About mmap and lseek

Posted on 2006-10-22
Last Modified: 2008-03-04
int f=open("myFile",O_RDONLY);
int n=lseek(f,0,SEEK_END);
void * mf=mmap(0,n,PROT_READ,MAP_PRIVATE,f,0);

1)Am I right that using lseek(f,0,SEEK_END); to get the file size will give you filesize + 1, and hence it is more correct to use lseek(f,-1,SEEK_END);? Also, is this a correct way to get the filesize?
2)After getting mf, how should I read the file inside the memory map file?
eg my file has
****myFile start****
line1 abc
line2 def
****myFile end****
How to get "line2 def", ie the second line?

Question by:william007
  • 3
  • 2
LVL 45

Accepted Solution

sunnycoder earned 500 total points
ID: 17782983
1. No ... Why would it give you length +1?
int n=lseek(f,0,SEEK_END);
is correct

note that this would position the head to end of file ... you must reposition it to beginning of file

2. mmap returns pointer to mmaped area ... simply treat it like a buffer in memory ... here is an example which should answer both your questions

int main() {          
      void * ret;
      int n;

      int f = open ("ha.txt", O_RDONLY);

      n = lseek (f,0,SEEK_END);
      printf ("%d\n",n);

      n = lseek (f,0,SEEK_SET);
      ret = mmap (0, 1024, PROT_READ,MAP_PRIVATE, f,0);
      printf ("%s\n", ((char *)ret) + 10);

Author Comment

ID: 17783003
For question 1,
this is because
I type "abcdef" in a file, it gives me length 7,
even nothing in a file, it gives me length 1,
If it is correct, what is the reason for the size? abcdef...should be 6 bytes and nothing in a file should be 0 byte....
LVL 45

Assisted Solution

sunnycoder earned 500 total points
ID: 17783007
EOF is also part of file .. else you would never know where the file ends!!!

Author Comment

ID: 17783049
I see:)

Author Comment

ID: 17794381

Featured Post

New! My Passport Wireless Pro Wi-Fi Mobile Storage

Portable wireless storage to offload, edit, and stream anywhere.

High-capacity, wireless mobile storage designed to accompany professional photographers and videographers in the field to easily offload, edit and stream captured photos and high-definition videos.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
Want to delete all my personal data 13 138
Problem to show menu 5 87
Unable to start eclipse ? 17 134
How to install SVN Command Line Client? 5 43
This tutorial is posted by Aaron Wojnowski, administrator at  To view more iPhone tutorials, visit This is a very simple tutorial on finding the user's current location easily. In this tutorial, you will learn ho…
Windows programmers of the C/C++ variety, how many of you realise that since Window 9x Microsoft has been lying to you about what constitutes Unicode ( They will have you believe that Unicode requires you to use…
The goal of this video is to provide viewers with basic examples to understand and use structures in the C programming language.
The goal of this video is to provide viewers with basic examples to understand how to use strings and some functions related to them in the C programming language.

863 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

22 Experts available now in Live!

Get 1:1 Help Now