Accessing External Shared Images from SAS

The SASCBTBL Attribute Table

Because the MODULE function invokes an external function that SAS knows nothing about, you must supply information about the function's arguments so that the MODULE function can validate them and convert them, if necessary. For example, suppose you want to invoke a routine that requires an integer as an argument. Because SAS uses floating-point values for all of its numeric arguments, the floating point value must be converted to an integer before you invoke the external routine. The MODULE function looks for this attribute information in an attribute table that is referred to by the SASCBTBL fileref.

What Is the SASCBTBL Attribute Table?

The attribute table is a sequential text file that contains descriptions of the routines you can invoke with the MODULE function. The table defines how the MODULE function should interpret supplied arguments when it is building a parameter list to pass to the called routine.

The MODULE function locates the table by opening the file that is referenced by the SASCBTBL fileref. If you do not define this fileref, the MODULE routines simply call the requested shared image routine without altering the arguments.

CAUTION:: Using the MODULE functions without defining an attribute table can cause SAS to crash or force you to reset your computer.
You need to use an attribute table for all external functions that you want to invoke.

Syntax of the Attribute Table

The attribute table should contain the following descriptions:

a description in a ROUTINE statement for each shared image routine you intend to call
descriptions in ARG statements for each argument associated with that routine

At any point in the attribute table file, you can create a comment using an asterisk (*) as the first non-blank character of a line or after the end of a statement (following the semicolon). You must end the comment with a semicolon.

ROUTINE Statement

The syntax of the ROUTINE statement is the following:

ROUTINE name MINARG=minarg MAXARG=maxarg

<CALLSEQ=BYVALUE | BYADDR>

<TRANSPOSE=YES | NO> <MODULE=shareable-image-name>

The following are descriptions of the ROUTINE statement attributes:

ROUTINE name

starts the ROUTINE statement. You need a ROUTINE statement for every shared image function that you intend to call. The value for name must match the routine name or ordinal you specified as part of the 'module' argument in the MODULE function, where module is the name of the shared image (if not specified by the MODULE attribute), followed by a comma, and then the routine name or ordinal. For example, to specify KERNEL32,GetPath in the MODULE function call, the ROUTINE name should be GetPath .

The name argument is required for the ROUTINE statement.

MINARG=minarg

specifies the minimum number of arguments to expect for the shared image routine. In most cases, this value will be the same as MAXARG, but some routines do allow a varying number of arguments. This is a required attribute.

MAXARG=maxarg

specifies the maximum number of arguments to expect for the shared image routine. This is a required attribute.

CALLSEQ=BYVALUE | BYADDR

indicates the calling sequence method used by the shared image routine. Specify BYVALUE for call-by-value and BYADDR for call-by-address. The default value is BYADDR.

FORTRAN and COBOL are call-by-address languages. C is usually call-by-value, although a specific routine might be implemented as call-by-address.

The MODULE function does not require that all arguments use the same calling method. You can identify any exceptions by using the BYVALUE and BYADDR options in the ARG statement.

TRANSPOSE=YES | NO

specifies whether SAS transposes matrices that have both more than one row and more than one column before it calls the shared image routine. This attribute applies only to routines called from within PROC IML with MODULEI, MODULEIC, and MODULEIN.

TRANSPOSE=YES is necessary when you are calling a routine that is written in a language that does not use row-major order to store matrices. (For example, FORTRAN uses column-major order.)

For example, consider the following matrix with three columns and two rows:

            columns
            1   2   3
       --------------
 rows  1 | 10  11  12
       2 | 13  14  15

PROC IML stores this matrix in memory sequentially as 10, 11, 12, 13, 14, 15. However, FORTRAN routines will expect this matrix as 10, 13, 11, 14, 12, 15.

The default value is NO.

MODULE=shareable-image-name

names the executable module (the shared image) in which the routine resides. The MODULE function searches the directories named by the SAS$LIBRARY environment variable. If you specify the MODULE attribute in the ROUTINE statement, then you do not need to include the module name in the module argument of the MODULE function (unless the shared image routine name you are calling is not unique in the attribute table). The MODULE function is described in MODULE Function: OpenVMS.

You can have multiple ROUTINE statements that use the same MODULE name. You can also have duplicate routine names that reside in different shared images.

specifies the type of value that the shared image routine returns. This value will be converted as appropriate, depending on whether you use MODULEC (which returns a character) or MODULEN (which returns a number). The following are the possible return value types:

SHORT: short integer.
USHORT: unsigned short integer.
LONG: long integer.
ULONG: unsigned long integer.
DOUBLE: double-precision floating-point number.
DBLPTR: pointer to a double-precision floating-point number (instead of using a floating-point register). See the documentation for your shared image routine to determine how it handles double-precision floating-point values.
PTR: character string.
[U]INT32: 32-bit unsigned integer.
[U]INT64: 64-bit unsigned integer.

