我正试图使用mmap
将图形存储在文件中,这样我读写速度更快,但我无法读取使用malloc创建的字段结构字段(也无法使它们成为数组)
问题是我无法从文件中读取存档的map[i].nodes->vertexKey
(我认为是因为它是使用malloc
创建的)
我的代码是:
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <string.h>
#define COUNT 10
#define FILESIZE ( COUNT * sizeof(struct vertex))
struct node{
int vertexKey ;
struct node *nextNode;
};
struct vertex {
int vertexKey;
struct node *nodes;
};
int readMmap(){
const char *filepath = "/tmp/mmapped.bin";
int fd = open(filepath, O_RDWR , (mode_t)0600);
if (fd == -1)
{
perror("Error opening file for writing");
exit(EXIT_FAILURE);
}
struct stat fileInfo = {0};
if (fstat(fd, &fileInfo) == -1)
{
perror("Error getting the file size");
exit(EXIT_FAILURE);
}
if (fileInfo.st_size == 0)
{
fprintf(stderr, "Error: File is empty, nothing to don");
exit(EXIT_FAILURE);
}
printf("File size is %jin", (intmax_t)fileInfo.st_size);
struct vertex *map = mmap(0, FILESIZE , PROT_READ, MAP_SHARED, fd, 0);
if (map == MAP_FAILED)
{
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
for (off_t i = 0; i < COUNT; i++)
{
printf("%d |", map[i].vertexKey );
// i can't read map[i].nodes->vertexKey
printf("%d n", map[i].nodes->vertexKey );
printf("n" );
}
// Don't forget to free the mmapped memory
if (munmap(map, fileInfo.st_size) == -1)
{
close(fd);
perror("Error un-mmapping the file");
exit(EXIT_FAILURE);
}
// Un-mmaping doesn't close the file, so we still need to do that.
close(fd);
return 0;
}
int writeMmap(){
const char *filepath = "/tmp/mmapped.bin";
int fd = open(filepath, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
if (fd == -1){
perror("Error opening file for writing");
exit(EXIT_FAILURE);
}
if (lseek(fd, FILESIZE-1, SEEK_SET) == -1){
close(fd);
perror("Error calling lseek() to 'stretch' the file");
exit(EXIT_FAILURE);
}
if (write(fd, "", 1) == -1){
close(fd);
perror("Error writing last byte of the file");
exit(EXIT_FAILURE);
}
struct vertex *map = mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (map == MAP_FAILED) {
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
for (size_t i = 0; i < COUNT; i++){
struct vertex ss ;
ss.vertexKey=i;
struct node *n1 = (struct node*)malloc(sizeof(struct node));
n1->nextNode =NULL ;
n1->vertexKey=i*10 ;
ss.nodes = n1 ;
map[i] = ss;
}
// Write it now to disk
if (msync(map, 100, MS_SYNC) == -1)
{
perror("Could not sync the file to disk");
}
// Don't forget to free the mmapped memory
if (munmap(map, 100) == -1)
{
close(fd);
perror("Error un-mmapping the file");
exit(EXIT_FAILURE);
}
// Un-mmaping doesn't close the file, so we still need to do that.
close(fd);
return 0;
}
这到底需要多快?使用内存映像作为持久格式是一种有问题的做法——如果可能的话,你需要它在更大的计划中取得相当大的胜利,它才有价值。
如果您想要数据的持久表示,那么该表示需要是自包含的。指针本身是不受支持的,但在它们的位置上,您可以将索引用于对象的表(实际上是数组)。如果索引是隐式的,那会更好,但这对您来说可能还不够。我很抱歉含糊其辞,但在提出任何细节之前,我需要更好地了解您的数据特征。