How to find the number of data mapped by mmap()?

Question

if mmap() was used to read a file, how can I find the number of data mapped by mmap().

float *map = (float *)mmap(NULL, FILESIZE, PROT_READ, MAP_SHARED, fd, 0);

Answer 1

The mmap system call does not read data. It just maps the data in your virtual address space (by indirectly configuring your MMU), and that virtual address space is changed by a successful mmap. Later, your program will read that data (or not). In your example, your program might later read map[356] if mmap has succeeded (and you should test against its failure).

Read carefully the documentation of mmap(2). The second argument (in your code, FILESIZE) defines the size of the mapping (in bytes). You might check that it is a multiple of sizeof(float) and divide it by sizeof(float) to get the number of elements in map that are meaningful and obtained from the file. The size of the mapping is rounded up to a multiple of pages. The man page of mmap(2) says:

A file is mapped in multiples of the page size. For a file that is not a multiple of the page size, the remaining memory is zeroed when mapped, and writes to that region are not written out to the file.

Data is mapped in pages. A page is usually 4096 bytes. Read more about paging.

The page size is returned by getpagesize(2) or by sysconf(3) with _SC_PAGESIZE (which usually gives 4096).

Consider reading some book like Operating Systems: Three Easy Pieces (freely downloadable) to understand how virtual memory works and what is a memory mapped file.

On Linux, the /proc/ filesystem (see proc(5)) is very useful to understand the virtual address space of some process: try cat /proc/$$/maps in your terminal, and read more to understand its output. For a process of pid 1234, try also cat /proc/1234/maps

From inside your process, you could even read sequentially the /proc/self/maps pseudo-file to understand its virtual address space, like here.