[英]C string to int without any libraries
I'm trying to write my first kernel module so I'm not able to include libraries for atoi, strtol, etc. How can I convert a string to int without these built-in functions?我正在尝试编写我的第一个内核模块,所以我无法包含 atoi、strtol 等库。如果没有这些内置函数,如何将字符串转换为 int? I tried:我试过了:
int num;
num = string[0] - '0';
which works for the first character, but if I remove the [0]
to try and convert the full string it gives me a warning: assignment makes integer from pointer without a cast.这适用于第一个字符,但如果我删除[0]
以尝试转换完整的字符串,它会给我一个警告:赋值从指针中生成整数而没有强制转换。 So what do I do?那我该怎么办?
When creating your own string to int function, make sure you check and protect against overflow.在为 int 函数创建自己的字符串时,请确保检查并防止溢出。 For example:例如:
/* an atoi replacement performing the conversion in a single
pass and incorporating 'err' to indicate a failed conversion.
passing NULL as error causes it to be ignored */
int strtoi (const char *s, unsigned char *err)
{
char *p = (char *)s;
int nmax = (1ULL << 31) - 1; /* INT_MAX */
int nmin = -nmax - 1; /* INT_MIN */
long long sum = 0;
char sign = *p;
if (*p == '-' || *p == '+') p++;
while (*p >= '0' && *p <= '9') {
sum = sum * 10 - (*p - '0');
if (sum < nmin || (sign != '-' && -sum > nmax)) goto error;
p++;
}
if (sign != '-') sum = -sum;
return (int)sum;
error:
fprintf (stderr, "strtoi() error: invalid conversion for type int.\n");
if (err) *err = 1;
return 0;
}
You can't remove the [0].您无法删除 [0]。 That means that you are subtracting '0' from the pointer string
, which is meaningless.这意味着您从指针string
中减去 '0' ,这是没有意义的。 You still need to dereference it: num = string[i] - '0';
您仍然需要取消引用它: num = string[i] - '0';
This function skips leading and trailing whitespace, handles one optional +
/ -
sign, and returns 0 on invalid input,此函数跳过前导和尾随空格,处理一个可选的+
/ -
符号,并在无效输入时返回 0,
// Convert standard null-terminated string to an integer
// - Skips leading whitespaces.
// - Skips trailing whitespaces.
// - Allows for one, optional +/- sign at the front.
// - Returns zero if any non-+/-, non-numeric, non-space character is encountered.
// - Returns zero if digits are separated by spaces (eg "123 45")
// - Range is checked against Overflow/Underflow (INT_MAX / INT_MIN), and returns 0.
int StrToInt(const char* s)
{
int minInt = 1 << (sizeof(int)*CHAR_BIT-1);
int maxInt = -(minInt+1);
char* w;
do { // Skip any leading whitespace
for(w=" \t\n\v\f\r"; *w && *s != *w; ++w) ;
if (*s == *w) ++s; else break;
} while(*s);
int sign = 1;
if ('-' == *s) sign = -1;
if ('+' == *s || '-' == *s) ++s;
long long i=0;
while('0' <= *s && *s <= '9')
{
i = 10*i + *s++ - '0';
if (sign*i < minInt || maxInt < sign*i)
{
i = 0;
break;
}
}
while (*s) // Skip any trailing whitespace
{
for(w=" \t\n\v\f\r"; *w && *s != *w; ++w) ;
if (*w && *s == *w) ++s; else break;
}
return (int)(!*s*sign*i);
}
A string is an array
of characters, represented by an address (aka pointer
).字符串是一个字符array
,由地址(又名pointer
)表示。
An pointer
has an value that might look something like 0xa1de2bdf
. pointer
的值可能类似于0xa1de2bdf
。 This value tells me where the start of the array is.这个值告诉我数组的开始在哪里。
You cannot subtract a pointer
type with a character
type (eg 0xa1de2bdf - 'b' does not really make sense).您不能用character
类型减去pointer
类型(例如 0xa1de2bdf - 'b' 没有真正意义)。
To convert a string to a number, you could try this:要将字符串转换为数字,您可以尝试以下操作:
//Find the length of the string
int len = 0;
while (str[len] != '\0') {
len++;
}
//Loop through the string
int num = 0, i = 0, digit;
for (i=0; i<len; i++) {
//Extract the digit
digit = ing[i] - '0';
//Multiply the digit with its correct position (ones, tens, hundreds, etc.)
num += digit * pow(10, (len-1)-i);
}
Of course if you are not allowed to use math.h
library, you could write your own pow(a,b)
function which gives you the value of a^b
.当然,如果您不允许使用math.h
库,您可以编写自己的pow(a,b)
函数,该函数为您提供a^b
的值。
int mypowfunc(int a, int b) {
int i=0, ans=1;
//multiply the value a for b number of times
for (i=0; i<b; i++) {
ans *= a;
}
return ans;
}
I have written the code above in a way that is simple to understand.我以一种易于理解的方式编写了上面的代码。 It assumes that your string has a null character ('\0') right behind the last useful character (which is good practice).它假定您的字符串在最后一个有用字符的后面有一个空字符 ('\0')(这是一种很好的做法)。
Also, you might want to check that the string is actually a valid string with only digits (eg '0', '1', '2', etc.).此外,您可能想要检查该字符串实际上是一个只有数字的有效字符串(例如,'0'、'1'、'2' 等)。 You could do this by including an if... else..
statement while looping through the string.您可以通过在遍历字符串时包含if... else..
