system call

This is a discussion on system call within the Linux Programming forums, part of the Platform Specific Boards category; Hi As I know, if I invoke a system call, it uses a special instruction (called as trap instruction) to ...

  1. #1
    Just kidding.... fnoyan's Avatar
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    system call

    Hi

    As I know, if I invoke a system call, it uses a special instruction (called as trap instruction) to change the kernel mode. When I use fork() to create a new process, I neither see any special instruction that changes the mode nor any jmp to any other function like sys_fork!

    Here is the gdb dump of the statically compiled C code that uses a fork call.
    Code:
    (gdb) disas fork
    Dump of assembler code for function fork:
    0x08053350 <fork+0>:    push   %ebp
    0x08053351 <fork+1>:    mov    $0x0,%edx
    0x08053356 <fork+6>:    mov    %esp,%ebp
    0x08053358 <fork+8>:    sub    $0x8,%esp
    0x0805335b <fork+11>:   test   %edx,%edx
    0x0805335d <fork+13>:   mov    %ebx,0xfffffffc(%ebp)
    0x08053360 <fork+16>:   je     0x8053375 <fork+37>
    0x08053362 <fork+18>:   movl   $0x80b05c0,(%esp)
    0x08053369 <fork+25>:   call   0x0
    0x0805336e <fork+30>:   mov    0xfffffffc(%ebp),%ebx
    0x08053371 <fork+33>:   mov    %ebp,%esp
    0x08053373 <fork+35>:   pop    %ebp
    0x08053374 <fork+36>:   ret
    0x08053375 <fork+37>:   mov    $0x2,%eax
    0x0805337a <fork+42>:   int    $0x80
    0x0805337c <fork+44>:   cmp    $0xfffff000,%eax
    0x08053381 <fork+49>:   mov    %eax,%ebx
    0x08053383 <fork+51>:   ja     0x8053389 <fork+57>
    0x08053385 <fork+53>:   mov    %ebx,%eax
    0x08053387 <fork+55>:   jmp    0x805336e <fork+30>
    0x08053389 <fork+57>:   neg    %ebx
    0x0805338b <fork+59>:   call   0x80489c0 <__errno_location>
    ---Type <return> to continue, or q <return> to quit---
    0x08053390 <fork+64>:   mov    %ebx,(%eax)
    0x08053392 <fork+66>:   mov    $0xffffffff,%ebx
    0x08053397 <fork+71>:   jmp    0x8053385 <fork+53>
    Thanks in advice....

  2. #2
    and the hat of int overfl Salem's Avatar
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    What's this look like?
    0x08053375 <fork+37>: mov $0x2,%eax
    0x0805337a <fork+42>: int $0x80
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
    Just kidding.... fnoyan's Avatar
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    Ahaa! I missed the point! Thanks...

    Next time I should check the code more carefully before posting to forum

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