If you do not specify the RETURNS attribute, you should invoke the routine with only the MODULE and MODULEI call routines. You will get unpredictable values if you omit the RETURNS attribute and invoke the routine using the MODULEN/MODULEIN or MODULEC/MODULEIC functions.

ARG Statement

The ROUTINE statement must be followed by as many ARG statements as you specified in the MAXARG= option. The ARG statements must appear in the order that the arguments will be specified within the MODULE routines.

The syntax for each ARG statement is

Here are the descriptions of the ARG statement attributes:

ARG argnum

defines the argument number. This a required attribute. Define the arguments in ascending order, starting with the first routine argument (ARG 1).

NUM | CHAR

defines the argument as numeric or character. This is a required attribute.

If you specify NUM here but pass the routine a character argument, the argument is converted using the standard numeric informat. If you specify CHAR here but pass the routine a numeric argument, the argument is converted using the BEST12 informat.

INPUT | OUTPUT | UPDATE

indicates the argument is either input to the routine, an output argument, or both. If you specify INPUT, the argument is converted and passed to the shared image routine. If you specify OUTPUT, the argument is not converted, but is updated with an outgoing value from the shared image routine. If you specify UPDATE, the argument is converted, passed to the shared image routine and updated with an outgoing value from the routine.

You can specify OUTPUT and UPDATE only with variable arguments (that is, no constants or expressions are allowed).

NOTREQD | REQUIRED

indicates whether the argument is required. If you specify NOTREQD, then the MODULE function can omit the argument. If other arguments follow the omitted argument, identify the omitted argument by including an extra comma as a placeholder. For example, to omit the second argument to routine XYZ, you would specify the following:

call module('XYZ',1,,3);

CAUTION:: Be careful when using NOTREQD; the shared image routine must not attempt to access the argument if it is not supplied in the call to MODULE. If the routine does attempt to access it, your system is likely to crash.

The REQUIRED attribute indicates that the argument is required and cannot be omitted. REQUIRED is the default value.

BYADDR | BYVALUE

indicates whether the argument is passed by reference or by value.

BYADDR is the default value unless CALLSEQ=BYVALUE was specified in the ROUTINE statement, in that case, BYVALUE is the default. Specify BYADDR when you are using a call-by-value routine that also has arguments to be passed by address.

FDSTART

indicates that the argument begins a block of values that are grouped into a structure whose pointer is passed as a single argument. Note that all subsequent arguments are treated as part of that structure until the MODULE function encounters another FDSTART argument.

FORMAT=format

names the format that presents the argument to the shared image routine. Any formats, supplied by SAS, PROC FORMAT style formats, or SAS/TOOLKIT formats are valid. Note that this format must have a corresponding valid informat if you specified the UPDATE or OUTPUT attribute for the argument.

The FORMAT= attribute is not required, but is recommended, because format specification is the primary purpose of the ARG statements in the attribute table.

CAUTION:: Using an incorrect format can produce invalid results or cause a system crash.

The Importance of the Attribute Table

The MODULE function relies heavily on the accuracy of the information in the attribute table. If this information is incorrect, unpredictable results can occur (including a system crash).

Consider an example routine xyz that expects two arguments: an integer and a pointer. The integer is a code indicating what action takes place. For example, action 1 means that a 20-byte character string is written into the area that is pointed to by the second argument, the pointer.

Now suppose you call xyz using the MODULE function, but you indicate in the attribute table that the receiving character argument is only 10 characters long:

routine xyz minarg=2 maxarg=2;
arg 1 input num byvalue format=ib4.;
arg 2 output char format=$char10.;

Regardless of the value given by the LENGTH statement for the second argument to MODULE, MODULE passes a pointer to a 10-byte area to the xyz routine. If xyz writes 20 bytes at that location, the 10 bytes of memory following the string provided by MODULE are overwritten, causing unpredictable results:

data _null_;
    length x $20;
    call module('xyz',1,x);
run;

The call might work fine, depending on which 10 bytes were overwritten. However, this might also cause you to lose data or cause your system to crash.

Also, note that the PEEKLONG and PEEKCLONG functions rely on the validity of the pointers you supply. If the pointers are invalid, it is possible that SAS could crash. For example, this code would cause a crash:

data _null_;
   length c $10;
     /* trying to copy from address 0!!!*/
   c = peekclong(0,10); 
run;

Note: SAS does not support return types for 64-bit pointers. [cautionend]

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