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libosmocore/src/utils.c

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/*
* (C) 2011 by Harald Welte <laforge@gnumonks.org>
* (C) 2011 by Sylvain Munaut <tnt@246tNt.com>
* (C) 2014 by Nils O. Selåsdal <noselasd@fiane.dyndns.org>
*
* All Rights Reserved
*
* SPDX-License-Identifier: GPL-2.0+
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*
*/
#include <stdbool.h>
#include <string.h>
#include <stdint.h>
#include <errno.h>
#include <stdio.h>
#include <inttypes.h>
#include <osmocom/core/utils.h>
#include <osmocom/core/bit64gen.h>
/*! \addtogroup utils
* @{
* various utility routines
*
* \file utils.c */
static char namebuf[255];
/*! get human-readable string for given value
* \param[in] vs Array of value_string tuples
* \param[in] val Value to be converted
* \returns pointer to human-readable string
*
* If val is found in vs, the array's string entry is returned. Otherwise, an
* "unknown" string containing the actual value is composed in a static buffer
* that is reused across invocations.
*/
const char *get_value_string(const struct value_string *vs, uint32_t val)
{
const char *str = get_value_string_or_null(vs, val);
if (str)
return str;
snprintf(namebuf, sizeof(namebuf), "unknown 0x%"PRIx32, val);
namebuf[sizeof(namebuf) - 1] = '\0';
return namebuf;
}
/*! get human-readable string or NULL for given value
* \param[in] vs Array of value_string tuples
* \param[in] val Value to be converted
* \returns pointer to human-readable string or NULL if val is not found
*/
const char *get_value_string_or_null(const struct value_string *vs,
uint32_t val)
{
int i;
for (i = 0;; i++) {
if (vs[i].value == 0 && vs[i].str == NULL)
break;
if (vs[i].value == val)
return vs[i].str;
}
return NULL;
}
/*! get numeric value for given human-readable string
* \param[in] vs Array of value_string tuples
* \param[in] str human-readable string
* \returns numeric value (>0) or negative numer in case of error
*/
int get_string_value(const struct value_string *vs, const char *str)
{
int i;
for (i = 0;; i++) {
if (vs[i].value == 0 && vs[i].str == NULL)
break;
if (!strcasecmp(vs[i].str, str))
return vs[i].value;
}
return -EINVAL;
}
/*! Convert BCD-encoded digit into printable character
* \param[in] bcd A single BCD-encoded digit
* \returns single printable character
*/
char osmo_bcd2char(uint8_t bcd)
{
if (bcd < 0xa)
return '0' + bcd;
else
return 'A' + (bcd - 0xa);
}
/*! Convert number in ASCII to BCD value
* \param[in] c ASCII character
* \returns BCD encoded value of character
*/
uint8_t osmo_char2bcd(char c)
{
if (c >= '0' && c <= '9')
return c - 0x30;
else if (c >= 'A' && c <= 'F')
return 0xa + (c - 'A');
else if (c >= 'a' && c <= 'f')
return 0xa + (c - 'a');
else
return 0;
}
/*! Convert BCD to string.
* The given nibble offsets are interpreted in BCD order, i.e. nibble 0 is bcd[0] & 0xf, nibble 1 is bcd[0] >> 4, nibble
* 3 is bcd[1] & 0xf, etc..
* \param[out] dst Output string buffer, is always nul terminated when dst_size > 0.
* \param[in] dst_size sizeof() the output string buffer.
* \param[in] bcd Binary coded data buffer.
* \param[in] start_nibble Offset to start from, in nibbles, typically 1 to skip the first nibble.
* \param[in] end_nibble Offset to stop before, in nibbles, e.g. sizeof(bcd)*2 - (bcd[0] & GSM_MI_ODD? 0:1).
* \param[in] allow_hex If false, return error if there are digits other than 0-9. If true, return those as [A-F].
* \returns The strlen that would be written if the output buffer is large enough, excluding nul byte (like
* snprintf()), or -EINVAL if allow_hex is false and a digit > 9 is encountered. On -EINVAL, the conversion is
* still completed as if allow_hex were passed as true. Return -ENOMEM if dst is NULL or dst_size is zero.
* If end_nibble <= start_nibble, write an empty string to dst and return 0.
