Hi all,
I'm trying to write a simple bit of test code to save a variable to a file. So each time the variable is written it is updated in the file. This simple code just increments a global variable counter, and calls the function. In the function I read the file getting the value of counter increament it and write it back to the file. I thought this would be fairly simple, however when I printf it always shows a value of counter =0

my function is

[source]
void savecounter(void)
{
FILE *fp;

counter[0]=0;
if((fp=fopen("/usr/local/counter", "w+b"))==NULL) /*count to events*/
{
printf("cannot open file\n");
return;
}
fread(counter, sizeof(counter[0]), sizeof(counter)/sizeof(counter[0]), fp); /*read counter from file*/
printf("counter %d\n",counter[0]); /*print the value*/
counter[0]++; /* inc &counter*/
rewind(fp); /*go to start of file*/
fwrite(counter, sizeof(counter[0]), sizeof(counter)/sizeof(counter[0]), fp); /*and write*/



fclose(fp);
}

[/source]

I call it from 5 times so would think I'd get counter = 0, counter =1 counter = 2.....


however all I get is counter = 0 everytime, I've tried taking out rewind, I've also called counter as & and used size of int, in my code.
so

FILE *fp;

counter=0;
if((fp=fopen("/usr/local/counter", "wb+"))==NULL) /*count to events*/
{
printf("cannot open file\n");
return;
}
fread(&counter, sizeof(int), 1, fp); /*read counter from file*/
printf("counter %d\n",counter); /*print the value*/
counter++; /* inc &counter*/
rewind(fp); /*go to start of file*/
fwrite(&counter, sizeof(int), 1, fp); /*and write*/
fclose(fp);


Can anyone help me out or point me in the direction. I should point out debugging in GDB show my file goes to 1 when I write, however when I read on the second function call he value is set to 0. Thanks in advance for any help.

man fopen and look at your savecounter open mode. Can you see what you are doing? even tho I can not see you using the function.
Counter seems to be a global pointer or an array, yet you NULL it before "count to events" and pass the address of the address?

Your problem lies not in fread nor fwrite, but in fopen. When you initially open with with "wb+" the existing file is truncated. Thus the fread() will always fail…

Maybe I am missing information you have then, personally I think there is problems with it as well as the mode.

Thanks it does seem to be my open, which I can't get to work with wb+ either, I have got it workig by opening the file with read only, then loading the data. Then closing the file. changeing the data, then opening the file for write only. This works a treat. but is slightly long winded, so I use
[source]
counter=0;
if((fp=fopen("/usr/local/counter", "rb"))==NULL)
{
printf("cannot open file\n");
return;
}

fread(&counter, sizeof(int), 1, fp);
printf("counter %d\n",counter);
fclose(fp);

if((fp=fopen("/usr/local/counter", "wb"))==NULL)
{
printf("cannot open file\n");
return;
}


counter++;

fwrite(&counter, sizeof(int), 1, fp);
fclose(fp);
[/source]

open file in read write mode
and use a rewind between reading and writing