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help with file opening /closing

Posted on 2005-04-08
Medium Priority
Last Modified: 2010-04-15
I have two functions A and B ,


   write some date to logfile.txt;


function B is called from function A, now in B i need to write some data to a file , say logfile.txt
now for some reason , I am not allowed to open the logfile  in either the main function or function A.  So I have these following options

1. open/close  the file in B()   in which case I will end up opening/closing the file each time B() is called

2. create the File* fp as a static variable  , so i can open it once and then each time check if its null like this
  static FILE *fp = NULL;
  if( !fp){  
    fp = fopen(....) ;
 do stuff with fp ;


In this case , I would not know when to close the file .


Do you guys have any other way ?
Question by:avizit
LVL 45

Expert Comment

ID: 13734031
Hi avizit,

Way 2 is the way to go.
Other options would be to use global variables which is not a good idea ...
It would be interesting to know of contraints which would not allow you to open the file in A()

LVL 11

Author Comment

ID: 13734094
>>It would be interesting to know of contraints which would not allow you to open the file in A()

the reason is ,
The call to B() in A() , is not a normal function call as in I don't insert call it explicitly. But I am modifying the interemediate code of the compiled program and inserting a call instruction [ which is a call to B() ]

the process is something like this

A() is in file fileA.c , and B() is in fileB.c
intially in A() there is no call to B() ,

while compiling fileA.c , the compiler inserts a call to B().
and later I link both the compiled object files.  B() is basically a logging function , which I may decide to change to do nothing , so i want everything to do with file pointers to be done in B() itself, otherwise there will be filepointer related junk on A() which may be totally unnecessary.

Hope I confused you enough :)

am leaving this question open for a few days , to see if ppl have other techniques .

LVL 45

Assisted Solution

sunnycoder earned 300 total points
ID: 13734403
I got your point ... unless you could modify main, there would not be much sense in opening and closing in A either. ... your way 2 looks appealing to me
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LVL 16

Accepted Solution

PaulCaswell earned 300 total points
ID: 13734577
You have the right method but, to close the file, use 'atexit'.

You have several options:

1. Make the file handle a global to the 'B' module or use some system-specific method of discovering the file handle from the file name.

2. Add a perameter to 'B' that indicates that it should close the file.

LVL 22

Assisted Solution

NovaDenizen earned 600 total points
ID: 13737256
Use a kind of instance pattern.

static FILE *logfileptr = NULL;
FILE *getLogFilePtr() {
    if (logfileptr == NULL) {
        logfileptr = fopen("logfilename", "wa");
    return logfileptr;

Then, in B, just call getLogFilePtr() to get the pointer.  This way can be used in multiple B-like functions.
LVL 11

Author Comment

ID: 13747410
oops i meant to accept nova denizen .. .. sorry ..
LVL 45

Expert Comment

ID: 13750330

Would you like me to reopen the question for you?
LVL 11

Author Comment

ID: 13750412
if no one objects i guess it doesn't really matter . i have awarded points as per what I wanted .
if anyone objects i am willing to undo my mistake .. but I hope its fine.

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