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READ(2) System Calls Manual READ(2)

read, readv, pread, preadv, extpread, extpreadv — read input

library “libc”

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

ssize_t
read(int d, void *buf, size_t nbytes);

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

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

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

ssize_t
extpread(int d, void *buf, size_t nbytes, int flags, off_t offset);

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

() 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]. The (), preadv(), extpread(), and extpreadv() calls perform the same function, 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.

The () and () allow a flags argument to also be passed in, controlling blocking/non-blocking and other features on a call-by-call basis, ignoring the related default for the descriptor. Allowed flags are:

Force the system call to operate in a blocking fashion.
Force the system call to operate in a non-blocking fashion.
Force append mode for the operation.
Force offset mode for the operation.

On objects capable of seeking, the () 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, (), readv(), (), 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.

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.

Read(), readv(), pread(), preadv(), extpread() and extpreadv() will succeed unless:

[]
D is not a valid file or socket descriptor open for reading.
[]
Buf points outside the allocated address space.
[]
An I/O error occurred while reading from the file system.
[]
A read from a slow device was interrupted before any data arrived by the delivery of a signal.
[]
The pointer associated with d was negative.
[]
The file was marked for non-blocking I/O, and no data were ready to be read.

In addition, readv(), preadv() and extpreadv() may return one of the following errors:

[]
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.
[]
Part of the iov points outside the process's allocated address space.

The pread(), preadv(), extpread() and extpreadv() calls may also return the following errors:

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

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

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”).

The extpread() and extpreadv() functions are DragonFly specific extensions.

The preadv() function call was added in DragonFly 1.5. 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.

March 2, 2018 DragonFly-5.6.1