Base64 编解码C语言实现

具体代码如下：

include <stdio.h>
const char base[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
char* base64_encode(const char* data, int data_len);
char *base64_decode(const char* data, int data_len);
static char find_pos(char ch);
int main(int argc, char* argv[])
{
    char *t = "那个abcd你好吗，哈哈，ANMOL";
    int i = 0;
    int j = strlen(t);
    char *enc = base64_encode(t, j);
    int len = strlen(enc);
    char *dec = base64_decode(enc, len);
    printf("noriginal: %sn", t);
    printf("nencoded : %sn", enc);
    printf("ndecoded : %sn", dec);
    free(enc);
    free(dec);
    return 0;
}
/* */
char *base64_encode(const char* data, int data_len)
{
    //int data_len = strlen(data);
    int prepare = 0;
    int ret_len;
    int temp = 0;
    char *ret = NULL;
    char *f = NULL;
    int tmp = 0;
    char changed[4];
    int i = 0;
    ret_len = data_len / 3;
    temp = data_len % 3;
    if (temp > 0)
    {
        ret_len += 1;
    }
    ret_len = ret_len*4 + 1;
    ret = (char *)malloc(ret_len);

    if ( ret == NULL)
    {
        printf("No enough memory.n");
        exit(0);
    }
    memset(ret, 0, ret_len);
    f = ret;
    while (tmp < data_len)
    {
        temp = 0;
        prepare = 0;
        memset(changed, '0', 4);
        while (temp < 3)
        {
            //printf("tmp = %dn", tmp);
            if (tmp >= data_len)
            {
                break;
            }
            prepare = ((prepare << 8) | (data[tmp] & 0xFF));
            tmp++;
            temp++;
        }
        prepare = (prepare<<((3-temp)*8));
        //printf("before for : temp = %d, prepare = %dn", temp, prepare);
        for (i = 0; i < 4 ;i++ )
        {
            if (temp < i)
            {
                changed[i] = 0x40;
            }
            else
            {
                changed[i] = (prepare>>((3-i)*6)) & 0x3F;
            }
            *f = base[changed[i]];
            //printf("%.2X", changed[i]);
            f++;
        }
    }
    *f = '0';

    return ret;

}
/* */
static char find_pos(char ch)
{
    char *ptr = (char*)strrchr(base, ch);//the last position (the only) in base[]
    return (ptr - base);
}
/* */
char *base64_decode(const char *data, int data_len)
{
    int ret_len = (data_len / 4) * 3;
    int equal_count = 0;
    char *ret = NULL;
    char *f = NULL;
    int tmp = 0;
    int temp = 0;
    char need[3];
    int prepare = 0;
    int i = 0;
    if (*(data + data_len - 1) == '=')
    {
        equal_count += 1;
    }
    if (*(data + data_len - 2) == '=')
    {
        equal_count += 1;
    }
    if (*(data + data_len - 3) == '=')
    {//seems impossible
        equal_count += 1;
    }
    switch (equal_count)
    {
    case 0:
        ret_len += 4;//3 + 1 [1 for NULL]
        break;
    case 1:
        ret_len += 4;//Ceil((6*3)/8)+1
        break;
    case 2:
        ret_len += 3;//Ceil((6*2)/8)+1
        break;
    case 3:
        ret_len += 2;//Ceil((6*1)/8)+1
        break;
    }
    ret = (char *)malloc(ret_len);
    if (ret == NULL)
    {
        printf("No enough memory.n");
        exit(0);
    }
    memset(ret, 0, ret_len);
    f = ret;
    while (tmp < (data_len - equal_count))
    {
        temp = 0;
        prepare = 0;
        memset(need, 0, 4);
        while (temp < 4)
        {
            if (tmp >= (data_len - equal_count))
            {
                break;
            }
            prepare = (prepare << 6) | (find_pos(data[tmp]));
            temp++;
            tmp++;
        }
        prepare = prepare << ((4-temp) * 6);
        for (i=0; i<3 ;i++ )
        {
            if (i == temp)
            {
                break;
            }
            *f = (char)((prepare>>((2-i)*8)) & 0xFF);
            f++;
        }
    }
    *f = '0';
    return ret;
}

这里添加了两个方法find_pos和base64_decode。前者是寻找给定字符在base数组中的位置的，使用了strrchr函数，寻找字符在字符串中最后一次的位置，由于总会存在并且仅存在一次，所以函数结果直接使用。base64_decode函数里边跟encode函数差不多，我没有作注释。

对于代码中给定的串，运行结果如下：

original: 那个abcd你好吗，哈哈，ANMOL
encoded : xMe49mFiY2TE47rDwvCjrLn+uf6jrEFOTU9M
decoded : 那个abcd你好吗，哈哈，ANMOL