DESCRIPTION
The read system call attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf. The readv system call 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]. The pread and preadv system calls 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; /* Base address. */
size_t iov_len; /* Length. */
};
Each iovec entry specifies the base address and length of an area in memory where data should be placed. The readv system call 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 pointer associated with d (see lseek(2)). Upon return from read, the 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 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.
RETURN VALUES
If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.
ERRORS
The read, readv, pread and preadv system calls will succeed unless:
