XOPEN
Written June 2020
XOPEN #ch, fspec$, mode, recsiz {, flags}
XOPEN is an alternative to the traditional OPEN statement, offering the following advantages:
| • | Replacement of the literal compiler keywords (e.g. WAIT'RECORD) with a flexible flags parameter, allowing a single generic, parameterized, XOPEN statement to be used for all variations. |
| • | Elimination of the requirement for the recnovar to remain in scope, by eliminating it entirely. Instead, you can specify the record number explicitly using the XREAD, XWRITE, XUNLOKR or SET'RECNO statements. |
Parameters
ch (Num) [in]
an integer expression (>0) specifying the file channel which will be needed for subsequent I/O operations on the file.
fspec (String) [in]
string expression containing file specification, in native or AMOS syntax. May contain embedded %env% variables, e.g. "%temp%\myfile.dat". If no directory information is included, the search path consists of the current directory, followed by the [p,0] directory.
mode (literal keyword) [in]
specifies the file access mode, and must be one of the following (case insensitive) compiler keywords:
Symbol |
Description |
|---|---|
RANDOM |
When LOKSER enabled, requests exclusive access; otherwise same as RANDOM'FORCED |
RANDOM'FORCED |
Shared access |
Note that unlike with the traditional OPEN, here the you can specify RANDOM and then use the flags argument to change it to RANDOM'FORCED, thereby eliminating the need for two separate literal variations of the XOPEN statement in your code.
recsiz (Num) [in]
integer expression specifying the record size—i.e. the number of bytes transferred in each subsequent READ and WRITE operations. See SPAN'BLOCKS below.
flags (B,4 or numeric expression) [in]
flags optionally specifies zero or more of the following modifiers:
Symbol |
Value |
Meaning |
|---|---|---|
FDVF_FORCED |
&h0010 |
Convert RANDOM to RANDOM'FORCED (shared) |
FDVF_EXCLUSIVE |
&h0800 |
Convert INDEXED to INDEXED'EXCLUSIVE |
FDVF_READONLY |
&h1000 |
READONLY |
FDVF_SPANBLOCKS |
&h2000 |
SPAN'BLOCKS |
FDVF_W_RECORD |
&h4000 |
WAIT'RECORD |
FDVF_W_FILE |
&h8000 |
WAIT'FILE |
Definition file: ashinc:addsfdv.def |
||
Note that if flags is expressed as a variable, it must be of type B,4. Since flags occupies the same position as recnovar in the traditional OPEN statement, requiring it to be of type B,4 here reduces the chance of accidentally confusing the two statements in your coding.
Example
XOPEN makes it possible to create a generic function for opening random files, such as the following:
call Fn'XOPEN'Random(ch=CH, fspec$="test.dat", recsiz=128, flags=FDVF_FORCED+FDVF_W_RECORD+FDVF_FILE)
XREAD #CH, 1 ! read rec # 1
...
Function Fn'XOPEN'Random(ch as b2:inputonly, fspec$ as s260:inputonly, recsiz as b2:inputonly, flags as b4:inputonly) as i4
XOPEN #ch, fspec$, RANDOM, recsiz, flags
EndFunction
The above function could easily be upgraded to trap errors and deal file locking conflicts, including logging, user interfacing, etc., allowing you to implement all of that logic in a single place with more modularity than would otherwise be possible with the traditional OPEN statement.
See Also
| • | OPEN |
