Does BaseUnix.FpIOCtl
meet your use case? Have a look at the BaseUnix documentation. I found an example of using it here (reposted below).
program testrpi;
{$mode objfpc}{$H+}
uses
baseUnix,
classes,
{$IFDEF UNIX}{$IFDEF UseCThreads}
cthreads,
{$ENDIF}{$ENDIF}
sysutils;
const
I2C_SLAVE = 1795;
var
buf : packed array [0..1] of char;
c : char;
devPath : string = '/dev/i2c-1';
handle : Cint;
iDevAddr : Cint = $04;
begin
try
handle := fpopen(devPath,O_RDWR);
fpIOCtl(handle, I2C_SLAVE, pointer(iDevAddr));
except
writeln('Error initalizing i2c');
halt;
end;
while true do begin
write('Enter digit 1-9:');
readln(c);
if (not(c in ['1'..'9'])) then begin
writeln('oops - try again');
continue;
end;
buf[0] := chr(ord(c) - ord('0'));
try
fpwrite(handle, buf, 1);
except
writeln('Error writing');
halt;
end; //try
buf[0] := #99;
sleep(10);
try
fpread(handle, buf, 1);
except
writeln('Error reading');
halt;
end; //try
writeln('buf=', ord(buf[0]));
end; //while
fpclose(handle);
end.