*/
int osmo_bcd2str(char *dst, size_t dst_size, const uint8_t *bcd, int start_nibble, int end_nibble, bool allow_hex)
{
char *dst_end = dst + dst_size - 1;
int nibble_i;
int rc = 0;
if (!dst || dst_size < 1)
return -ENOMEM;
for (nibble_i = start_nibble; nibble_i < end_nibble && dst < dst_end; nibble_i++, dst++) {
uint8_t nibble = bcd[nibble_i >> 1];
if ((nibble_i & 1))
nibble >>= 4;
nibble &= 0xf;
if (!allow_hex && nibble > 9)
rc = -EINVAL;
*dst = osmo_bcd2char(nibble);
}
*dst = '\0';
if (rc < 0)
return rc;
return OSMO_MAX(0, end_nibble - start_nibble);
}
/*! Parse a string containing hexadecimal digits
* \param[in] str string containing ASCII encoded hexadecimal digits
* \param[out] b output buffer
* \param[in] max_len maximum space in output buffer
* \returns number of parsed octets, or -1 on error
*/
int osmo_hexparse(const char *str, uint8_t *b, int max_len)
{
char c;
uint8_t v;
const char *strpos;
unsigned int nibblepos = 0;
memset(b, 0x00, max_len);
for (strpos = str; (c = *strpos); strpos++) {
/* skip whitespace */
if (c == ' ' || c == '\t' || c == '\n' || c == '\r')
continue;
/* If the buffer is too small, error out */
if (nibblepos >= (max_len << 1))
return -1;
if (c >= '0' && c <= '9')
v = c - '0';
else if (c >= 'a' && c <= 'f')
v = 10 + (c - 'a');
else if (c >= 'A' && c <= 'F')
v = 10 + (c - 'A');
else
return -1;
b[nibblepos >> 1] |= v << (nibblepos & 1 ? 0 : 4);
nibblepos ++;
}
/* In case of uneven amount of digits, the last byte is not complete
* and that's an error. */
if (nibblepos & 1)
return -1;
return nibblepos >> 1;
}
static char hexd_buff[4096];
static const char hex_chars[] = "0123456789abcdef";
static char *_osmo_hexdump(const unsigned char *buf, int len, char *delim)
{
int i;
char *cur = hexd_buff;
hexd_buff[0] = 0;
for (i = 0; i < len; i++) {
const char *delimp = delim;
int len_remain = sizeof(hexd_buff) - (cur - hexd_buff);
if (len_remain < 3)
break;
*cur++ = hex_chars[buf[i] >> 4];
*cur++ = hex_chars[buf[i] & 0xf];
while (len_remain > 1 && *delimp) {
*cur++ = *delimp++;
len_remain--;
}
*cur = 0;
}
hexd_buff[sizeof(hexd_buff)-1] = 0;
return hexd_buff;
}
/*! Convert a sequence of unpacked bits to ASCII string
* \param[in] bits A sequence of unpacked bits
* \param[in] len Length of bits
*/
char *osmo_ubit_dump(const uint8_t *bits, unsigned int len)
{
int i;
if (len > sizeof(hexd_buff)-1)
len = sizeof(hexd_buff)-1;
memset(hexd_buff, 0, sizeof(hexd_buff));
for (i = 0; i < len; i++) {
char outch;
switch (bits[i]) {
case 0:
outch = '0';
break;
case 0xff:
outch = '?';
break;
case 1:
outch = '1';
break;
default:
outch = 'E';
break;
}
hexd_buff[i] = outch;
}
hexd_buff[sizeof(hexd_buff)-1] = 0;
return hexd_buff;
}
/*! Convert binary sequence to hexadecimal ASCII string
* \param[in] buf pointer to sequence of bytes
* \param[in] len length of buf in number of bytes
* \returns pointer to zero-terminated string
*
* This function will print a sequence of bytes as hexadecimal numbers,
* adding one space character between each byte (e.g. "1a ef d9")
*
* The maximum size of the output buffer is 4096 bytes, i.e. the maximum
* number of input bytes that can be printed in one call is 1365!
