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92 lines
1.8 KiB
Groff
92 lines
1.8 KiB
Groff
.TH READ 3
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.SH NAME
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read, readn, write, pread, pwrite \- read or write file
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.PP
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.B
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long read(int fd, void *buf, long nbytes)
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.PP
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.B
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long readn(int fd, void *buf, long nbytes)
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.PP
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.B
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long write(int fd, void *buf, long nbytes)
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.PP
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.B
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long pread(int fd, void *buf, long nbytes, vlong offset)
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.PP
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.B
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long pwrite(int fd, void *buf, long nbytes, vlong offset)
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.SH DESCRIPTION
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.I Read
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reads
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.I nbytes
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bytes of data
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from the offset in the file associated with
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.I fd
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into memory at
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.IR buf .
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The offset is advanced by the number of bytes read.
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It is not guaranteed
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that all
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.I nbytes
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bytes will be read; for example
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if the file refers to the console, at most one line
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will be returned.
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In any event the number of bytes read is returned.
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A return value of
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0 is conventionally interpreted as end of file.
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.PP
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.I Readn
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is just like read, but does successive
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.I read
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calls until
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.I nbytes
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have been read, or a read system call
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returns a non-positive count.
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.PP
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.I Write
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writes
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.I nbytes
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bytes of data starting at
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.I buf
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to the file associated with
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.I fd
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at the file offset.
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The offset is advanced by the number of bytes written.
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The number of characters actually written is returned.
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It should be regarded as an error
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if this is not the same as requested.
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.PP
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.I Pread
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and
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.I Pwrite
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equivalent to a
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.MR seek (3)
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to
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.I offset
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followed by a
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.I read
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or
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.IR write .
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By combining the operations in a single atomic call, they more closely
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match the 9P protocol
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(see
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.IR intro (9p))
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and, more important,
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permit multiprocess programs to execute multiple concurrent
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read and write operations on the same file descriptor
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without interference.
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.SH SOURCE
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.B \*9/src/lib9/readn.c
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.SH SEE ALSO
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.MR intro (3) ,
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.IR open (3),
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.MR dup (3) ,
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.MR pipe (3)
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.SH DIAGNOSTICS
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These functions set
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.IR errstr .
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