本文介绍Linux 4.4内核IS_ERR()
函数。
文件:include/linux/err.h
,定义如下:
#define MAX_ERRNO 4095
## 对于64位系统,判断x是否在0xffff ffff ffff f001 ~ 0xffff ffff ffff ffff
#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
static inline long __must_check PTR_ERR(__force const void *ptr)
{
return (long) ptr;
}
static inline bool __must_check IS_ERR(__force const void *ptr)
{
return IS_ERR_VALUE((unsigned long)ptr);
}
一、功能
功能:判断传入的指针ptr
是否有效。
Linux内核指针包括:有效指针、空指针(NULL)和错误指针。
Linux内核用最后4K空间保存指针的出错码,64位的系统地址为:0xffff ffff ffff f001 ~ 0xffff ffff ffff ffff
。
如果是错误指针,配合PTR_ERR()
函数判断错误代码。
文件:include/uapi/asm-generic/errno-base.h
,定义如下:
#define EPERM 1 /* Operation not permitted */
#define ENOENT 2 /* No such file or directory */
#define ESRCH 3 /* No such process */
#define EINTR 4 /* Interrupted system call */
#define EIO 5 /* I/O error */
#define ENXIO 6 /* No such device or address */
#define E2BIG 7 /* Argument list too long */
#define ENOEXEC 8 /* Exec format error */
#define EBADF 9 /* Bad file number */
#define ECHILD 10 /* No child processes */
#define EAGAIN 11 /* Try again */
#define ENOMEM 12 /* Out of memory */
...
文件:include/uapi/asm-generic/errno.h
,定义如下:
#define EDEADLK 35 /* Resource deadlock would occur */
#define ENAMETOOLONG 36 /* File name too long */
#define ENOLCK 37 /* No record locks available */
#define ENOSYS 38 /* Invalid system call number */
#define ENOTEMPTY 39 /* Directory not empty */
#define ELOOP 40 /* Too many symbolic links encountered */
...
文件:include/linux/errno.h
,定义如下:
#define ERESTARTSYS 512
#define ERESTARTNOINTR 513
#define ERESTARTNOHAND 514 /* restart if no handler.. */
#define ENOIOCTLCMD 515 /* No ioctl command */
#define ERESTART_RESTARTBLOCK 516 /* restart by calling sys_restart_syscall */
#define EPROBE_DEFER 517 /* Driver requests probe retry */
...
二、说明
1)inline
:内联函数。内联函数的代码会直接嵌入到调用它的位置,调用几次就嵌入几次。
2)__must_check
:指调用函数一定要处理函数的返回值,否则编译器会给出警告。
3)__force
:表示变量可进行强制转换。
4)unlikely
后续专门介绍。
4)(unsigned long)-MAX_ERRNO
:用补码的方式表示-4095,64位系统为0xffff ffff ffff f001
。
三、示例
文件:drivers/gpu/drm/rockchip/rockchip_drm_drv.c
printk(KERN_ERR" ==== unsigned long 0x%lx \n", (unsigned long)-MAX_ERRNO);
...
private->devfreq = devfreq_get_devfreq_by_phandle(dev, 0);
printk(KERN_ERR"==== IS_ERR(private->devfreq) %d , private->devfreq %p\n", \
IS_ERR(private->devfreq), private->devfreq);
## 1、判断是否有效指针
if (IS_ERR(private->devfreq)) {
## 2、判断错误码
if (PTR_ERR(private->devfreq) == -EPROBE_DEFER) {
parent_np = of_parse_phandle(np, "devfreq", 0);
if (parent_np &&
of_device_is_available(parent_np)) {
...
} else {
## 3、运行到此处
dev_info(dev, "dmc is disabled\n");
}
} else {
...
}
...
}
...
private->hdmi_pll.pll = devm_clk_get(dev, "hdmi-tmds-pll");
printk(KERN_ERR"==== IS_ERR(private->hdmi_pll.pll) %d , private->hdmi_pll.pll %p\n", \
IS_ERR(private->hdmi_pll.pll), private->hdmi_pll.pll);
程序运行后,相关日志如下:
[ 2.270871] ==== unsigned long 0xfffffffffffff001
[ 2.271322] ==== IS_ERR(private->devfreq) 1 , private->devfreq fffffffffffffdfb
[ 2.271993] rockchip-drm display-subsystem: dmc is disabled
[ 2.272504] ==== IS_ERR(private->hdmi_pll.pll) 0 , private->hdmi_pll.pll ffffffc0782e6340
通过上面日志可以看出:
1)private->devfreq
值为0xfffffffffffffdfb
(值为-517),对应错误码EPROBE_DEFER
,此时IS_ERR
返回1。
2)private->hdmi_pll.pll
值为0xffffffc0782e6340
,是有效指针,IS_ERR
返回0。
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