C++实现文件下载

基于curl实现文件下载

curl第三方库的安装

sudo apt-get install libcurl4-openssl-dev

实现文件下载思路

1. 使用curl发送请求获取需要下载的文件的大小
2. 再次使用curl下载文件

代码

#include <cstring>
#include <iostream>
#include <curl/curl.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<stdio.h>
#include<fcntl.h>
#include<sys/mman.h>
#include <unistd.h>
using namespace std;

struct fileInfo
{
    char *fileptr;
    int offset;
    /* data */
};


size_t writeFunc(void *ptr, size_t size, size_t memb, void *userdata)
{
    fileInfo *info = (fileInfo *)userdata;

    // printf("aaaaaa");
    memcpy(info->fileptr + info->offset, ptr, size * memb);
    info->offset += size * memb;
    return size * memb;
}

double getDownloadFileLength(const char* url){
    double length;
    CURL *curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_HEADER, 1);
    curl_easy_setopt(curl, CURLOPT_NOBODY, 1);

    CURLcode res = curl_easy_perform(curl);
    if (res == CURLE_OK)
    {
        curl_easy_getinfo(curl, CURLINFO_CONTENT_LENGTH_DOWNLOAD, &length);
    }else{
        length = -1;
    }
    curl_easy_cleanup(curl);
    return length;
}

int download(const char *url, const char *filename)
{
    long length = getDownloadFileLength(url);
    printf("file lenght:%ld\n", length);
    int fd = open(filename,O_RDWR | O_CREAT | S_IWUSR);
    if(fd==-1){
        return -1;
    }
    if(-1==lseek(fd, length-1, SEEK_SET)){
        perror("lseek");
        close(fd);
        return -1;
    }

    if(1!=write(fd,"",1)){
        perror("write");
        close(fd);
        return -1;
    }

    char* fileptr = (char*)mmap(NULL, length, PROT_READ | PROT_WRITE,MAP_SHARED, fd, 0);
    if(fileptr==MAP_FAILED){
        perror("mmap");
        close(fd);
        return -1;
    }

    fileInfo *info = (fileInfo*)malloc(sizeof(fileInfo));
    if(info==NULL){
        munmap(fileptr, length);
        close(fd);
        return -1;
    }
    info->fileptr = fileptr;
    info->offset = 0;

    CURL *curl = curl_easy_init();
    curl_easy_setopt(curl, CURLOPT_URL, url);
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeFunc);
    curl_easy_setopt(curl,CURLOPT_WRITEDATA,info);

    CURLcode res = curl_easy_perform(curl);
    if (res != CURLE_OK)
    {
        printf("res %d\n", res);
    }
    curl_easy_cleanup(curl);
    free(info);
    munmap(fileptr, length);
    close(fd);
    return 0;
}

int main()
{
    cout << "hello" << endl;
    download("https://releases.ubuntu.com/22.04/ubuntu-22.04.4-live-server-amd64.iso.zsync","ubuntu.zsync");
    return 0;
}