linux 系统调用返回 1,Linux内核系统调用返回-1而不是{-1,-256}
我是一个内核新手,面临着一个奇怪的问题.我编写了一个概念验证计算器系统调用,虽然它适用于大多数计算,但当SUBTRACTION结果介于-1到-256之间时它返回-1.如果有人可以对可能发生的事情有所了解,那就会很感激.下面是系统调用代码.
SYSCALL_DEFINE3(calc, int, a, int, b , char, op) {
int res_int;
switch(op) {
case '+': res_int = a + b;
break;
case '-': res_int = a - b;
break;
case '*': res_int = a * b;
break;
case '/': res_int = (a*1000) / b;
break;
}
printk(KERN_INFO "KERNEL CALC RESULT : %d %c %d = %ld",a, op, b, res_int);
return res_int;
}
编辑：
内核版本：Android Linux Kernel 3.10.xxx.
平台：Nexus7 ARM EABI.
我不明白的是它失败的原因. errno完全没用,因为它将-res_int设置为errno.另外,我不明白为什么只有当res_int为{-1,-256}时才会失败.
a = 1200,b = 1300 op =’ – ‘=> res_int = -100是printk打印-100的示例,但在我的用户空间应用程序中,我收到-1.
看起来当res_int为{-1,-256}时,errno被设置为-res_int.
root@android:/data/local # ./calc
Please enter request in 'num1 oper num2' format:
2.45 - 2.2
returned from syscall with res_int = 250
errno = 22, strerror(errno) = Invalid argument
Calculator result = 0.250000
root@android:/data/local # ./calc
Please enter request in 'num1 oper num2' format:
2.2 - 2.45
returned from syscall with res_int = -1
errno = 250, strerror(errno) = Unknown error 250
Calculator result = -0.001000
root@android:/data/local #