5. Strange and Interesting System Calls

• one strerror-equivalent per system call
• boundary conditions
• multiple causes for the same errno value
• multiple errno values for the same cause
• coders@slug mailing list

5.1 ENOMEDIUM

• of course, they are all frauds; slide
• Native English speakers don't know that “media” is a plural, let alone that “medium” is its singular.
• this strerror(3) string is almost content-free.
• surely this could be more informative; slide
• O_NONBLOCK
• it also groks floppies, magnetic tapes, MMC card readers, etc
• disc-with-a-c vs disk-with-a-k

5.2 EFAULT

• one pointer is easy; slide
• must not disturb process (thread) signal handlers
• bounded memory: mincore(2)
• strings: lstat(2)

5.3 EMFILE

• The message includes the limit; slide
• can't use /proc
• sysconf(3) is very handy
• of course, it could be a single-open device that is already open or mounted

5.4 ENFILE

• The message includes the limit; slide
• can't use /proc
• sysconf(3) is very handy
• this is when you read kernel sources
• system call with no [e]glibc binding
  CTL_FS, FS_MAXFILE

5.5 EPERM vs ENOSYS

• also EINVAL sometimes
• same cause, different errno(3) values; slide
• same errno(3) value, different causes; slide
• wishlist: pathconf for "is this fops pointer non-null"

5.6 errno(3) is not always set

• slide
• somtimes they set errno(3), sometimes they don't.
• traps for young players (old ones, too)
• not going into each one, read the paper
• who knew that fileno(3) could fail?

5.7 ENOSPC

• slide
• print the mount point
• work in progress (inodes vs data vs directory entries)
• special devices already includes device name...
• &...need to add device size, without altering state

5.8 EROFS

• file systems print the mount point; slide
• devices are more specific
• work in progress: filesys case could drill down
• work in progress: not all devices this accurate
  ...because a CD-ROM is not writable
  ...because the memory card has the write protect tab set
  ...because the ½ inch magnetic tape does not have a write ring
  

5.9 rename(2)

• not talking about mv<(1)
• rename(2) directory over directory; slide
• destination must be empty
• implies all the rmdir(2) errors as well
• but not files over directories, or vice versa
• who knew: dup2(2) is the same
• ...implies all the unlink(2) errors as well