I am learning about mapping files in to memory and I saw in an example that you would point a charter pointer to the address that mmap returned and you could use putchar to display the contents of the file.


I got to thinking what about a a file with a link list so i put a file pointer to the address that mmap returned and tried to use fread and fwrite on it and I kept getting segment faults....

the code for reading the linked list works if not using mmap but i was curious on how to get fread and fwrite to work with mapped files, or why it will not work with mapped files.

Sounds like you don't get it what mmap() actually is.

mmap() is an alternative way to read/write files. Alternative to fread()/fwrite().

Once a file is mmap()-ed reading and writing to the file is much more convenient than using read()/write()/fread()/fwrite().

You can read/write bytes to the files just by reading/writing an array of bytes. This is much more easier/convenient than fread()/fwrite() and/or linked lists.

On the downside opening and mmap()-ing the file is little more programming work. Also it may be a little slower than read()/write(), and appending to a file can be a bit troublesome.

So why making your program more complicated and slower by laying a linked list on top of an array which is more convenient than linked list already. And why fread()/fwrite() when you can just read/assign values to an array (which will end up inside the file at the right position)?

Or to put it the other way around: why mmap() a file if you prefer to fread()/fwrite() to it?

Just choose: Either read/write to/from an array (which actually is the bytes in the file), or fread()/frwite() the "old-fashioned" way.

Here is some example code for reading/writing to a file as an array of bytes.

In addition to the useful info from Hko, here's another way to look at it. fread and fwrite provide linear access to a file, whereas mmap provides random access. Additionally, fread is a wrapper in the C library that implements a buffer on top of read. In between fread and read, the C library interprets the input to allow it to be treated as text, e.g. for use with fgets. In the same way you can't mix mmap and fread, you can't mix read and fread. This is because fread assumes it's the only thing accessing the contents of the file; if that isn't the case, things don't work as they should (as you've seen.)
Kevin Barry