Some applications may need to read or write data to multiple buffers,
which are separated in memory. Although this can be done easily enough
with multiple calls to read and write, it is inefficient
because there is overhead associated with each kernel call.
Instead, many platforms provide special high-speed primitives to perform
these scatter-gather operations in a single kernel call. The GNU C
library will provide an emulation on any system that lacks these
primitives, so they are not a portability threat. They are defined in
These functions are controlled with arrays of iovec structures,
which describe the location and size of each buffer.
— Data Type: struct iovec
The iovec structure describes a buffer. It contains two fields:
The writev function gathers data from the buffers described in
vector, which is taken to be count structures long, and writes
them to filedes. As each buffer is written, it moves on to the
Like readv, writev may stop midstream under the same
conditions write would.
The return value is a count of bytes written, or -1 indicating an
error. The possible errors are the same as in write.
Note that if the buffers are small (under about 1kB), high-level streams
may be easier to use than these functions. However, readv and
writev are more efficient when the individual buffers themselves
(as opposed to the total output), are large. In that case, a high-level
stream would not be able to cache the data effectively.
Published under the terms of the GNU General Public License