About the file read and write

Dear all,
     I am now trying to write some codes for file read write on Zaurus
5500. I used the linux system call 'write(int fd, char* buf, int
length)' to write to a file that I created using the 'open("file",
O_CREATE|O_APPEND, 0777)'. I succeeded to open the file and get the
fd number, however, I can't write data into the opened file, but if i
changed the codes to use the Qt file read write functions, that's ok.
I want to ask if there is a problem in using the linux system call to
do the file read write, or it needs some extra coding method to
achieve if i don't use the Qt functions but linux system calls?
Thanks a lot!

Regards,
Roy