I use fgets to grab a line to a string, and I checked the pointer before and after I use fgets. I want to know why the pointer remained the same, instead of add 200?
Code:#include <stdio.h> int main (){ FILE *new; char string[100]; new = fopen ("jt.rules", "r"); printf ("start address: %d\n", new); fgets (string, 100, new); printf ("end address: %d\n", new); fclose (new); return 0; }
output:
start address: 1024748
end address: 1024748
Found this while googling
FILE objects are usually created by a call to either fopen or tmpfile, which both return a reference to one of these objects.
The content of a FILE object is not meant to be read from outside the functions of the cstdio library; In fact, its main purpose is to be referenced as an argument in all stream-involving functions of this library to identify the stream to be affected.
Its memory allocation is automatically performed by either fopen or tmpfile, and is the responsibility of the library to free the resources once the stream has been closed using fclose or other means.
Last edited by Doorsdown; 10-15-2007 at 09:58 PM.
Because you have no idea what you're measuring.Originally Posted by lukewang
The FILE * isn't going to change. It's pointing to the location in memory where information is stored on the file. Where this information is stored isn't going to change. The information itself -- or to be more exact, parts of the information -- will change.
Thanks for answering my question. But I'm still confused with how the pointer works. For example, if I use fgetc function for 5 times, how can I know where is my current position?
You're not supposed to know. It's compiler and system specific. Check stdio.h and the other headers that your compiler may use and see if you can find the FILE struct definition. Remember, different compilers have different versions of it, so you should not make guarentees about this. Use the standard functions as much as possible.
If you need more power, then you should look into operating system specific ways of handling files (which the C standard library functions actually use internally).
