不清楚为什么mmap没有按照我的想法去做。 C ++ Linux

Question

I trying to make a log file class, the process I want to follow is:

• Open the file (Or create it)
• Map the file on memory
• Close de file
• Write to memory

The file will have a size of 1024KB (SIZE_KB constant).

So far this is what I'm doing:

Im creating the file with read and write permissions for the owner (S_IRUSR | S_IWUSR) and read permissions for the rest (S_IRGRP | S_IROTH).

// Open the file. If the file doesnt exists it will create it
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
int fd = open(path.c_str(), O_CREAT | O_RDWR, mode);

After that, I check if fd is -1 :

if (fd < 0)
    throw std::system_error(errno, std::system_category(), "couldnt open history");

#ifdef _DEBUG_
    std::clog << "History with file descriptor " << fd << " opened" << std::endl;
#endif

Now, I map the file, but first I need to set a variable length with the size of the file which must be a mutiple of sysconf(_SC_PAGE_SIZE) :

size_t length = (int)ceil(SIZE_KB*1024.0/sysconf(_SC_PAGE_SIZE))*sysconf(_SC_PAGE_SIZE);  
/* ceil( size / page_size)*page_size*/

#ifdef _DEBUG_
    std::clog << "Length is " << length << " bytes" << std::endl;
#endif

Mapping, block_start is a private char pointer:

block_start = (char*)mmap(NULL, length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

• Extra question: M_SHARED option make this part of the memory accesible to other processes (?). Why is it also used to make the SO save the changes made in the virtual memory to the actual file ?.

Check for errors:

#ifdef _DEBUG_  
    std::clog << Returned: " << (int*)block_start << std::endl;
#endif

if (block_start == MAP_FAILED)
    throw std::system_error(errno, std::system_category(), "couldnt map memory");

#ifdef _DEBUG_
    std::clog << "History memory mapped" << std::endl;
#endif

And close the file:

int result = close(fd);
if (result < 0)
    throw std::system_error(errno, std::system_category(), "error closing history");

#ifdef _DEBUG_
    std::clog << "History with file descriptor " << fd << " closed" << std::endl;
#endif

Now, I should be able to add information to mapped memory, so I tried doing:

std::cout << "Attemping to write on first" << std::endl;
*block_start = 'A';
std::cout << "End" << std::endl;

(Inside the constructor)

This is the output I have:

History with file descriptor 3 opened
Length is 1048576 bytes
Returned: 0x7f7e9160a000
History memory mapped
History with file descriptor 3 closed
Attemping to write on first
Bus error (core dumped)

I think it probably have to do with the file size, since the created file have 0 size, but Im telling mmap to map a size of SIZE_KB*1024 bytes, so why is this not working ?

-rw-r--r-- 1 dark dark 0 Dec 13 16:15 /home/dark/talk.log

Answer 1

Because mmap will not map beyond the size of the file. If you're mmaping an empty file, your effective mmap size is 0.

You need to set the size of the file, via truncate() or ftruncate(), before mmaping it, and the size of the file, and the mmapped size, must agree.