*/
char *osmo_hexdump(const unsigned char *buf, int len)
{
return _osmo_hexdump(buf, len, " ");
}
/*! Convert binary sequence to hexadecimal ASCII string
* \param[in] buf pointer to sequence of bytes
* \param[in] len length of buf in number of bytes
* \returns pointer to zero-terminated string
*
* This function will print a sequence of bytes as hexadecimal numbers,
* without any space character between each byte (e.g. "1aefd9")
*
* The maximum size of the output buffer is 4096 bytes, i.e. the maximum
* number of input bytes that can be printed in one call is 2048!
*/
char *osmo_hexdump_nospc(const unsigned char *buf, int len)
{
return _osmo_hexdump(buf, len, "");
}
/* Compat with previous typo to preserve abi */
char *osmo_osmo_hexdump_nospc(const unsigned char *buf, int len)
#if defined(__MACH__) && defined(__APPLE__)
;
#else
__attribute__((weak, alias("osmo_hexdump_nospc")));
#endif
#include "../config.h"
#ifdef HAVE_CTYPE_H
#include <ctype.h>
/*! Convert an entire string to lower case
* \param[out] out output string, caller-allocated
* \param[in] in input string
*/
void osmo_str2lower(char *out, const char *in)
{
unsigned int i;
for (i = 0; i < strlen(in); i++)
out[i] = tolower((const unsigned char)in[i]);
out[strlen(in)] = '\0';
}
/*! Convert an entire string to upper case
* \param[out] out output string, caller-allocated
* \param[in] in input string
*/
void osmo_str2upper(char *out, const char *in)
{
unsigned int i;
for (i = 0; i < strlen(in); i++)
out[i] = toupper((const unsigned char)in[i]);
out[strlen(in)] = '\0';
}
#endif /* HAVE_CTYPE_H */
/*! Wishful thinking to generate a constant time compare
* \param[in] exp Expected data
* \param[in] rel Comparison value
* \param[in] count Number of bytes to compare
* \returns 1 in case \a exp equals \a rel; zero otherwise
*
* Compare count bytes of exp to rel. Return 0 if they are identical, 1
* otherwise. Do not return a mismatch on the first mismatching byte,
* but always compare all bytes, regardless. The idea is that the amount of
* matching bytes cannot be inferred from the time the comparison took. */
int osmo_constant_time_cmp(const uint8_t *exp, const uint8_t *rel, const int count)
{
int x = 0, i;
for (i = 0; i < count; ++i)
x |= exp[i] ^ rel[i];
/* if x is zero, all data was identical */
return x? 1 : 0;
}
/*! Generic retrieval of 1..8 bytes as big-endian uint64_t
* \param[in] data Input data as byte-array
* \param[in] data_len Length of \a data in octets
* \returns uint64_t of \a data interpreted as big-endian
*
* This is like osmo_load64be_ext, except that if data_len is less than
* sizeof(uint64_t), the data is interpreted as the least significant bytes
* (osmo_load64be_ext loads them as the most significant bytes into the
* returned uint64_t). In this way, any integer size up to 64 bits can be
* decoded conveniently by using sizeof(), without the need to call specific
* numbered functions (osmo_load16, 32, ...). */
uint64_t osmo_decode_big_endian(const uint8_t *data, size_t data_len)
{
uint64_t value = 0;
while (data_len > 0) {
value = (value << 8) + *data;
data += 1;
data_len -= 1;
}
return value;
}
/*! Generic big-endian encoding of big endian number up to 64bit
* \param[in] value unsigned integer value to be stored
* \param[in] data_len number of octets
* \returns static buffer containing big-endian stored value
*
* This is like osmo_store64be_ext, except that this returns a static buffer of
* the result (for convenience, but not threadsafe). If data_len is less than
* sizeof(uint64_t), only the least significant bytes of value are encoded. */
uint8_t *osmo_encode_big_endian(uint64_t value, size_t data_len)
{
static uint8_t buf[sizeof(uint64_t)];
OSMO_ASSERT(data_len <= ARRAY_SIZE(buf));
osmo_store64be_ext(value, buf, data_len);
return buf;
}
/*! Copy a C-string into a sized buffer
* \param[in] src source string
* \param[out] dst destination string
* \param[in] siz size of the \a dst buffer
* \returns length of \a src
*
* Copy at most \a siz bytes from \a src to \a dst, ensuring that the result is
* NUL terminated. The NUL character is included in \a siz, i.e. passing the
* actual sizeof(*dst) is correct.
