Cant call an assembly function in c++ code

meerokh

I am trying to implement coroutines in c++ (I had to reinvent the wheel for my project). coroutine.h

class coroutine {
public:
void (*action1)(int);
Stack local_stack;
coroutine(void (*action)(int ), int id); ///< will create coroutine
~coroutine();
static void yield();
};

coroutine.cpp

coroutine *global_coro;
fcontext_t context_array[2];
coroutine :: coroutine(void (*action)(int), int id)
{
global_coro=this;
action1=action;
fcontext_t f_ctx = make_fcontext(global_coro->local_stack.local_stack, 1000, global_coro->action1);//Save the current context of coroutine for the sake of making switch context.
// make_fcontext is in .S file taken from boost::context
context_array[0]=f_ctx; // Global Array to store the coroutine context so that i can switch the context
}

coroutine::~coroutine(){}

void coroutine::yield(){

transfer_t tr = jump_fcontext(context_of_current_corotine, context_of_next_coroutine ); //Also in .S file

Following is my stack class which allocates a block of memory to every coroutine from a global Memory pool stack.h

extern MemoryPool memPoolObj;

class Stack {
public:
void *local_stack;
MemoryPool& m_memPool=memPoolObj;
Stack();
~Stack();
};

Stack.cpp

#include "Stack.h"
MemoryPool memPoolObj;
Stack::Stack() {
auto *local_stack= m_memPool.Allocate();
}
Stack::~Stack() {}

Context.h

typedef void* fcontext_t;

struct transfer_t
{
fcontext_t fctx;
void * data;
};

extern "C"
transfer_t jump_fcontext( fcontext_t const to, void * vp);
extern "C"
fcontext_t make_fcontext( void * sp, std::size_t size, void (* fn)( int) );

Main.cpp

#include "coroutine.h"
void workPackage(int id){
printf("Coroutine with id %d is called\n", id);
coroutine::yield();
printf("Coroutine with id %d is resumed after first yield\n", id);
coroutine::yield();
printf("Coroutine with id %d is resumed after second yield\n", id);
}

int main() {
coroutine Coro1(workPackage, 1);
coroutine Coro2(workPackage, 2);
printf("Main is finished \n");
}

My program compiles but gives segmentation fault during execution. Valgrind gives the following information and i am not able to solve the problem. I will provide the assembly file also if needed. Any help would be appreciated

Use of uninitialised value of size 8
==24145== at 0x10922B: make_fcontext (in /home/user1/eclipse-workspace/coroutines/Debug/coroutines)
==24145== by 0x10912D

Lost User

Where is the code for make_fcontext, which is shown as the place where the error occurs?

meerokh

My project structure in eclipse is as follows

Coroutines
Src
Debug

Code for make_fcontext is in src folder in file "make_x86_64_sysv_elf_gas.S"

.text
.globl make_fcontext
.type make_fcontext,@function
.align 16
make_fcontext:
/* first arg of make_fcontext() == top of context-stack */
movq %rdi, %rax

/\* shift address in RAX to lower 16 byte boundary \*/
andq  $-16, %rax

/\* reserve space for context-data on context-stack \*/
/\* on context-function entry: (RSP -0x8) % 16 == 0 \*/
leaq  -0x40(%rax), %rax

/\* third arg of make\_fcontext() == address of context-function \*/
/\* stored in RBX \*/
movq  %rdx, 0x28(%rax)

/\* save MMX control- and status-word \*/
stmxcsr  (%rax)
/\* save x87 control-word \*/
fnstcw   0x4(%rax)

/\* compute abs address of label trampoline \*/
leaq  trampoline(%rip), %rcx
/\* save address of trampoline as return-address for context-function \*/
/\* will be entered after calling jump\_fcontext() first time \*/
movq  %rcx, 0x38(%rax)

/\* compute abs address of label finish \*/
leaq  finish(%rip), %rcx
/\* save address of finish as return-address for context-function \*/
/\* will be entered after context-function returns \*/
movq  %rcx, 0x30(%rax)

ret /\* return pointer to context-data \*/

trampoline:
/* store return address on stack */
/* fix stack alignment */
push %rbp
/* jump to context-function */
jmp *%rbx

finish:
/* exit code is zero */
xorq %rdi, %rdi
/* exit application */
call _exit@PLT
hlt
.size make_fcontext,.-make_fcontext

/* Mark that we don't need executable stack. */
.section .note.GNU-stack,"",%progbits

Lost User

You will need to use the information from the stack trace to figure out which line caused the SEGV, and which value has not been set.

Joe Woodbury

A quick glance brings up a problem here:

Stack::Stack() {
auto *local_stack= m_memPool.Allocate();
}

Perhaps I misunderstand the intention, but shouldn't this be:

Stack::Stack()
: local_stack(m_memPool.Allocate())
{}

Lost User

Good catch; I missed that completely.

meerokh

Thanks alot it worked