[C++知识库]C++生成随机字符串，UUID

Linux平台利用dev/urandom来生成随机数，再转成字符串，以下代码生成的字符串长度为两倍len.

dev/random和/dev/urandom是Linux系统中提供的随机伪设备，这两个设备的任务，是提供永不为空的随机字节数据流。很多解密程序与安全应用程序（如SSH Keys,SSL Keys等）需要它们提供的随机数据流。

#include"log_uuid.h"

#include <stdlib.h>
#include <unistd.h>
#include<fcntl.h>

std::string getUUID(unsigned int len) {
    std::string str;
    unsigned char* buffer = new unsigned char[len+1];
    char* uuid = new char [2*len+1];
    int fd = open("/dev/urandom", O_RDONLY);
    if (read(fd, buffer, len) == len) {
        
        for(int i=0;i<len;i++)
            sprintf(uuid+i*2, "%02X",buffer[i]);
        uuid[2*len] = '\0';
        str = uuid;
        
    } else {
        printf("Error: GetUnique %d\n", __LINE__);
    }
    delete [] buffer;
    delete [] uuid;
    return str;
}

另一方法，生成最大长度为maxlen的随机长度的随机字符串

std::string randstr(int max_len)
{
    int real_len = rand() % max_len;
    if(real_len == 0) return "";
    char* str = (char*)malloc(real_len);
    
    for (int i = 0; i < real_len; ++i)
    {
        switch ((rand() % 3))
        {
        case 1:
            str[i] = 'A' + rand() % 26;
            break;
        case 2:
            str[i] = 'a' + rand() % 26;
            break;
        default:
            str[i] = '0' + rand() % 10;
            break;
        }
    }
    str[real_len -1] = '\0';
    std::string string = str;
    free(str);
    return string;
}