*/
size_t osmo_strlcpy(char *dst, const char *src, size_t siz)
{
size_t ret = src ? strlen(src) : 0;
if (siz) {
size_t len = (ret >= siz) ? siz - 1 : ret;
if (src)
memcpy(dst, src, len);
dst[len] = '\0';
}
return ret;
}
/*! Validate that a given string is a hex string within given size limits.
* Note that each hex digit amounts to a nibble, so if checking for a hex
* string to result in N bytes, pass amount of digits as 2*N.
* \param str A nul-terminated string to validate, or NULL.
* \param min_digits least permitted amount of digits.
* \param max_digits most permitted amount of digits.
* \param require_even if true, require an even amount of digits.
* \returns true when the hex_str contains only hexadecimal digits (no
* whitespace) and matches the requested length; also true
* when min_digits <= 0 and str is NULL.
*/
bool osmo_is_hexstr(const char *str, int min_digits, int max_digits,
bool require_even)
{
int len;
/* Use unsigned char * to avoid a compiler warning of
* "error: array subscript has type 'char' [-Werror=char-subscripts]" */
const unsigned char *pos = (const unsigned char*)str;
if (!pos)
return min_digits < 1;
for (len = 0; *pos && len < max_digits; len++, pos++)
if (!isxdigit(*pos))
return false;
if (len < min_digits)
return false;
/* With not too many digits, we should have reached *str == nul */
if (*pos)
return false;
if (require_even && (len & 1))
return false;
return true;
}
/*! Determine if a given identifier is valid, i.e. doesn't contain illegal chars
* \param[in] str String to validate
* \param[in] sep_chars Permitted separation characters between identifiers.
* \returns true in case \a str contains only valid identifiers and sep_chars, false otherwise
*/
bool osmo_separated_identifiers_valid(const char *str, const char *sep_chars)
{
/* characters that are illegal in names */
static const char illegal_chars[] = "., {}[]()<>|~\\^`'\"?=;/+*&%$#!";
unsigned int i;
size_t len;
/* an empty string is not a valid identifier */
if (!str || (len = strlen(str)) == 0)
return false;
for (i = 0; i < len; i++) {
if (sep_chars && strchr(sep_chars, str[i]))
continue;
/* check for 7-bit ASCII */
if (str[i] & 0x80)
return false;
if (!isprint((int)str[i]))
return false;
/* check for some explicit reserved control characters */
if (strchr(illegal_chars, str[i]))
return false;
}
return true;
}
/*! Determine if a given identifier is valid, i.e. doesn't contain illegal chars
* \param[in] str String to validate
* \returns true in case \a str contains valid identifier, false otherwise
*/
bool osmo_identifier_valid(const char *str)
{
return osmo_separated_identifiers_valid(str, NULL);
}
/*! Return the string with all non-printable characters escaped.
* \param[in] str A string that may contain any characters.
* \param[in] len Pass -1 to print until nul char, or >= 0 to force a length.
* \param[inout] buf string buffer to write escaped characters to.
* \param[in] bufsize size of \a buf.
* \returns buf containing an escaped representation, possibly truncated, or str itself.
*/
const char *osmo_escape_str_buf(const char *str, int in_len, char *buf, size_t bufsize)
{
int in_pos = 0;
int next_unprintable = 0;
int out_pos = 0;
char *out = buf;
/* -1 to leave space for a final \0 */
int out_len = bufsize-1;
if (!str)
return "(null)";
if (in_len < 0)
in_len = strlen(str);
while (in_pos < in_len) {
for (next_unprintable = in_pos;
next_unprintable < in_len && isprint((int)str[next_unprintable])
&& str[next_unprintable] != '"'
&& str[next_unprintable] != '\\';
next_unprintable++);
if (next_unprintable == in_len
&& in_pos == 0)
return str;
while (in_pos < next_unprintable && out_pos < out_len)
out[out_pos++] = str[in_pos++];
if (out_pos == out_len || in_pos == in_len)
goto done;
switch (str[next_unprintable]) {
#define BACKSLASH_CASE(c, repr) \
case c: \
if (out_pos > out_len-2) \
goto done; \
out[out_pos++] = '\\'; \
out[out_pos++] = repr; \
break
BACKSLASH_CASE('\n', 'n');
BACKSLASH_CASE('\r', 'r');
BACKSLASH_CASE('\t', 't');
BACKSLASH_CASE('\0', '0');
BACKSLASH_CASE('\a', 'a');
BACKSLASH_CASE('\b', 'b');
BACKSLASH_CASE('\v', 'v');
BACKSLASH_CASE('\f', 'f');
BACKSLASH_CASE('\\', '\\');
BACKSLASH_CASE('"', '"');
#undef BACKSLASH_CASE
default:
out_pos += snprintf(&out[out_pos], out_len - out_pos, "\\%u", (unsigned char)str[in_pos]);
if (out_pos > out_len) {
out_pos = out_len;
goto done;
}
break;
}
in_pos ++;
}
done:
out[out_pos] = '\0';
return out;
}
/*! Return the string with all non-printable characters escaped.
