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I want to write a physical memory to a file. The memory itself will not be touched again, thus I want to use O_DIRECT to gain the best write performance. My first idea was to open /dev/mem and mmap the memory and write everything to a file, which is opened with O_DIRECT . The write call fails ( EFAULT ) on the memory-address returned by mmap. If I do not use O_DIRECT , it results in a memcpy . #include <cstdint> #include <iostream> #include <unistd.h> #include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <stdio.h> #include <errno.h> #include <malloc.h> #include <sys/mman.h> #define PRINT_ERRNO_REASON(reason) \ case reason: { std::cout << #reason << std::endl; } break; void write_page_aligned_buffer(int out_fd) { const ssize_t PAGE_SIZE = getpagesize(); void* page_aligned_buffer = memalign(PAGE_SIZE, PAGE_SIZE); if(!page_aligned_buffer) { std::cout << "Could not allocate page aligned buffer." << std::endl; return; } std::cout << "Allocated a buffer at address " << page_aligned_buffer << "." << std::endl; if(write(out_fd, page_aligned_buffer, PAGE_SIZE) < 0) { std::cout << "Could not write page-aligned user buffer to tmp-file. Quitting..." << std::endl; std::cout << "Reason of fail is "; switch(errno) { PRINT_ERRNO_REASON(EAGAIN); PRINT_ERRNO_REASON(EBADF); PRINT_ERRNO_REASON(EFAULT); PRINT_ERRNO_REASON(EFBIG); PRINT_ERRNO_REASON(EINTR); PRINT_ERRNO_REASON(EINVAL); PRINT_ERRNO_REASON(EIO); PRINT_ERRNO_REASON(ENOSPC); PRINT_ERRNO_REASON(EPIPE); default: std::cout << "Unknown" << std::endl;

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