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@(#)read.2 8.4 (Berkeley) 2/26/94
.Dd February 26, 1994 .Dt READ 2 .Os BSD 4 .Sh NAME .Nm read , .Nm readv .Nd read input .Sh SYNOPSIS .Fd #include <sys/types.h> .Fd #include <sys/uio.h> .Fd #include <unistd.h> .Ft ssize_t .Fn read "int d" "void *buf" "size_t nbytes" .Ft ssize_t .Fn readv "int d" "const struct iovec *iov" "int iovcnt" .Sh DESCRIPTION .Fn Read attempts to read .Fa nbytes of data from the object referenced by the descriptor .Fa d into the buffer pointed to by .Fa buf . .Fn Readv performs the same action, but scatters the input data into the .Fa iovcnt buffers specified by the members of the .Fa iov array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
p For .Fn readv , the .Fa iovec structure is defined as:
p d -literal -offset indent -compact struct iovec { void *iov_base; size_t iov_len; }; .Ed
p Each .Fa iovec entry specifies the base address and length of an area in memory where data should be placed. .Fn Readv will always fill an area completely before proceeding to the next.
p On objects capable of seeking, the .Fn read starts at a position given by the pointer associated with .Fa d (see .Xr lseek 2 ) . Upon return from .Fn read , the pointer is incremented by the number of bytes actually read.
p 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.
p Upon successful completion, .Fn read and .Fn readv 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.
p .Sh 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 .Va errno is set to indicate the error. .Sh ERRORS .Fn Read and .Fn readv will succeed unless: l -tag -width Er t Bq Er EBADF .Fa D is not a valid file or socket descriptor open for reading. t Bq Er EFAULT .Fa Buf points outside the allocated address space. t Bq Er EIO An I/O error occurred while reading from the file system. t Bq Er EINTR A read from a slow device was interrupted before any data arrived by the delivery of a signal. t Bq Er EINVAL The pointer associated with .Fa d was negative. t Bq Er EAGAIN The file was marked for non-blocking I/O, and no data were ready to be read. .El
p In addition, .Fn readv may return one of the following errors: l -tag -width Er t Bq Er EINVAL .Fa Iovcnt was less than or equal to 0, or greater than 16. t Bq Er EINVAL One of the .Fa iov_len values in the .Fa iov array was negative. t Bq Er EINVAL The sum of the .Fa iov_len values in the .Fa iov array overflowed a 32-bit integer. t Bq Er EFAULT Part of the .Fa iov points outside the process's allocated address space. .El .Sh SEE ALSO .Xr dup 2 , .Xr fcntl 2 , .Xr open 2 , .Xr pipe 2 , .Xr select 2 , .Xr socket 2 , .Xr socketpair 2 .Sh STANDARDS .Fn Read is expected to conform to IEEE Std 1003.1-1988
q Dq Tn POSIX . .Sh HISTORY The .Fn readv function call appeared in x 4.2 . A .Nm read function call appeared in Version 6 AT&T UNIX.