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.TH READ 2

.SH NAME

read, readn, write, pread, pwrite \- read or write file

.SH SYNOPSIS

.B #include <u.h>

.br

.B #include <libc.h>

.PP

.B

long read(int fd, void *buf, long nbytes)

.PP

.B

long readn(int fd, void *buf, long nbytes)

.PP

.B

long write(int fd, void *buf, long nbytes)

.PP

.B

long pread(int fd, void *buf, long nbytes, vlong offset)

.PP

.B

long pwrite(int fd, void *buf, long nbytes, vlong offset)

.SH DESCRIPTION

.I Read

reads

.I nbytes

bytes of data

from the offset in the file associated with

.I fd

into memory at

.IR buf .

The offset is advanced by the number of bytes read.

It is not guaranteed

that all

.I nbytes

bytes will be read; for example

if the file refers to the console, at most one line

will be returned.

In any event the number of bytes read is returned.

A return value of

.PP

.I Readn

is just like read, but does successive

.I read

calls until

.I nbytes

have been read, or a read system call

returns a non-positive count.

.PP

.I Write

writes

.I nbytes

bytes of data starting at

.I buf

to the file associated with

.I fd

at the file offset.

The offset is advanced by the number of bytes written.

The number of characters actually written is returned.

It should be regarded as an error

if this is not the same as requested.

.PP

.I Pread

and

.I Pwrite

are equivalent to a

.IR seek (2)

to

.I offset

followed by a

.I read

or

.IR write .

By combining the operations in a single atomic call, they more closely

match the 9P protocol

(see

.IR intro (5))

and, more important,

permit multiprocess programs to execute multiple concurrent

read and write operations on the same file descriptor

without interference.

.SH SOURCE

.B /sys/src/libc/9syscall

.br

.B /sys/src/libc/port/readn.c

.SH SEE ALSO

.IR intro (2),

.IR dirread (2),

.IR dup (2),

.IR open (2),

.IR pipe (2),

.IR readv (2)

.SH DIAGNOSTICS

These functions set

.IR errstr .