read() attempts to read
nbytes of data from the object referenced by the descriptor
d into the buffer pointed to by
buf.
readv() performs the same action, but scatters the input data into the
iovcnt buffers specified by the members of the
iov array: iov[0], iov[1], ..., iov[iovcnt-1].
pread() and
preadv() perform the same functions, but read from the specified position in the file without modifying the file pointer.
For
readv() and
preadv(), the
iovec structure is defined as:
struct iovec {
void *iov_base;
size_t iov_len;
};
Each
iovec entry specifies the base address and length of an area in memory where data should be placed.
readv() will always fill an area completely before proceeding to the next.
On objects capable of seeking, the
read() starts at a position given by the file pointer associated with
d (see
lseek(2)). Upon return from
read(), the file pointer is incremented by the number of bytes actually read.
Objects that are not capable of seeking always read from the current position. The value of the file pointer associated with such an object is undefined.
Upon successful completion,
read(),
readv(),
pread(), and
preadv() return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case.