READ(2) FreeBSD System Calls Manual READ(2)
NAME
read, readv, pread, preadv - read input
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
ssize_t
read(int fd, void *buf, size_t nbytes);
ssize_t
pread(int fd, void *buf, size_t nbytes, off_t offset);
#include <sys/uio.h>
ssize_t
readv(int fd, const struct iovec *iov, int iovcnt);
ssize_t
preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
DESCRIPTION
The read() system call attempts to read nbytes of data from the object
referenced by the descriptor fd into the buffer pointed to by buf. The
readv() system call 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 pread() and preadv() system
calls perform the same functions, but read from the specified position in
the file without modifying the file pointer.
For readv() and preadv(), the iovec structure is defined as:
struct iovec {
void *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. The readv() system call will always
fill an area completely before proceeding to the next.
On objects capable of seeking, the read() starts at a position given by
the pointer associated with fd (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, read(), 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.
In accordance with IEEE Std 1003.1-2004 ("POSIX.1"), both read(2) and
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 read(2) or write(2), syscalls, each call will see either
all of the changes of the other call, or none of them. The FreeBSD
kernel implements this guarantee by locking the file ranges affected by
the calls.
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 errno is set to indicate the error.
ERRORS
The read(), readv(), pread() and preadv() system calls will succeed
unless:
[EBADF] The fd argument is not a valid file or socket
descriptor open for reading.
[ECONNRESET] The fd argument refers to a socket, and the remote
socket end is forcibly closed.
[EFAULT] The buf argument points outside the allocated address
space.
[EIO] An I/O error occurred while reading from the file
system.
[EINTEGRITY] Corrupted data was detected while reading from the
file system.
[EBUSY] Failed to read from a file, e.g. /proc/<pid>/regs
while <pid> is not stopped
[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.
[EINVAL] The pointer associated with fd was negative.
[EAGAIN] The file was marked for non-blocking I/O, and no data
were ready to be read.
[EISDIR] The file descriptor is associated with a directory.
Directories may only be read directly by root if the
filesystem supports it and the
security.bsd.allow_read_dir sysctl MIB is set to a
non-zero value. For most scenarios, the readdir(3)
function should be used instead.
[EOPNOTSUPP] The file descriptor is associated with a file system
and file type that do not allow regular read
operations on it.
[EOVERFLOW] The file descriptor is associated with a regular file,
nbytes is greater than 0, offset is before the end-of-
file, and offset is greater than or equal to the
offset maximum established for this file system.
[EINVAL] The value nbytes is greater than INT_MAX.
In addition, readv() and preadv() may return one of the following errors:
[EINVAL] The iovcnt argument was less than or equal to 0, or
greater than IOV_MAX.
[EINVAL] One of the iov_len values in the iov array was
negative.
[EINVAL] The sum of the iov_len values in the iov array
overflowed a 32-bit integer.
[EFAULT] Part of the iov array points outside the process's
allocated address space.
The pread() and preadv() system calls may also return the following
errors:
[EINVAL] The offset value was negative.
[ESPIPE] The file descriptor is associated with a pipe, socket,
or FIFO.
SEE ALSO
dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2),
socket(2), socketpair(2), fread(3), readdir(3)
STANDARDS
The read() system call is expected to conform to IEEE Std 1003.1-1990
("POSIX.1"). The readv() and pread() system calls are expected to
conform to X/Open Portability Guide Issue 4, Version 2 ("XPG4.2").
HISTORY
The preadv() system call appeared in FreeBSD 6.0. The pread() function
appeared in AT&T System V Release 4 UNIX. The readv() system call
appeared in 4.2BSD. The read() function appeared in Version 1 AT&T UNIX.
FreeBSD 13.1-RELEASE-p6 June 4, 2020 FreeBSD 13.1-RELEASE-p6
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