内存映射文件中对象的排序

Question

I have a structure as below:

struct XX
{
   int x;
   char szT[200];
   int y;
} ;

I have a file that stores several of these XX objects, written through fwrite call. Now when I read the file as a memory mmaped file using mmap, I use as:

// sz = size of the file in bytes
// fd = file descriptor of the file opened through fopen in O_RDWR mode
char *p = (char *) mmap(0,sz,PROT_READ | PROT_WRITE,MAP_SHARED,fd,0);
unsigned int N = (sz / sizeof(XX)); // number of objects

Thus, if I have N objects of type XX , I may access the first object as:

XX *px = (XX*) p;

And, i-th object I can access as px + i where i <= N . May I use std::sort to sort the contents of a memory mapped file being accessed as px being the pointer of first record, and px+i pointing to i-th record. I don't like to store px, px+1, px+2 etc in a vector of XX pointers as vector<XX*> , as I can access the records directly from the memory-mapped file. Please suggest.

Answer 1

Read documentation of std::sort

The following compiles cleanly. It sorts a raw array with a peculiar lambda (comparing squares of numbers)

 #include <algorithm>
 #include <functional>

 void sort_array(int *p, size_t n) {
   std::sort(p, p+n, [=](int x, int y) { return x*x < y*y; });
 }    

You should be able to adapt that to your situation:

 void sort_xx (XX*p, size_t n) {
   std::sort(p, p+n, [=](const XX& x, const XX& y) 
    { return strcmp(x.szT, y.szT)<0; });
 }

Read more about lambda expressions in C++ . [=] is a capture-list by value (actually by copy) notation (for all closed variables, theoretically the only one here being strcmp which the compiler is likely to optimize - so won't close it naively; probably the lambda-abstraction and its application would be inlined by an optimizing compiler).