Thread: fread() on a bad disk?

Hi all,

I have the following code which hangs in fread() when one of my disks goes "bad" or is pulled from system unplanned (I need to support this case)

Code:

mbr = fopen(fullpath, "r");
        if (!mbr) {
                fprintf(stderr, "ERROR: Can't open device %s
\n",
                        fullpath);
                return 0;
        }

        count = fread(buffer, 1, 512, mbr);
        if (count < 512) {
                  return and print error
        }

any ideas why/when fread() would just never return??

fyi, this is on Redhat linux.....

I tried using read() instead but still get the hang.....

Originally Posted by vart

does buffer contain the 512 bytes?

I have it defined as:

Code:

unsigned char buffer[512]

Originally Posted by zacs7

why not use sizeof buffer instead?, so you don't make mistakes, and if you change the size of buffer there's no need to change it anywhere else!

I will try that, but I'm guessing you mean for future.
Or do you think me assuming the size is 512 might be wrong?

Originally Posted by zacs7

why not use sizeof buffer instead?, so you don't make mistakes, and if you change the size of buffer there's no need to change it anywhere else!

also your check is dodge... your assuming your going to read all 512 bytes? What if there aren't 512 bytes in the file then if(count < 512) will trigger a false error.

that was just a hack for now. I never get to that line anyway since I hang in the fread()....

How big is what you are reading?

Originally Posted by zacs7

why not use sizeof buffer instead?, so you don't make mistakes, and if you change the size of buffer there's no need to change it anywhere else!

also your check is dodge... your assuming your going to read all 512 bytes? What if there aren't 512 bytes in the file then if(count < 512) will trigger a false error.

ran with sizeof(buffer) instead. Got same result. (hung in fread())

I printed the sizeof(buffer) in gdb before the hang, it was 512 as expected.

Originally Posted by markcole

How big is what you are reading?

I'm opening up a disk device. Trying to figure out if its a dos disk or not (on linux)

Originally Posted by annied

any ideas why/when fread() would just never return??

Why WOULD it ever return? Huh?

If you ask for 512 bytes, fread() is going to give you 512 bytes. Since the disk is physically unavailable, this is impossible. So fread() is going to wait forever. fread() is based on the underlying read() system call, and this call isn't going to return.

Why would you expect it to not hang, I don't get it.

Originally Posted by brewbuck

Why WOULD it ever return? Huh?

If you ask for 512 bytes, fread() is going to give you 512 bytes. Since the disk is physically unavailable, this is impossible. So fread() is going to wait forever. fread() is based on the underlying read() system call, and this call isn't going to return.

Why would you expect it to not hang, I don't get it.

thought maybe it would timeout at some point so any application doing a read or scan of devices wouldn't end up hanging.

Originally Posted by brewbuck

Why WOULD it ever return? Huh?

If you ask for 512 bytes, fread() is going to give you 512 bytes. Since the disk is physically unavailable, this is impossible. So fread() is going to wait forever. fread() is based on the underlying read() system call, and this call isn't going to return.

Why would you expect it to not hang, I don't get it.

in addition to my last response, what about in scenarios where we have failover and such.
If a disk goes bad, we should be able to recover.

Originally Posted by annied

in addition to my last response, what about in scenarios where we have failover and such.
If a disk goes bad, we should be able to recover.

Then you need to write the code correctly, using non-blocking reads and such. The OS is capable of telling you when a disk is pulled, learn how to do that.