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

2025-02-15

Dynamic memory allocation is often associated with the heap, but in Linux, you can leverage files for this purpose. Here’s how you can achieve it:

1. Open a file using the `open` syscall:

int fd = open("memory_file", O_RDWR | O_CREAT, 0644);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}

1. Resize the file to fit the object with lseek:

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

2. Map the file into process memory using mmap:

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

3. Use placement new to construct the object in that memory:

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

5. Handle cleanup properly to avoid resource leaks:

obj->~MyClass(); // Explicitly call destructor
if (munmap(addr, size) == -1) {
perror("munmap");
}
close(fd);

For a practical example, explore the Compiler Explorer: https://lnkd.in/evEGZDp2.

What Undercode Say

Dynamic memory allocation using files in Linux is a powerful technique that can be particularly useful in scenarios where you need persistent memory or shared memory across processes. By leveraging the open, lseek, mmap, and `munmap` syscalls, you can manage memory in a way that is both flexible and efficient. This method is especially beneficial in systems programming where performance and resource management are critical.

Here are some additional Linux commands and tips to enhance your understanding:

• Check memory usage: Use `free -m` to display memory usage in megabytes.
• Monitor system calls: Use `strace -c ` to trace system calls made by a process.
• List open files: Use `lsof` to list all open files and the processes that opened them.
• Shared memory: Explore `shm_open` and `shm_unlink` for POSIX shared memory objects.
• File permissions: Use `chmod` to modify file permissions and `chown` to change file ownership.

For more advanced memory management techniques, consider diving into the Linux kernel documentation or exploring resources like the Linux Programmer’s Manual (man mmap). Additionally, tools like `valgrind` can help detect memory leaks and improve your code’s robustness.

By mastering these techniques, you can optimize your applications for better performance and resource utilization, making you a more effective systems programmer.

References:

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