The final cut. |
How (explicit) "rollback" works? To discover this kind of magic we're cooking hello_world3.c example.
Example 2-3. hello_world2.c
1 #include <jf_file.h>
2 int main()
3 {
4 int rc;
5 jf_file_t jf;
6 size_t write;
7 rc = jf_file_open(&jf, NULL, "jf_tut_foo", "w", NULL);
8 if (JF_RC_OK != rc)
9 return 1;
10 rc = jf_file_printf(&jf, &write, "%s", "Hello");
11 if (JF_RC_OK != rc)
12 return 1;
13 rc = jf_file_rollback(&jf);
14 if (JF_RC_OK != rc)
15 return 1;
16 rc = jf_file_printf(&jf, &write, "%s", " world!\n");
17 if (JF_RC_OK != rc)
18 return 1;
19 rc = jf_file_commit(&jf);
20 if (JF_RC_OK != rc)
21 return 1;
22 rc = jf_file_close(&jf);
23 if (JF_RC_OK != rc)
24 return 1;
25 printf("Hello world III program is OK!\n");
26 return 0;
27 }
Hello world III code explanation
to use libjf a program must include at least jf_file.h header file
declare object jf of type jf_file_t: jf is a "journaled file object"
open (create & open) for write ("w") a journaled
file of name jf_tut_foo and associate it to
object jf; second argument
(NULL) indicates a private journal
must be used, fifth argument (NULL) tells
default parameters must be used
write to journaled file string "Hello"; second argument
is used to
retrieve the number of bytes stored to journaled file, other
arguments mimic fprintf function
rollback changes previously operated on journaled file jf: write of string "Hello" will be discarded (undone)
write to journaled file different stuff (string " world!\n")
commit changes previously operated on journaled file jf
close journaled file
Use a command like this to compile hello_world3.c source code:
libtool --mode=link gcc -Wall -I/opt/libjf/include -L/opt/libjf/lib -ljf \
-o hello_world3 hello_world3.c
remove temporary files wrote by first and/or second episode of
hello world saga:
tiian@linux:~/tutorial> rm -f jf_tut_foo*
and run hello_world3 program:
tiian@linux:~/tutorial> ./hello_world3
Hello world III program is OK!
Take a look to journaled file and associated private journal:
tiian@linux:~/tutorial> ls -la jf_tut_foo*
-rw-r--r-- 1 tiian users 8 2005-08-11 14:11 jf_tut_foo
-rw-r--r-- 1 tiian users 8306 2005-08-11 14:11 jf_tut_foo.jf
inspect journaled file jf_tut_foo:
tiian@linux:~/tutorial> cat jf_tut_foo
world!
The first string ("Hello") was not stored in journaled file
because an explicit rollback was performed; inspect journal content:
tiian@linux:~/tutorial> jf_report -df -j jf_tut_foo.jf
<?xml version="1.0" encoding="UTF-8"?>
<journal>
<header magic_number='0x41524153' version='1' file_id_mask='0x8'
file_id_mask_shift='3' size_mask='0xfffffff0' size_mask_shift='4'
file_size='4194304' file_num='3' rotation_threshold='0.800'
ctrl_recs='36' journal_recs='8278' />
<journaled_file_table max_files='2' number_of_files='2'
file_table='0x804b170'>
<file id='0' name='jf_tut_foo.jf' last_pos='8306' last_size='0'
status='0' last_uc_pos='0' last_uc_size='8306' stream='0x804b008' />
<file id='1' name='jf_tut_foo' last_pos='8' last_size='8' status='2'
last_uc_pos='0' last_uc_size='0' stream='(nil)' />
</journaled_file_table>
<records>
<append jrn_rec_off='8278' file_id='1' size='8' offset='0'>
<data type='redo' format='hex'>20 77 6f 72 6c 64 21 0a </data>
<data type='redo' format='text'> world! </data>
</append>
<commit jrn_rec_off='8302' file_id='1'/>
</records>
</journal>