Question

I have a simple task to accomplish with this routine where, all it has to do is, open the file, append data from a buffer & close.
I am using ‘open’ & ‘write’ for that purpose on a linux machine. Although the return code after ‘write()’ is positive, the file size does not increase and it is always empty. I ma pulling my hair to figure out what the issue with the below code. Thought some fresh eyes can shed some light.

#define BIT_Q_FILE  ".\\bitq.dat"

int BQWrite(void *p)
{
int fd ;
int rc = -1 ;

fd = open(BIT_Q_FILE, O_RDWR | O_APPEND ) ;

if (fd < 0)
    return -1;


memset(&BITQBuff,0,sizeof(typeBITQFile));
memcpy(&BITQBuff.pBitQueue,p,sizeof(typeBITQueue));

rc = write(fd, &BITQBuff,sizeof(typeBITQFile)) ;

close(fd) ;

if(rc!=sizeof(typeBITQFile))
{
    return -1;
}

rc = sizeof(typeBITQueue);

return rc ;
}

Answer 1

I got your problem right here:

#define BIT_Q_FILE  ".\\bitq.dat"

You’ve hit a trifecta of Windows-to-Unix porting gotchas:

• The directory separator on Unix is /, not \.
• But Unix is perfectly happy to let you put \ in the middle of a file name. (The only bytes — and I really mean bytes, not characters — that cannot appear in a pathname component are those with the values 0x2F and 0x00.)
• Unix is also perfectly happy to let a file name begin with a dot; however, by default ls does not print any file names that begin with a dot.

So you are expecting data to be written to a file named bitq.dat in the current directory, but it is actually being written to a file named .\bitq.dat, still in the current directory. That file is hidden by default, so it looks like the data is disappearing into thin air. ls -a will reveal the hidden file, and rm .\\bitq.dat will delete it. To fix your code, just change the define to

#define BIT_Q_FILE "bitq.dat"

It is not necessary to put a leading ./ on the path passed to open.

This may not be the only problem with your code, but I don’t see anything else obviously wrong. If you need more help, please post a new question with a complete, minimal test program that people can compile and run for themselves.