语句来做到这一点。
In modern kernels you want to use kstrto*
:在现代内核中,您想使用kstrto*
:
http://lxr.free-electrons.com/source/include/linux/kernel.h#L274 http://lxr.free-electrons.com/source/include/linux/kernel.h#L274
274 /**
275 * kstrtoul - convert a string to an unsigned long
276 * @s: The start of the string. The string must be null-terminated, and may also
277 * include a single newline before its terminating null. The first character
278 * may also be a plus sign, but not a minus sign.
279 * @base: The number base to use. The maximum supported base is 16. If base is
280 * given as 0, then the base of the string is automatically detected with the
281 * conventional semantics - If it begins with 0x the number will be parsed as a
282 * hexadecimal (case insensitive), if it otherwise begins with 0, it will be
283 * parsed as an octal number. Otherwise it will be parsed as a decimal.
284 * @res: Where to write the result of the conversion on success.
285 *
286 * Returns 0 on success, -ERANGE on overflow and -EINVAL on parsing error.
287 * Used as a replacement for the obsolete simple_strtoull. Return code must
288 * be checked.
289 */
" not able to include libraries" --> Unclear if code is allowed access to INT_MAX
, INT_MIN
. “无法包含库”-> 不清楚是否允许代码访问INT_MAX
、 INT_MIN
。 There is no way to determine the minimum/maximum signed integer in a completely portable fashion without using the language provided macros like INT_MAX
, INT_MIN
.如果不使用语言提供的宏,如INT_MAX
、 INT_MIN
,就无法以完全可移植的方式确定最小/最大有符号整数。
Use INT_MAX
, INT_MIN
is available.使用INT_MAX
, INT_MIN
可用。 Else we could guess the char
width is 8. We could guess there are no padding bits.否则我们可以猜测char
宽度是 8。我们可以猜测没有填充位。 We could guess that integers are 2's complement.我们可以猜测整数是 2 的补码。 With these reasonable assumptions , minimum and maximum are defined below.有了这些合理的假设,最小值和最大值定义如下。
Note: Shifting into the sign bit is undefined behavior (UB), so don't do that.注意:移入符号位是未定义的行为(UB),所以不要这样做。
Let us add another restriction: make a solution that works for any signed integer from signed char
to intmax_t
.让我们添加另一个限制:制定一个适用于从signed char
到intmax_t
的任何有符号整数的解决方案。 This disallows code from using a wider type, as there may not be a wider type.这不允许代码使用更宽的类型,因为可能没有更宽的类型。
typedef int Austin_int;
#define Austin_INT_MAXMID ( ((Austin_int)1) << (sizeof(Austin_int)*8 - 2) )
#define Austin_INT_MAX (Austin_INT_MAXMID - 1 + Austin_INT_MAXMID)
#define Austin_INT_MIN (-Austin_INT_MAX - 1)
int Austin_isspace(int ch) {
const char *ws = " \t\n\r\f\v";
while (*ws) {
if (*ws == ch) return 1;
ws++;
}
return 0;
}
// *endptr points to where parsing stopped
// *errorptr indicates overflow
Austin_int Austin_strtoi(const char *s, char **endptr, int *errorptr) {
int error = 0;
while (Austin_isspace(*s)) {
s++;
}
char sign = *s;
if (*s == '-' || *s == '+') {
s++;
}
Austin_int sum = 0;
while (*s >= '0' && *s <= '9') {
int ch = *s - '0';
if (sum <= Austin_INT_MIN / 10 &&
(sum < Austin_INT_MIN / 10 || -ch < Austin_INT_MIN % 10)) {
sum = Austin_INT_MIN;
error = 1;
} else {
sum = sum * 10 - ch;
}
s++;
}
if (sign != '-') {
if (sum < -Austin_INT_MAX) {
sum = Austin_INT_MAX;
error = 1;
} else {
sum = -sum;
}
}
if (endptr) {
*endptr = (char *) s;
}
if (errorptr) {
*errorptr = error;
}
return sum;
}
The above depends on C99 or later in the Austin_INT_MIN Austin_INT_MIN % 10
part.以上取决于Austin_INT_MIN Austin_INT_MIN % 10
部分中的 C99 或更高版本。
This is the cleanest and safest way I could come up with这是我能想到的最干净、最安全的方法
int str_to_int(const char * str, size_t n, int * int_value) {
int i;
int cvalue;
int value_muliplier = 1;
int res_value = 0;
int neg = 1; // -1 for negative and 1 for whole.
size_t str_len; // String length.
int end_at = 0; // Where loop should end.
if (str == NULL || int_value == NULL || n <= 0)
return -1;
// Get string length
str_len = strnlen(str, n);
if (str_len <= 0)
return -1;
// Is negative.
if (str[0] == '-') {
neg = -1;
end_at = 1; // If negative 0 item in 'str' is skipped.
}
// Do the math.
for (i = str_len - 1; i >= end_at; i--) {
cvalue = char_to_int(str[i]);
// Character not a number.
if (cvalue == -1)
return -1;
// Do the same math that is down below.
res_value += cvalue * value_muliplier;
value_muliplier *= 10;
}
/*
* "436"
* res_value = (6 * 1) + (3 * 10) + (4 * 100)
*/
*int_value = (res_value * neg);
return 0;
}
int char_to_int(char c) {
int cvalue = (int)c;
// Not a number.
// 48 to 57 is 0 to 9 in ascii.
if (cvalue < 48 || cvalue > 57)
return -1;
return cvalue - 48; // 48 is the value of zero in ascii.
}
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