如何在C中正确地将十六进制字符串转换为Byte？

Question

我需要将包含十六进制值的字符串转换为字节数组。虽然already here已经回答了这个问题，但我得到了以下错误：

warning: ISO C90 does not support the ‘hh’ gnu_scanf length modifier [-Wformat]

因为我不喜欢警告，而省略hh只会产生另一个警告

warning: format ‘%x’ expects argument of type ‘unsigned int *’, but argument 3 has type ‘unsigned char *’ [-Wformat]

我的问题是:如何做好这件事？为了完成，我再次在这里发布示例代码：

#include <stdio.h>

int main(int argc, char **argv)
{
    const char hexstring[] = "deadbeef10203040b00b1e50", *pos = hexstring;
    unsigned char val[12];
    size_t count = 0;

     /* WARNING: no sanitization or error-checking whatsoever */
    for(count = 0; count < sizeof(val)/sizeof(val[0]); count++) {
        sscanf(pos, "%2hhx", &val[count]);
        pos += 2 * sizeof(char);
    }

    printf("0x");
    for(count = 0; count < sizeof(val)/sizeof(val[0]); count++)
        printf("%02x", val[count]);
    printf("\n");

    return(0);
}

Answer 1

您可以使用strtol()代替。

只需替换这一行：

sscanf(pos, "%2hhx", &val[count]);

通过以下方式：

char buf[10];
sprintf(buf, "0x%c%c", pos[0], pos[1]);
val[count] = strtol(buf, NULL, 0);

UPDATE：您可以避免使用此片段的sprintf()：

char buf[5] = {"0", "x", pos[0], pos[1], 0};
val[count] = strtol(buf, NULL, 0);

Answer 2

您可以将编译器切换到C99模式( hh长度修饰符在C99中是标准化的)，也可以使用unsigned int临时变量：

unsigned int byteval;
if (sscanf(pos, "%2x", &byteval) != 1)
{
    /* format error */
}
val[count] = byteval;

Answer 3

为什么不使用sscanf，strol等来做呢?下面是HexToBin，作为一个自由蜜蜂，BinToHex。(注意，最初有通过错误日志系统返回的枚举错误代码，而不是简单的-1返回。)

unsigned char HexChar (char c)
{
    if ('0' <= c && c <= '9') return (unsigned char)(c - '0');
    if ('A' <= c && c <= 'F') return (unsigned char)(c - 'A' + 10);
    if ('a' <= c && c <= 'f') return (unsigned char)(c - 'a' + 10);
    return 0xFF;
}

int HexToBin (const char* s, unsigned char * buff, int length)
{
    int result;
    if (!s || !buff || length <= 0) return -1;

    for (result = 0; *s; ++result)
    {
        unsigned char msn = HexChar(*s++);
        if (msn == 0xFF) return -1;
        unsigned char lsn = HexChar(*s++);
        if (lsn == 0xFF) return -1;
        unsigned char bin = (msn << 4) + lsn;

        if (length-- <= 0) return -1;
        *buff++ = bin;
    }
    return result;
}

void BinToHex (const unsigned char * buff, int length, char * output, int outLength)
{
    char binHex[] = "0123456789ABCDEF";

    if (!output || outLength < 4) return;
    *output = '\0';

    if (!buff || length <= 0 || outLength <= 2 * length)
    {
        memcpy(output, "ERR", 4);
        return;
    }

    for (; length > 0; --length, outLength -= 2)
    {
        unsigned char byte = *buff++;

        *output++ = binHex[(byte >> 4) & 0x0F];
        *output++ = binHex[byte & 0x0F];
    }
    if (outLength-- <= 0) return;
    *output++ = '\0';
}