Not by the Heap Alone: Dynamic Memory Allocation Using Files in Linux

2025-02-11

Dynamic memory allocation is a fundamental concept in programming, often associated with the heap. However, in Linux, you can leverage files for dynamic memory allocation, offering a unique approach to memory management. Here’s how you can achieve this:

Steps to Allocate Memory Using a File in Linux

1. Open a File Using the `open` Syscall

Use the `open` syscall to create or open a file. This file will act as the memory storage.

int fd = open("memory_file", O_RDWR | O_CREAT, 0666);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}

2. Resize the File with `lseek`

Resize the file to fit the object you want to store.

off_t size = 1024; // Example size
if (lseek(fd, size - 1, SEEK_SET) == -1) {
perror("lseek");
close(fd);
exit(EXIT_FAILURE);
}
write(fd, "", 1); // Ensure the file is the correct size

1. Map the File into Process Memory Using `mmap`
   Use `mmap` to map the file into the process’s address space.

   void* mapped_memory = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
   if (mapped_memory == MAP_FAILED) {
   perror("mmap");
   close(fd);
   exit(EXIT_FAILURE);
   }
   

4. Use Placement `new` to Construct the Object

Construct the object in the mapped memory using placement new.

MyClass* obj = new(mapped_memory) MyClass();

5. Handle Cleanup Properly

Ensure proper cleanup to avoid resource leaks. Use a custom deleter with `shared_ptr` to unmap the memory and close the file.

auto deleter = [fd, size](MyClass* obj) {
obj->~MyClass(); // Call the destructor
munmap(obj, size); // Unmap the memory
close(fd); // Close the file
};
std::shared_ptr<MyClass> shared_obj(obj, deleter);

Example Code

Here’s a complete example demonstrating the process:

#include <iostream>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <memory>

class MyClass {
public:
MyClass() { std::cout << "MyClass constructed!\n"; }
~MyClass() { std::cout << "MyClass destroyed!\n"; }
};

int main() {
const off_t size = 1024;
int fd = open("memory_file", O_RDWR | O_CREAT, 0666);
if (fd == -1) {
perror("open");
return EXIT_FAILURE;
}

if (lseek(fd, size - 1, SEEK_SET) == -1) {
perror("lseek");
close(fd);
return EXIT_FAILURE;
}
write(fd, "", 1);

void* mapped_memory = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (mapped_memory == MAP_FAILED) {
perror("mmap");
close(fd);
return EXIT_FAILURE;
}

MyClass* obj = new(mapped_memory) MyClass();

auto deleter = [fd, size](MyClass* obj) {
obj->~MyClass();
munmap(obj, size);
close(fd);
};
std::shared_ptr<MyClass> shared_obj(obj, deleter);

return 0;
}

What Undercode Say

Dynamic memory allocation using files in Linux is a powerful technique that extends beyond traditional heap-based approaches. By leveraging system calls like open, lseek, and mmap, developers can map files directly into process memory, enabling efficient memory management and persistence. This method is particularly useful for applications requiring large data structures or shared memory across processes.

To further explore Linux memory management, consider these commands and tools:
– pmap: View memory mappings of a process.

pmap <pid>

– free: Display system memory usage.

free -h

– vmstat: Report virtual memory statistics.

vmstat 1

– strace: Trace system calls and signals.

strace ./your_program

For more advanced memory management techniques, refer to the Linux `mmap` documentation:
https://man7.org/linux/man-pages/man2/mmap.2.html

By mastering these tools and techniques, you can optimize memory usage and build robust applications on Linux.

References:

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