posix.md

POSIX Support Library

Context switching

#include <sys/ucontext.h>

void my_context_function();

static int ctx_stack[200] {};

int main() {
  ucontext_t link_ctx;
  
  ucontext_t ctx;
  ctx.uc_stack.ss_sp = ctx_stack;
  ctx.uc_stack.ss_size = sizeof(ctx_stack);
  ctx.uc_link = &link_ctx;
  
  makecontext(&ctx, my_context_function, 0);
  
  /* Do some stuff */
  
  swapcontext( &link_ctx, &ctx ); /* Swap from `link_ctx` to `ctx` */
  
  /* Continue doing stuff */
}

void my_context_function() {
  /* Do other stuff */
}

Signature	Description
int getcontext(ucontext_t* ucp)	Copies the current machine context into ucp
int setcontext(const ucontext_t* ucp)	Sets the current context to ucp
int swapcontext(ucontext_t* oucp, const ucontext_t* ucp)	Writes current context into oucp, and switches to ucp
void makecontext(ucontext_t* ucp, void(*func)(void), int argc, ...)	Modifies context ucp to invoke func with setcontext. Before invoking, the caller must allocate a new stack for this context and assign its address to ucp->uc_stack, and set a successor context to ucp->uc_link.

ucontext_t is a structure with a pointer to the next context to return to (uc_link), the stack creation parameters (stack_t), and the opaque machine context (uc_mcontext).

typedef struct ucontext_t {
    struct ucontext_t* uc_link;
    stack_t uc_stack;
    mcontext_t uc_mcontext;
} ucontext_t;

Time functions

Requires RTC hardware, and for the RTC hardware to be first initialized with __agbabi_rtc_init.

#include <agbabi.h>

#include <sys/time.h> /* `gettimeofday` `settimeofday` */
#include <time.h> /* C time API also available */

int main() {
    if (__agbabi_rtc_init() == 0) {
        /* RTC available for time related functions */
    }
}

Signature	Description
int gettimeofday(struct timeval* tv, void* tz)
int settimeofday(const struct timeval* tv, const struct timezone* tz)