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.\"     @(#)kvm_read.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: src/lib/libkvm/kvm_read.3,v 1.6.2.3 2001/12/17 10:08:30 ru Exp $
.\" $DragonFly: src/lib/libkvm/kvm_read.3,v 1.4 2008/09/07 08:36:54 swildner Exp $
.\"
.Dd January 8, 2006
.Dt KVM_READ 3
.Os
.Sh NAME
.Nm kvm_read ,
.Nm kvm_readstr ,
.Nm kvm_write
.Nd read or write kernel virtual memory
.Sh LIBRARY
.Lb libkvm
.Sh SYNOPSIS
.In sys/types.h
.In kvm.h
.Ft ssize_t
.Fn kvm_read "kvm_t *kd" "unsigned long addr" "void *buf" "size_t nbytes"
.Ft "char *"
.Fn kvm_readstr "kvm_t *kd" "unsigned long addr" "char *buf" "size_t *len"
.Ft ssize_t
.Fn kvm_write "kvm_t *kd" "unsigned long addr" "const void *buf" "size_t nbytes"
.Sh DESCRIPTION
The
.Fn kvm_read ,
.Fn kvm_readstr
and
.Fn kvm_write
functions are used to read and write kernel virtual memory (or a crash
dump file). See
.Fn kvm_open 3
or
.Fn kvm_openfiles 3
for information regarding opening kernel virtual memory and crash dumps.
.Pp
The
.Fn kvm_read
function transfers
.Fa nbytes
bytes of data from
the kernel space address
.Fa addr
to
.Fa buf .
Conversely,
.Fn kvm_write
transfers data from
.Fa buf
to
.Fa addr .
Unlike their SunOS counterparts, these functions cannot be used to
read or write process address spaces.
.Pp
The
.Fn kvm_readstr
function exists for convenience to read NUL terminated strings
from the kernel address space.
If
.Fa buf
is
.Dv NULL ,
.Fn kvm_readstr
will allocate a sufficiently large buffer, which needs to be
deallocated via
.Xr free 3
by the caller.
If
.Fa len
is not
.Dv NULL ,
.Fn kvm_readstr
will interpret the value it is pointing to as the size of
.Fa buf
and will store the size of the complete string at
.Fa addr .
Note that if only
.Fa buf
is too small to hold the complete string,
.Fn kvm_readstr
will return
.Dv NULL
but set
.Fa len
to the size needed.
.Sh RETURN VALUES
For
.Fn kvm_read
and
.Fn kvm_write
the number of bytes actually transferred is returned, if the request
was successful.
Otherwise, -1 is returned.
.Pp
For
.Fn kvm_readstr
.Dv NULL
is returned on failure.
Upon success, the address of the string is returned, which will be
.Fa buf
if this was supplied.
.Sh SEE ALSO
.Xr kvm 3 ,
.Xr kvm_close 3 ,
.Xr kvm_getargv 3 ,
.Xr kvm_getenvv 3 ,
.Xr kvm_geterr 3 ,
.Xr kvm_getprocs 3 ,
.Xr kvm_nlist 3 ,
.Xr kvm_open 3 ,
.Xr kvm_openfiles 3