Mapping an Ordinary File

• To map an ordinary file to a process's memory, use the mmap (Memory MAPped) call.
  • The first argument is the address at which you would like Linux to map the file into your process's address space; the value NULL allows Linux to choose an available start address.
  • The second argument is the length of the map in bytes.
  • The third argument specifies the protection on the mapped address range. The protection consists of a bitwise or of
    PROT_READ, PROT_WRITE, and PROT_EXEC.
  • The fourth argument is a flag value that specifies additional options. The flag value is a bitwise or of these constraints:
    • MAP_PRIVATE Writes to the memory range should not be written back to the attached file, but to a private copy of the file. No other process sees these writes. This mode may not be used with MAP_SHARED.
    • MAP_SHARED Writes are immediately reflected in the underlying file rather than buffering writes. Use this mode when using mapped memory for IPC. This mode may not be used with MAP_PRIVATE.
  • The fifth argument is a file descriptor opened to the file to be mapped.
  • The last argument is the offset from the beginning of the file from which to start the map.
• When you are finished with a memory mapping, release it by using munmap. Pass it the start address and length of the mapped memory region. Linux automatically unmaps mapped regions when a process terminates.
• The following http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/mmap-write.ccode (see Fig. 6.3.1) generates a random number and writes it to a memory-mapped file.
• The following http://siber.cankaya.edu.tr/SystemsProgramming/cfiles/mmap-read.ccode (see Fig. 6.3.1) reads the number, prints it, and replaces it in the memory-mapped file with double the value.

  Figure 6.5: Write a Random Number to a Memory-Mapped File

  

  Figure 6.6: Read an Integer from a Memory-Mapped File, and Double It

  

  $ ./mmap-write /tmp/integer-file 
  $ cat /tmp/integer-file 
  42 
  $ ./mmap-read /tmp/integer-file 
  value: 42 
  $ cat /tmp/integer-file 
  84