READ(2) System Calls Manual READ(2)

read, readv, pread, preadv — read input

Standard C Library (libc, -lc)

#include <unistd.h>

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

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

#include <sys/uio.h>

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

ssize_t
preadv(int d, const struct iovec *iov, int iovcnt, 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]. () 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 {
	void *iov_base;
	size_t iov_len;
};

Each iovec entry specifies the base address and length of an area in memory where data should be placed. () will always fill an area completely before proceeding to the next.

On objects capable of seeking, the () starts at a position given by the file pointer associated with d (see lseek(2)). Upon return from read(), the file 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 file 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(), and preadv() will succeed unless:

[]
The file was marked for non-blocking I/O, and no data were ready to be read.
[]
d is not a valid file or socket descriptor open for reading.
[]
buf points outside the allocated address space.
[]
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. See sigaction(2) for more information on the interaction between signals and system calls.
[]
The file pointer associated with d was negative; or the total length of the I/O is more than can be expressed by the ssize_t return value.
[]
An I/O error occurred while reading from the file system.
[]
d refers to a directory and the implementation does not allow the directory to be read using read() or pread(). The readdir() function should be used instead.
[]
A message was not delivered because it would have overflowed the buffer.

In addition, readv() and preadv() may 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 {IOV_MAX}; or one of the iov_len values in the iov array was negative; or the sum of the iov_len values in the iov array overflowed a 32-bit integer.

The 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.

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

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

The preadv() function call appeared in NetBSD 1.4. The pread() function call appeared in AT&T System V Release 4 UNIX. The readv() function call appeared in 4.2BSD. The read() function call appeared in Version 1 AT&T UNIX.

Error checks should explicitly test for -1. Code such as

	while ((nr = read(fd, buf, sizeof(buf))) > 0)

is not maximally portable, as some platforms allow for nbytes to range between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an error-free read() may appear as a negative number distinct from -1. Proper loops should use

	while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
September 2, 2019 NetBSD 11.0