* Call osmo_escape_str_buf() with a static buffer.
* \param[in] str A string that may contain any characters.
* \param[in] len Pass -1 to print until nul char, or >= 0 to force a length.
* \returns buf containing an escaped representation, possibly truncated, or str itself.
*/
const char *osmo_escape_str(const char *str, int in_len)
{
return osmo_escape_str_buf(str, in_len, namebuf, sizeof(namebuf));
}
/*! Like osmo_escape_str(), but returns double-quotes around a string, or "NULL" for a NULL string.
* This allows passing any char* value and get its C representation as string.
* \param[in] str A string that may contain any characters.
* \param[in] in_len Pass -1 to print until nul char, or >= 0 to force a length.
* \returns buf containing a quoted and escaped representation, possibly truncated.
*/
const char *osmo_quote_str_buf(const char *str, int in_len, char *buf, size_t bufsize)
{
const char *res;
int l;
if (!str)
return "NULL";
if (bufsize < 3)
return "<buf-too-small>";
buf[0] = '"';
res = osmo_escape_str_buf(str, in_len, buf + 1, bufsize - 2);
/* if osmo_escape_str_buf() returned the str itself, we need to copy it to buf to be able to
* quote it. */
if (res == str) {
/* max_len = bufsize - two quotes - nul term */
int max_len = bufsize - 2 - 1;
if (in_len >= 0)
max_len = OSMO_MIN(in_len, max_len);
/* It is not allowed to pass unterminated strings into osmo_strlcpy() :/ */
strncpy(buf + 1, str, max_len);
buf[1 + max_len] = '\0';
}
l = strlen(buf);
buf[l] = '"';
buf[l+1] = '\0'; /* both osmo_escape_str_buf() and max_len above ensure room for '\0' */
return buf;
}
/*! Like osmo_quote_str_buf() but returns the result in a static buffer.
* The static buffer is shared with get_value_string() and osmo_escape_str().
* \param[in] str A string that may contain any characters.
* \param[in] in_len Pass -1 to print until nul char, or >= 0 to force a length.
* \returns static buffer containing a quoted and escaped representation, possibly truncated.
*/
const char *osmo_quote_str(const char *str, int in_len)
{
return osmo_quote_str_buf(str, in_len, namebuf, sizeof(namebuf));
}
/*! perform an integer square root operation on unsigned 32bit integer.
* This implementation is taken from "Hacker's Delight" Figure 11-1 "Integer square root, Newton's
* method", which can also be found at http://www.hackersdelight.org/hdcodetxt/isqrt.c.txt */
uint32_t osmo_isqrt32(uint32_t x)
{
uint32_t x1;
int s, g0, g1;
if (x <= 1)
return x;
s = 1;
x1 = x - 1;
if (x1 > 0xffff) {
s = s + 8;
x1 = x1 >> 16;
}
if (x1 > 0xff) {
s = s + 4;
x1 = x1 >> 8;
}
if (x1 > 0xf) {
s = s + 2;
x1 = x1 >> 4;
}
if (x1 > 0x3) {
s = s + 1;
}
g0 = 1 << s; /* g0 = 2**s */
g1 = (g0 + (x >> s)) >> 1; /* g1 = (g0 + x/g0)/2 */
/* converges after four to five divisions for arguments up to 16,785,407 */
while (g1 < g0) {
g0 = g1;
g1 = (g0 + (x/g0)) >> 1;
}
return g0;
}
/*! Convert a string to lowercase, while checking buffer size boundaries.
