7.8. errno 中报告的错误

使用原始接口，几乎任何字符串都可能被意外地传递给 write()，并可能（但很少）发生一些糟糕的事情。如果检测到某些错误，则很可能 EIO 会被放入 errno 中。

不幸的是，这种情况仍然可能发生在 write() 中，因为它既可以接受原始的 struct sg_header，也可以接受本说明中描述的更新的 sg_io_hdr_t。然而，由于 SG_IO ioctl() 只接受 sg_io_hdr_t 结构，因此随机字符串被解释为命令的可能性较小。由于 sg_io_hdr_t 接口进行了更多的错误检查，它尝试给出更精确的 errno 值，以帮助用户查明问题。[诚然，其中一些 errno 值是从大量可用值中任意选择的。]

在大多数情况下，当对 sg 文件描述符的系统调用失败时，该调用将返回 -1。在应用程序检测到系统调用失败后，它应该读取 "errno" 变量中的值（在执行任何其他系统调用之前）。应用程序应包含 <errno.h> 头文件。

下面是一个 errno 值表，指示哪些对 sg 的调用会生成它们以及错误的含义。write() 调用用 "w" 表示，read() 调用用 "r" 表示，open() 调用用 "o" 表示。

errno which_calls Meaning ----- ----------- ---------------------------------------------- EACCES <some ioctls> Root permission (more precisely CAP_SYS_ADMIN or CAP_SYS_RAWIO) required. Also may occur during an attempted write to /proc/scsi/sg files. EAGAIN r The file descriptor is non-blocking and the request has not been completed yet. EAGAIN w,SG_IO SCSI sub-system has (temporarily) run out of command blocks. EBADF w File descriptor was not open()ed O_RDWR. EBUSY o Someone else has an O_EXCL lock on this device. EBUSY w With mmap-ed IO, the reserved buffer already in use. EBUSY <some ioctls> Attempt to change something (e.g. reserved buffer size) when the resource was in use. EDOM w,SG_IO Too many requests queued against this file descriptor. Limit is SG_MAX_QUEUE active requests. If sg_header interface is being used then the default queue depth is 1. Use SG_SET_COMMAND_Q ioctl() to increase it. EFAULT w,r,SG_IO Pointer to user space invalid. <most ioctls> EINVAL w,r Size given as 3rd argument not large enough for the sg_io_hdr_t structure. Both direct and mmap-ed IO selected. EIO w Size given as 3rd argument less than size of old header structure (sg_header). Additionally a write() with the old header will yield this error for most detected malformed requests. EIO r A read() with the older sg_header structure yields this value for some errors that it detects. EINTR o While waiting for the O_EXCL lock to clear this call was interrupted by a signal. EINTR r,SG_IO While waiting for the request to finish this call was interrupted by a signal. EINTR w [Very unlikely] While waiting for an internal SCSI resource this call was interrupted by a signal. EMSGSIZE w,SG_IO SCSI command size ('cmd_len') was too small (i.e. < 6) or too large ENODEV o Tried to open() a file with no associated device. [Perhaps sg has not been built into the kernel or is not available as a module?] ENODEV o,w,r,SG_IO SCSI device has detached, awaiting cleanup. User should close fd. Poll() will yield POLLHUP. ENOENT o Given filename not found. ENOMEM o [Very unlikely] Kernel was not even able to find enough memory for this file descriptor's context. ENOMEM w,SG_IO Kernel unable to find memory for internal buffers. This is usually associated with indirect IO. For mmap-ed IO 'dxfer_len' greater than reserved buffer size. Lower level (adapter) driver does not support enough scatter gather elements for requested data transfer. ENOSYS w,SG_IO 'interface_id' of a sg_io_hdr_t object was _not_ 'S'. ENXIO o "remove-single-device" may have removed this device. ENXIO o, w,r,SG_IO Internal error (including SCSI sub-system busy doing error processing - e.g. SCSI bus reset). When a SCSI device is offline, this is the response. This can be bypassed by opening O_NONBLOCK. EPERM o Can't use O_EXCL when open()ing with O_RDONLY EPERM w,SG_IO File descriptor open()-ed O_RDONLY but O_RDWR <some ioctls> access mode needed for this operation.