Summary GNU C Library Implementation of the Write() and Exit() API Call

Entry into the Linux kernel, any most other OS kernels, employ a trap instruction that emulates a hardware interrupt. (Unfortunately, Intel manuals name this instruction after the hardware interrupt mechanism and cause endless confusion by calling it the interrupt or "int" instruction instead of a trap instruction).

Thus, the C language I/O request

write (FileDescriptor, BufferAddress, BufferSize)
system service is implemented with the
int $0x80
(the trap instruction with vector 80 hex specified).
C Library implementation of write

       movl    BufferSize, %edx     ;third argument - byte count
movl BufferAddress, %ecx ;second argument - pointer to user space buffer
movl FileDescriptor, %ebx ;first argument - usually 1 (stdout)
movl $4, %eax ;system call number - offset in syscall table to sys_write
int $0x80 ;trap into kernel
C library implementation of exit (terminate) user program

       movl    $0, %ebx             ;only one argument - exit code
movl $1, %eax ;offset in syscall table to sys_exit
int $0x80 ;trap into kernel

Detailed GNU C Library Implementation of Any API Call

From the GNU C Library glibc-2.3.2/sysdeps/unix/sysv/linux/i386/sysdep.h file:

/* The original calling convention for system calls on Linux/i386 is
   to use int $0x80.  */

#ifdef I386_USE_SYSENTER
# ifdef SHARED
# else
#  define ENTER_KERNEL call *_dl_sysinfo
# endif
# define ENTER_KERNEL int $0x80

/* Linux takes system call arguments in registers:

        syscall number  %eax         call-clobbered
        arg 1           %ebx         call-saved
        arg 2           %ecx         call-clobbered
        arg 3           %edx         call-clobbered
        arg 4           %esi         call-saved
        arg 5           %edi         call-saved

   The stack layout upon entering the function is:

        20(%esp)        Arg# 5
        16(%esp)        Arg# 4
        12(%esp)        Arg# 3
         8(%esp)        Arg# 2
         4(%esp)        Arg# 1
          (%esp)        Return address

   (Of course a function with say 3 arguments does not have entries for
   arguments 4 and 5.)

   The following code tries hard to be optimal.  A general assumption
   (which is true according to the data books I have) is that

        2 * xchg        is more expensive than  pushl + movl + popl

   Beside this a neat trick is used.  The calling conventions for Linux
   tell that among the registers used for parameters %ecx and %edx need
   not be saved.  Beside this we may clobber this registers even when
   they are not used for parameter passing.

   As a result one can see below that we save the content of the %ebx
   register in the %edx register when we have less than 3 arguments
   (2 * movl is less expensive than pushl + popl).

   Second unlike for the other registers we don't save the content of
   %ecx and %edx when we have more than 1 and 2 registers resp.

   The code below might look a bit long but we have to take care for
   the pipelined processors (i586).  Here the `pushl' and `popl'
   instructions are marked as NP (not pairable) but the exception is
   two consecutive of these instruction.  This gives no penalty on
   other processors though.  */

#undef  DO_CALL
#define DO_CALL(syscall_name, args)                                           \
    PUSHARGS_##args                                                           \
    DOARGS_##args                                                             \
    movl $SYS_ify (syscall_name), %eax;                                       \
    ENTER_KERNEL                                                              \

/* Note the above macros "PUSHARGS_", "DOARGS_" and "POPARGS_" combined
   with the actual number arguments specified in the DO_CALL macro
   invocation to yeild one of the following macro sets: */

#define PUSHARGS_0      /* No arguments to push.  */
#define DOARGS_0        /* No arguments to frob.  */
#define POPARGS_0       /* No arguments to pop.  */
#define _PUSHARGS_0     /* No arguments to push.  */
#define _DOARGS_0(n)    /* No arguments to frob.  */
#define _POPARGS_0      /* No arguments to pop.  */

#define PUSHARGS_1      movl %ebx, %edx; PUSHARGS_0
#define DOARGS_1        _DOARGS_1 (4)
#define POPARGS_1       POPARGS_0; movl %edx, %ebx
#define _PUSHARGS_1     pushl %ebx; _PUSHARGS_0
#define _DOARGS_1(n)    movl n(%esp), %ebx; _DOARGS_0(n-4)
#define _POPARGS_1      _POPARGS_0; popl %ebx

#define PUSHARGS_2      PUSHARGS_1
#define DOARGS_2        _DOARGS_2 (8)
#define POPARGS_2       POPARGS_1
#define _PUSHARGS_2     _PUSHARGS_1
#define _DOARGS_2(n)    movl n(%esp), %ecx; _DOARGS_1 (n-4)
#define _POPARGS_2      _POPARGS_1

#define PUSHARGS_3      _PUSHARGS_2
#define DOARGS_3        _DOARGS_3 (16)
#define POPARGS_3       _POPARGS_3
#define _PUSHARGS_3     _PUSHARGS_2
#define _DOARGS_3(n)    movl n(%esp), %edx; _DOARGS_2 (n-4)

#define _POPARGS_3      _POPARGS_2
#define PUSHARGS_4      _PUSHARGS_4
#define DOARGS_4        _DOARGS_4 (24)
#define POPARGS_4       _POPARGS_4
#define _PUSHARGS_4     pushl %esi; _PUSHARGS_3
#define _DOARGS_4(n)    movl n(%esp), %esi; _DOARGS_3 (n-4)
#define _POPARGS_4      _POPARGS_3; popl %esi

#define PUSHARGS_5      _PUSHARGS_5
#define DOARGS_5        _DOARGS_5 (32)
#define POPARGS_5       _POPARGS_5
#define _PUSHARGS_5     pushl %edi; _PUSHARGS_4
#define _DOARGS_5(n)    movl n(%esp), %edi; _DOARGS_4 (n-4)
#define _POPARGS_5      _POPARGS_4; popl %edi

#else   /* !__ASSEMBLER__ */

ENTRY (syscall)

        PUSHARGS_5              /* Save register contents.  */
        _DOARGS_5(36)           /* Load arguments.  */
        movl 16(%esp), %eax     /* Load syscall number into %eax.  */
        ENTER_KERNEL            /* Do the system call.  */
        POPARGS_5               /* Restore register contents.  */
        cmpl $-4095, %eax       /* Check %eax for error.  */
        jae SYSCALL_ERROR_LABEL /* Jump to error handler if error.  */
        ret                     /* Return to caller.  */

/* Linux uses a negative return value to indicate syscall errors,
   unlike most Unices, which use the condition codes' carry flag.

   Since version 2.1 the return value of a system call might be
   negative even if the call succeeded.  E.g., the `lseek' system call
   might return a large offset.  Therefore we must not anymore test
   for < 0, but test for a real error by making sure the value in %eax
   is a real error number.  Linus said he will make sure the no syscall
   returns a value in -1 .. -4095 as a valid result so we can savely
   test with -4095.  */