A text_io problem: ORA-302000

Question

Hi,all:
I have 2 apps on form.
The first one is to read from file, and the second is to write on the file.
Firstly, i have created the file on the server, and the file(abc.txt)'s permission is rwxr--r--.
Then i tested the first one, it can read from file well.
But when i tested the second, it throwed a exception: ORA-302000.
I was so confused. why it can read from file with text_io, but can not write?
Can anyone help me to solve this problem, Thanks so much.
Here's my code:
1.WHEN-BUTTON-PRESSED(to read): works fine
DECLARE
     in_file text_io.file_type;
     line_bufer varchar2(80);
     lv_strtmp varchar2(100);
     errnum NUMBER                := ERROR_CODE;
     errtxt VARCHAR2(80) := ERROR_TEXT;
     errtyp VARCHAR2(3)      := ERROR_TYPE;
BEGIN
     in_file := text_io.fopen('/home/cn01278/abc.txt', 'r');
     text_io.Get_Line(in_file,line_bufer);
     lv_strtmp := substr(line_bufer, 1,10);
     :blk_text.txt_content := :blk_text.txt_read_err || lv_strtmp;
     text_io.Fclose(in_file);
END;
2.WHEN-BUTTON-PRESSED(to write): thown an exception
DECLARE
          out_file text_io.file_type;
          line_bufer varchar2(80);
          lv_strtmp varchar2(100);
          lv_content varchar2(100);
          errnum NUMBER := ERROR_CODE;
          errtxt VARCHAR2(80) := ERROR_TEXT;
          errtyp VARCHAR2(3) := ERROR_TYPE;
BEGIN
     message('111',acknowledge);
          out_file := text_io.fopen('/home/cn01278/abc.txt', 'w');
          message('222',acknowledge);
          text_io.put('123');
          text_io.new_line;
          text_io.put_line(out_file, 'abcd...');
     text_io.fclose(out_file);
END;
3.ON-ERROR Trigger
DECLARE
     errnum NUMBER := ERROR_CODE;
     errtxt VARCHAR2(80) := ERROR_TEXT;
     errtyp VARCHAR2(3) := ERROR_TYPE;
BEGIN
message('error:',acknowledge);
     Message(errtyp||'-'||TO_CHAR(errnum)||': '||errtxt,acknowledge);
     --:blk_text.txt_err := errtyp||'-'||TO_CHAR(errnum)||': '||errtxt;
     text_io.fclose(out_file);
     RAISE Form_Trigger_Failure;
END;
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Answer 1

Thanks for attension.
the problem is resolved, it's a access/right problem with the file(abc.txt).
after i change the file permission from rwxr--r-- to rwxrw-rw-, then it works fine.
But I have some confuse about it.
1.The file must exist firstly. if not, then read or wirte the file will cause the ORA-302000 error. Can the file not exist? Can anyone tell me how to create file(abc.txt) with text_io if the file not exist.
2.If the file was created by A, and A have rw rights, but the group owner have no rw rights, then you run the form by A(connect DB by A), it would cause ORA-302000 error too(both read and write).Why the group owner must have rw rights, or the form can not works fine?
By the way, my DB account and my App server(Linux) account are same(both A).