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C sockets

The following C program calls select() to get a list of client sockets that have data in the socket buffer (using UNIX domain datagram sockets). It takes four parameters on call from a NASM program. The parameter fds_array[] is a 3-element integer array created in NASM and passed into the program.

int select_clientsockets(int nfds, fd_set rfds, int fds_array[], int fd_count){

    int fds_ready;
    int abc;
    fd_count = 3;

    FD_ZERO(&rfds);
    FD_SET(0,&rfds);

    fds_ready = select(3, &rfds, NULL, NULL, 0);

    for (int i = 0; i < fd_count; i++){

        fds_array[i] = 0;

        if (FD_ISSET(i, &rfds)) {
            fds_array[i] = 1; }
    }

    for (int i = 0; i < fd_count; i++){
        abc = fds_array[i];
        printf("Value of array at %d, is %dn", i, abc );
    }

    return 0;
}

The data are passed in from NASM in rdi, rsi, rdx and rcx. :

mov rdi,7 ; range of fd #s
lea rsi,[socket_client_fds]
lea rdx,[fds_ready]
mov rcx,3 ; number of open client sockets
call [rel select_clientsockets wrt ..got]

As written, the program hangs. When I step through it with gdb, it does its calculations correctly and the values in fds_array are as expected. But at the return line (return 0), gdb reports this:

0x00007ffff720cfed in select_clientsockets (
    nfds=<error reading variable: Cannot access memory at address   0xfffffffffffffffc>, rfds=<error reading variable: Cannot access memory     at address 0x10>,
    fds_array=<error reading variable: Cannot access memory at address  0xfffffffffffffff0>,
    fd_count=<error reading variable: Cannot access memory at address   0xffffffffffffffec>) at POSIX_Datagram_Socket.c:139

So it says the variables nfds, rfds and fds_array cannot be accessed. But I just finished stepping through it and everything was correct.

My research turned up a number of Stack Overflow questions on that gdb error message, but they’re all fact-specific and I didn’t find any that fit these facts.

Thanks for any ideas on how to fix this error on exit.

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Answer

Let’s look at this call…

mov rdi,7 ; range of fd #s
lea rsi,[socket_client_fds]
lea rdx,[fds_ready]
mov rcx,3 ; number of open client sockets
call [rel select_clientsockets wrt ..got]

In the System V AMD64 ABI, the arguments would be passed in:

  • rdi – ndfs
  • rsi – fds_array
  • rsp..rsp+127 – rfds
  • rdx – fd_count

It looks like you’re passing rfds as if it were a pointer. It’s not. It goes on the stack.

If this were Windows, it would be passed as a pointer. However, it sounds like you’re not using Windows, since you mention Unix domain sockets (although AF_UNIX is actually available on Windows).

If you ever need help calling a C function from assembly, just use Godbolt.

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