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READ(2) System Calls Manual READ(2)

pread, read, preadv, readv — read input

Standard C Library (libc, -lc)

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>

ssize_t
pread(int d, void *buf, size_t nbyte, off_t offset);

ssize_t
read(int fildes, void *buf, size_t nbyte);

ssize_t
preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);

ssize_t
readv(int d, const struct iovec *iov, int iovcnt);

() attempts to read nbyte bytes of data from the object referenced by the descriptor fildes 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 () and (), the iovec structure is defined as:

struct iovec {
	char   *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. () and () will always fill an area completely before proceeding to the next.

On objects capable of seeking, the () starts at a position given by the pointer associated with fildes (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, (), 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.

() and () will fail if the parameter nbyte exceeds INT_MAX, and they do not attempt a partial read.

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.

The read(), readv(), pread(), and preadv() calls will succeed unless:

[]
The file was marked for non-blocking I/O, and no data were ready to be read.
[]
fildes is not a valid file or socket descriptor open for reading.
[]
Buf points outside the allocated address space.
[]
A read from a slow device was interrupted before any data arrived by the delivery of a signal.
[]
The pointer associated with fildes was negative.
[]
An I/O error occurred while reading from the file system.
[]
The process group is orphaned.
[]
The file is a regular file, nbyte is greater than 0, the starting position is before the end-of-file, and the starting position is greater than or equal to the offset maximum established for the open file descriptor associated with fildes.
[]
An attempt is made to read a directory.
[]
An attempt to allocate a memory buffer fails.
[]
Insufficient memory is available.
[]
An action is requested of a device that does not exist.
[]
A requested action cannot be performed by the device.
[]
An attempt to read a remote file through NFS that has already been deleted in the server.
[]
The connection timed out while reading a remote file from a soft mounted NFS volume (see mount_nfs(8)).
[]
The file is a “dataless” file that requires materialization and materialization timed out or encountered some other temporary failure.
[]
The file is a “dataless” file that requires materialization and the I/O policy of the current thread or process disallows dataless file materialization (see getiopolicy_np(3)).

pread() and preadv() calls may also return the following errors:

[]
The specified file offset is invalid.
[]
The file descriptor is associated with a pipe, socket, or FIFO.

The read() call may also return the following errors:

[]
The connection is closed by the peer during a read attempt on a socket.
[]
A read is attempted on an unconnected socket.
[]
A transmission timeout occurs during a read attempt on a socket.

The read() and pread() call may also return the following error:

[]
The value provided for nbyte exceeds INT_MAX.

The readv() and preadv() calls may also return one of the following errors:

[]
Part of the iov points outside the process's allocated address space.
[]
Iovcnt was less than or equal to 0, or greater than 16.
[]
One of the iov_len values in the iov array was negative.
[]
The sum of the iov_len values in the iov array overflowed a 32-bit integer.

#include <sys/types.h> #include <sys/uio.h> #include <unistd.h>

The include files <sys/types.h> and <sys/uio.h> are necessary for all functions.

dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2), compat(5)

The read() function call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”). The readv() and pread() functions are expected to conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”). preadv() is nonstandard.

The pread() function call appeared in AT&T System V Release 4 UNIX. The readv() function call appeared in 4.2BSD. A read() function call appeared in Version 6 AT&T UNIX.

June 3, 2021 macOS