making one function to use local variabe of other function.

Makefile

@@ -0,0 +1,7 @@
+all: context_asm.S ctx_switch.c
+	gcc -c context_asm.S -o context_asm.o -ggdb3 
+	gcc -c ctx_switch.c -o ctx_switch.o -ggdb3 
+	gcc ctx_switch.o context_asm.o -o run -ggdb3
+
+clean:
+	rm -f context_asm.o ctx_switch.o run

README

@@ -0,0 +1,2 @@
+Can one function access the local variable of other? No, right?
+Look the code and figure out how function "funa" is using local variable "i" of funb.

context_asm.S

@@ -0,0 +1,47 @@
+.extern cur_ctx
+.globl my_yeild 
+
+
+#ifdef __x86_64__
+
+my_yeild:
+	movq    cur_ctx, %rsi
+        movq    %rsp, 0(%rsi)
+        /* Save the callee saved registers */
+	movq    %r12, 8(%rsi)
+        movq    %r13, 16(%rsi)
+        movq    %r14, 24(%rsi)
+        movq    %r15, 32(%rsi)
+
+my_set:
+        movq    %rdi, cur_ctx /*argument will be in rdi */
+        /* reload the registers */
+        movq    0(%rdi), %rsp
+        movq    8(%rdi), %r12
+        movq    16(%rdi), %r13
+        movq    24(%rdi), %r14
+        movq    32(%rdi), %r15
+        ret
+
+#endif
+
+#ifdef __i386__
+
+my_yeild:
+	movl cur_ctx, %eax
+	movl 	%esp, 0(%eax)
+        /* Save the callee saved registers */
+	movl 	%ebx, 4(%eax)
+	movl 	%esi, 8(%eax)
+	movl 	%edi, 12(%eax)
+
+my_set:
+	movl 	4(%esp), %eax /* agrument will be on stack*/
+	movl	%eax, cur_ctx
+	/* reload the registers */
+	movl    0(%eax), %esp
+	movl    4(%eax), %ebx
+	movl    8(%eax), %esi
+	movl    12(%eax), %edi
+	ret
+#endif

ctx_switch.c

@@ -0,0 +1,64 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef size_t uintptr_t;
+
+#ifdef __x86_64__
+	#define NUM_REG 4
+#elif __i386__
+	#define NUM_REG 3
+#else
+	#define NUM_REG 0
+#endif
+
+struct stack_context {
+    uintptr_t sp_reg;
+    uintptr_t other_reg[NUM_REG];
+}actx, bctx, pctx;
+
+struct stack_context *cur_ctx;
+#define STACK_SIZE 4096
+
+unsigned int astack[STACK_SIZE];
+unsigned int bstack[STACK_SIZE];
+
+extern void my_yeild(struct stack_context *rctx);
+extern void funa(void);
+extern void funb(void);
+
+void funa(void)
+{
+    int i = 0;
+    my_yeild(&bctx);
+    while (i++ < 20) {
+        printf("a %d\n", i);
+        sleep(1);
+        my_yeild(&bctx);
+    }
+    exit(0);
+}
+
+void funb(void)
+{
+    int i = 0;
+    while (i++ < 20) {
+        printf("b %d\n", i);
+        sleep(1);
+        my_yeild(&actx);
+    }
+    exit(0);
+}
+
+int main()
+{
+    cur_ctx = &pctx;
+    uintptr_t *sp = (uintptr_t *) ((((uintptr_t)&astack[STACK_SIZE]) & -16L) - sizeof(uintptr_t));
+    *sp = (uintptr_t)funa;
+    actx.sp_reg = (uintptr_t)sp;
+
+    sp = (uintptr_t *) ((((uintptr_t)&bstack[STACK_SIZE]) & -16L) - sizeof(uintptr_t));
+    *sp = (uintptr_t)funb;
+    bctx.sp_reg = (uintptr_t) sp;
+    my_yeild(&actx); 
+    return 0;
+}