Erlang: Fail to make a gen_server:call() - erlang

Question

Given the following code:
-module(appender_server).
-behaviour(gen_server).
-export([start_link/1, stop/0]).
-export([append_synched/2]).
-export([init/1, handle_call/3, handle_cast/2]).
start_link(FilePath) ->
gen_server:start_link({local, ?MODULE}, ?MODULE, FilePath, []).
init(FilePath) ->
{ok, TheFile} = file:open(FilePath, [append]), % File is created if it does not exist.
io_lib:print(TheFile).
append_synched(FilePath, Text) ->
gen_server:call(?MODULE, {append_synched, FilePath, Text}).
handle_call({append_synched, FilePath, Text}, _From, LoopData) ->
io:fwrite("Hello!~n", []),
io:fwrite("LoopData:~p~n", LoopData),
io:fwrite("~p~n", FilePath),
io:fwrite("~p~n", Text).
handle_cast(append_asynched, Str) ->
{ok, theFile} = file:write_file(theFile, Str).
stop() ->
gen_server:cast(?MODULE, stop).
I then use the erlang shell to call the following and the get the errors:
1> cd("c:/Users/TalTe/WebAppsProjects/fileappender/src/").
c:/Users/TalTe/WebAppsProjects/fileappender/src
ok
2> c(appender_server).
{ok,appender_server}
3> appender_server:start_link("c:/temp/file.txt").
{error,{bad_return_value,"<0.88.0>"}}
4> =CRASH REPORT==== 2-Dec-2020::23:30:19.687000 ===
crasher:
initial call: appender_server:init/1
pid: <0.87.0>
registered_name: appender_server
exception exit: {bad_return_value,"<0.88.0>"}
in function gen_server:init_it/6 (gen_server.erl, line 409)
ancestors: [<0.79.0>]
message_queue_len: 0
messages: []
links: [<0.79.0>]
dictionary: []
trap_exit: false
status: running
heap_size: 376
stack_size: 28
reductions: 257
neighbours:
neighbour:
pid: <0.79.0>
registered_name: []
initial_call: {erlang,apply,2}
current_function: {io,execute_request,2}
ancestors: []
message_queue_len: 0
links: [<0.78.0>,<0.87.0>]
trap_exit: false
status: waiting
heap_size: 1598
stack_size: 26
reductions: 5365
current_stacktrace: [{io,execute_request,2,[{file,"io.erl"},{line,571}]},
{shell,exprs,7,[{file,"shell.erl"},{line,693}]},
{shell,eval_exprs,7,[{file,"shell.erl"},{line,642}]},
{shell,eval_loop,3,[{file,"shell.erl"},{line,627}]}]
** exception error: {bad_return_value,"<0.88.0>"}
4> appender_server:append_synched("c:/temp:/file.txt", "Hello").
** exception exit: {noproc,{gen_server,call,
[appender_server,
{append_synched,"c:/temp:/file.txt","Hello"}]}}
in function gen_server:call/2 (gen_server.erl, line 238)
5>
Please help me understand what am I doing wrong here?
BTW, the file was created despite the error on step 3.

Answer 1

The init callback should return {ok, SomeState}. In your case the return value of init is the return value of io_lib:print()