Write blocks of data to a file


#include <unistd.h>

int writeblock( int fd,
                size_t blksize,
                unsigned block,
                int numblks,
                const void *buff );


The file descriptor for the file you want to write in.
The number of bytes in each block of data.
The block number from which to start writing. Blocks are numbered starting at 0.
The number of blocks to write.
A pointer to a buffer that contains the blocks of data that you want to write.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The writeblock() function writes numblks blocks of data to the file associated with the open file descriptor, fd, from the buffer pointed to by buff, starting at block number block.

This function is useful for direct updating of raw blocks on a block special device (for example, raw disk blocks), but you can also use it for high-speed updating (for example, of database files). The speed gain is through the combined seek/write implicit in this call.

If numblks is zero, writeblock() returns zero, and has no other results.

If successful, writeblock() returns the number of blocks actually written to the disk associated with fd. This number is never greater than numblks, but could be less than numblks if one of the following occurs:

If a write error occurs on the first block and one of the sync flags is set, writeblock() returns -1 and sets errno to EIO.

If one of the sync flags is set, writeblock() doesn't return until the blocks are actually transferred to the disk. If neither of the flags is set, writeblock() places the blocks in the cache and schedules them for writing as soon as possible, but returns before the writing takes place.

Note: In the latter instance, it's impossible for the application to know if the write succeeded or not (due to system failures or bad disk blocks). Using the sync flags significantly impacts the performance of writeblock(), but guarantees that the data can be recovered.


The number of blocks actually written. If an error occurred, writeblock() returns -1, sets errno to indicate the error, and doesn't change the contents of the buffer pointed to by buff.


The fd argument isn't a valid file descriptor that's open for writing.
One of the following occurred:
  • An attempt was made to write a file that exceeds the maximum file size, and there was no room for any bytes to be written.
  • The file is a regular file, nbytes is greater than 0, and the starting position is greater than or equal to the offset maximum established in the open file description associated with fildes.
The write 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.
The starting position is negative or beyond the maximum file size.
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.
There's no free space remaining on the device containing the file.
The lseek() or write() function isn't implemented for the filesystem specified by the file descriptor.
A request was made of a nonexistent device, or the request was outside the capabilities of the device.
The resulting file offset is a value that can't be represented correctly in an object of type off_t.
The file descriptor is associated with a pipe or FIFO.


QNX Neutrino

Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes