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IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .Dd February 10, 2024 .Dt READ 2 .Os .Sh NAME .Nm read , .Nm readv , .Nm pread , .Nm preadv .Nd read input .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In unistd.h .Ft ssize_t .Fn read "int fd" "void *buf" "size_t nbytes" .Ft ssize_t .Fn pread "int fd" "void *buf" "size_t nbytes" "off_t offset" .In sys/uio.h .Ft ssize_t .Fn readv "int fd" "const struct iovec *iov" "int iovcnt" .Ft ssize_t .Fn preadv "int fd" "const struct iovec *iov" "int iovcnt" "off_t offset" .Sh DESCRIPTION The .Fn read system call attempts to read .Fa nbytes of data from the object referenced by the descriptor .Fa fd into the buffer pointed to by .Fa buf . The .Fn readv system call 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]. The .Fn pread and .Fn preadv system calls perform the same functions, but read from the specified position in the file without modifying the file pointer. .Pp For .Fn readv and .Fn preadv , the .Fa iovec structure is defined as: .Pp .Bd -literal -offset indent -compact struct iovec { void *iov_base; /* Base address. */ size_t iov_len; /* Length. */ }; .Ed .Pp Each .Fa iovec entry specifies the base address and length of an area in memory where data should be placed. The .Fn readv system call will always fill an area completely before proceeding to the next. .Pp On objects capable of seeking, the .Fn read starts at a position given by the pointer associated with .Fa fd (see .Xr lseek 2 ) . Upon return from .Fn read , the pointer is incremented by the number of bytes actually read. .Pp 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. .Pp Upon successful completion, .Fn read , .Fn readv , .Fn pread and .Fn 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. .Pp In accordance with .St -p1003.1-2004 , both .Xr read 2 and .Xr write 2 syscalls are atomic with respect to each other in the effects on file content, when they operate on regular files. If two threads each call one of the .Xr read 2 or .Xr write 2 , syscalls, each call will see either all of the changes of the other call, or none of them. The .Fx kernel implements this guarantee by locking the file ranges affected by the calls. .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 The .Fn read , .Fn readv , .Fn pread and .Fn preadv system calls will succeed unless: .Bl -tag -width Er .It Bq Er EBADF The .Fa fd argument is not a valid file or socket descriptor open for reading. .It Bq Er ECONNRESET The .Fa fd argument refers to a socket, and the remote socket end is forcibly closed. .It Bq Er EFAULT The .Fa buf argument points outside the allocated address space. .It Bq Er EIO An I/O error occurred while reading from the file system. .It Bq Er EINTEGRITY Corrupted data was detected while reading from the file system. .It Bq Er EBUSY Failed to read from a file, e.g. /proc//regs while is not stopped .It Bq Er EINTR A read from a slow device (i.e.\& one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data arrived. .It Bq Er EINVAL The pointer associated with .Fa fd was negative. .It Bq Er EAGAIN The file was marked for non-blocking I/O, and no data were ready to be read. .It Bq Er EISDIR The file descriptor is associated with a directory. Directories may only be read directly by root if the filesystem supports it and the .Dv security.bsd.allow_read_dir sysctl MIB is set to a non-zero value. For most scenarios, the .Xr readdir 3 function should be used instead. .It Bq Er EOPNOTSUPP The file descriptor is associated with a file system and file type that do not allow regular read operations on it. .It Bq Er EOVERFLOW The file descriptor is associated with a regular file, .Fa nbytes is greater than 0, .Fa offset is before the end-of-file, and .Fa offset is greater than or equal to the offset maximum established for this file system. .It Bq Er EINVAL The value .Fa nbytes is greater than .Dv SSIZE_MAX (or greater than .Dv INT_MAX , if the sysctl .Va debug.iosize_max_clamp is non-zero). .El .Pp In addition, .Fn readv and .Fn preadv may return one of the following errors: .Bl -tag -width Er .It Bq Er EINVAL The .Fa iovcnt argument was less than or equal to 0, or greater than .Dv IOV_MAX . .It Bq Er EINVAL One of the .Fa iov_len values in the .Fa iov array was negative. .It Bq Er EINVAL The sum of the .Fa iov_len values in the .Fa iov array is greater than .Dv SSIZE_MAX (or greater than .Dv INT_MAX , if the sysctl .Va debug.iosize_max_clamp is non-zero). .It Bq Er EFAULT Part of the .Fa iov array points outside the process's allocated address space. .El .Pp The .Fn pread and .Fn preadv system calls may also return the following errors: .Bl -tag -width Er .It Bq Er EINVAL The .Fa offset value was negative. .It Bq Er ESPIPE The file descriptor is associated with a pipe, socket, or FIFO. .El .Sh SEE ALSO .Xr dup 2 , .Xr fcntl 2 , .Xr getdirentries 2 , .Xr open 2 , .Xr pipe 2 , .Xr select 2 , .Xr socket 2 , .Xr socketpair 2 , .Xr fread 3 , .Xr readdir 3 .Sh STANDARDS The .Fn read system call is expected to conform to .St -p1003.1-90 . The .Fn readv and .Fn pread system calls are expected to conform to .St -xpg4.2 . .Sh HISTORY The .Fn preadv system call appeared in .Fx 6.0 . The .Fn pread function appeared in .At V.4 . The .Fn readv system call appeared in .Bx 4.2 . The .Fn read function appeared in .At v1 .