Read blocks of data from a file
int readblock( int fd,
void *buff );
- The file descriptor for the file you want to read from.
- The number of bytes in each block of data.
- The block number from which to start reading.
Blocks are numbered starting at 0.
- The number of blocks to read. If numblks is zero,
readblock() returns zero and has no other results.
If numblks is less than zero, or if
numblks * blksize is greater than
INT_MAX (see <limits.h>),
the function returns -1 and sets errno to EINVAL.
- A pointer to a buffer where the function can store the data that it
Use the -l c option to
to link against this library.
This library is usually included automatically.
The readblock() function reads numblks blocks of data
from the file associated with the open file descriptor fd,
into the buffer pointed to by buff, starting at block number
The blksize argument specifies the size of a block, in bytes.
This function is useful for direct access to raw blocks on a
block special device (for example, raw disk blocks) but may also be used for
high-speed access to database files, for example.
The speed gain is through the combined seek/read implicit in this call.
On successful completion, readblock() returns the number of
blocks actually read and placed in the buffer. This number is never
greater than numblks. The value returned may be less than
numblks if one of the following occurs:
- The number of blocks left before the end-of-file is less than
- The process requests more blocks than implementation limits allow to be
read in a single atomic operation.
- A read error occurred after reading at least one block.
If a read error occurs on the first block, readblock() returns
-1 and sets errno to EIO.
The number of blocks actually read. If an error occurs, -1 is returned,
errno is set to indicate the error, and the
contents of the buffer pointed to by buff are left unchanged.
- The fd argument isn't a valid file descriptor open for reading.
- The read operation was interrupted by a signal, and either no data was
transferred, or the resource manager responsible for that file doesn't
report partial transfers.
- An attempt was made to read a negative number of blocks or read a number of bytes
that exceeds the allowable limit.
- One of the following:
- A physical I/O error occurred (for example, a bad block on a disk).
The precise meaning is device-dependent.
- The filesystem resides on a removable media device, and the media has been forcibly removed.
- The lseek() or read() function isn't implemented
for the filesystem specified by the file descriptor.
- The file descriptor is associated with a pipe or FIFO.