• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 840
  • Last Modified:

About mmap and lseek

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?

0
william007
Asked:
william007
  • 3
  • 2
2 Solutions
 
sunnycoderCommented:
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);
}
0
 
william007Author Commented:
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....
0
 
sunnycoderCommented:
EOF is also part of file .. else you would never know where the file ends!!!
0
 
william007Author Commented:
I see:)
0
 
william007Author Commented:
Thanks=)
0

Featured Post

Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

  • 3
  • 2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now