* The result written to \a dest is guaranteed to be nul terminated if \a dest_len > 0.
* If dest == src, the string is converted in-place, if necessary truncated at dest_len - 1 characters
* length as well as nul terminated.
* Note: similar osmo_str2lower(), but safe to use for src strings of arbitrary length.
* \param[out] dest Target buffer to write lowercase string.
* \param[in] dest_len Maximum buffer size of dest (e.g. sizeof(dest)).
* \param[in] src String to convert to lowercase.
* \returns Length of \a src, like osmo_strlcpy(), but if \a dest == \a src at most \a dest_len - 1.
*/
size_t osmo_str_tolower_buf(char *dest, size_t dest_len, const char *src)
{
size_t rc;
if (dest == src) {
if (dest_len < 1)
return 0;
dest[dest_len - 1] = '\0';
rc = strlen(dest);
} else {
if (dest_len < 1)
return strlen(src);
rc = osmo_strlcpy(dest, src, dest_len);
}
for (; *dest; dest++)
*dest = tolower(*dest);
return rc;
}
/*! Convert a string to lowercase, using a static buffer.
* The resulting string may be truncated if the internally used static buffer is shorter than src.
* The internal buffer is at least 128 bytes long, i.e. guaranteed to hold at least 127 characters and a
* terminating nul.
* See also osmo_str_tolower_buf().
* \param[in] src String to convert to lowercase.
* \returns Resulting lowercase string in a static buffer, always nul terminated.
*/
const char *osmo_str_tolower(const char *src)
{
static char buf[128];
osmo_str_tolower_buf(buf, sizeof(buf), src);
return buf;
}
/*! Convert a string to uppercase, while checking buffer size boundaries.
* The result written to \a dest is guaranteed to be nul terminated if \a dest_len > 0.
* If dest == src, the string is converted in-place, if necessary truncated at dest_len - 1 characters
* length as well as nul terminated.
* Note: similar osmo_str2upper(), but safe to use for src strings of arbitrary length.
* \param[out] dest Target buffer to write uppercase string.
* \param[in] dest_len Maximum buffer size of dest (e.g. sizeof(dest)).
* \param[in] src String to convert to uppercase.
* \returns Length of \a src, like osmo_strlcpy(), but if \a dest == \a src at most \a dest_len - 1.
*/
size_t osmo_str_toupper_buf(char *dest, size_t dest_len, const char *src)
{
size_t rc;
if (dest == src) {
if (dest_len < 1)
return 0;
dest[dest_len - 1] = '\0';
rc = strlen(dest);
} else {
if (dest_len < 1)
return strlen(src);
rc = osmo_strlcpy(dest, src, dest_len);
}
for (; *dest; dest++)
*dest = toupper(*dest);
return rc;
}
/*! Convert a string to uppercase, using a static buffer.
* The resulting string may be truncated if the internally used static buffer is shorter than src.
* The internal buffer is at least 128 bytes long, i.e. guaranteed to hold at least 127 characters and a
* terminating nul.
* See also osmo_str_toupper_buf().
* \param[in] src String to convert to uppercase.
* \returns Resulting uppercase string in a static buffer, always nul terminated.
*/
const char *osmo_str_toupper(const char *src)
{
static char buf[128];
osmo_str_toupper_buf(buf, sizeof(buf), src);
return buf;
}
/*! Calculate the Luhn checksum (as used for IMEIs).
* \param[in] in Input digits in ASCII string representation.
* \param[in] in_len Count of digits to use for the input (14 for IMEI).
* \returns checksum char (e.g. '3'); negative on error
*/
const char osmo_luhn(const char* in, int in_len)
{
int i, sum = 0;
/* All input must be numbers */
for (i = 0; i < in_len; i++) {
if (!isdigit(in[i]))
return -EINVAL;
}
/* Double every second digit and add it to sum */
for (i = in_len - 1; i >= 0; i -= 2) {
int dbl = (in[i] - '0') * 2;
if (dbl > 9)
dbl -= 9;
sum += dbl;
}
/* Add other digits to sum */
for (i = in_len - 2; i >= 0; i -= 2)
sum += in[i] - '0';
/* Final checksum */
return (sum * 9) % 10 + '0';
}
/*! @} */
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