions. It provides an efficient mechanism for grouping dynamically allocated objects so that they can be freed all at once rather than individually.

In 2024, as part of changes made by Martijn Dekker <martijn@inlv.org> for ksh 93u+m, the return types of stkalloc(), stkset(), stkseek() and stkfreeze() were changed from char* to void*, the stkoverflow() function was added, and the stkinstall() function was deprecated.

AUTHOR
David Korn

SEE ALSO
exit(2), longjmp(3), malloc(3), sfio(3)