系統調用是os操作系統提供的服務,用戶程序通過各種系統調用,來引用內核提供的各種服務,系統調用的執行讓用戶程序陷入內核,該陷入動作由swi軟中斷完成.
#if defined(__thumb__) //thumb模式
#define __syscall(name) \
"push {r7}\n\t" \
"mov r7, #" __sys1(__NR_##name) "\n\t" \
"swi 0\n\t" \
"pop {r7}"
#else //arm模式
#define __syscall(name) "swi\t" __sys1(__NR_##name) "\n\t"
#endif
#define __sys2(x) #x
#define __sys1(x) __sys2(x)
#define __NR_SYSCALL_BASE 0x900000 //此為OS_NUMBER << 20運算值
#define __NR_open (__NR_SYSCALL_BASE+ 5) //0x900005
舉一個例子來說:open系統調用,庫函數最終會調用__syscall(open),宏展開之後為swi #__NR_open,即,swi #0x900005觸發中斷,中斷號0x900005存放在[lr,#-4]地址中,處理器跳轉到arch/arm/kernel/entry-common.S中vector_swi讀取[lr,#-4]地址中的中斷號,之後查詢arch/arm/kernel/entry-common.S中的sys_call_table系統調用表,該表內容在arch/arm/kernel/calls.S中定義,__NR_open在表中對應的順序號為
...
.long SYMBOL_NAME(sys_open) //第5個
...
將sys_call_table[5]中內容傳給pc,系統進入sys_open函數,處理實質的open動作
注:用到的一些函數數據所在文件,如下所示
arch/arm/kernel/calls.S聲明了系統調用函數
include/asm-arm/unistd.h定義了系統調用的調用號規則
vector_swi定義在arch/arm/kernel/entry-common.S
vector_IRQ定義在arch/arm/kernel/entry-armv.S
vector_FIQ定義在arch/arm/kernel/entry-armv.S
arch/arm/kernel/entry-common.S中對sys_call_table進行了定義:
.type sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S" //將calls.S中的內容順序鏈接到這裡
__syscall_start:
源程序：
ENTRY(vector_swi)
save_user_regs
zero_fp
get_scno //將[lr,#-4]中的中斷號轉儲到scno(r7)
arm710_bug_check scno, ip
#ifdef CONFIG_ALIGNMENT_TRAP
ldr ip, __cr_alignment
ldr ip, [ip]
mcr p15, 0, ip, c1, c0 @ update control register
#endif
enable_irq ip
str r4, [sp, #-S_OFF]! @ push fifth arg
get_current_task tsk
ldr ip, [tsk, #TSK_PTRACE] @ check for syscall tracing
bic scno, scno, #0xff000000 @ mask off SWI op-code
//#define OS_NUMBER 9[entry-header.S]
//所以對於上面示例中open系統調用號scno=0x900005
//eor scno,scno,#0x900000
//之後scno=0x05
eor scno, scno, #OS_NUMBER << 20 @ check OS number
//sys_call_table項為calls.S的內容
adr tbl, sys_call_table @ load syscall table pointer
tst ip, #PT_TRACESYS @ are we tracing syscalls?
bne __sys_trace
adrsvc al, lr, ret_fast_syscall @ return address
cmp scno, #NR_syscalls @ check upper syscall limit
//執行sys_open函數
ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine
add r1, sp, #S_OFF
2: mov why, #0 @ no longer a real syscall
cmp scno, #ARMSWI_OFFSET
eor r0, scno, #OS_NUMBER << 20 @ put OS number back
bcs SYMBOL_NAME(arm_syscall)
b SYMBOL_NAME(sys_ni_syscall) @ not private func
/*
* This is the really slow path. We're going to be doing
* context switches, and waiting for our parent to respond.
*/
__sys_trace:
add r1, sp, #S_OFF
mov r0, #0 @ trace entry [IP = 0]
bl SYMBOL_NAME(syscall_trace)
/*
//2007-07-01 gliethttp [entry-header.S]
//Like adr, but force SVC mode (if required)
.macro adrsvc, cond, reg, label
adr\cond \reg, \label
.endm
//對應反匯編:
//add lr, pc, #16 ; lr = __sys_trace_return
*/
adrsvc al, lr, __sys_trace_return @ return address
add r1, sp, #S_R0 + S_OFF @ pointer to regs
cmp scno, #NR_syscalls @ check upper syscall limit
ldmccia r1, {r0 - r3} @ have to reload r0 - r3
ldrcc pc, [tbl, scno, lsl #2] @ call sys_* routine
b 2b
__sys_trace_return:
str r0, [sp, #S_R0 + S_OFF]! @ save returned r0
mov r1, sp
mov r0, #1 @ trace exit [IP = 1]
bl SYMBOL_NAME(syscall_trace)
b ret_disable_irq
.align 5
#ifdef CONFIG_ALIGNMENT_TRAP
.type __cr_alignment, #object
__cr_alignment:
.word SYMBOL_NAME(cr_alignment)
#endif
.type sys_call_table, #object
ENTRY(sys_call_table)
#include "calls.S"