Entry points

An entry point in a program module is a location that is known by name to the operating system and which can be referenced by or receive control from another module. In PDS and PDSE libraries entry points are represented by directory entries; entry points in z/OS UNIX files are each represented by a file name in the z/OS UNIX directory structure.

There are five types of entry points in program modules:

Primary entry point. This is the point that receives control when the module is invoked by its primary, or member, name. The primary name is the name that was specified on the NAME control statement or the SYSLMOD dd-statement when the module was created.
Alternate entry point. Alternate entry points are locations, other than the primary entry, which can receive control or be referenced from another module. An alternate entry point is defined during binding by use of an ALIAS control statement (or ADDAlias API function) that specifies the name of an external label in the program.
True alias. A true alias is another name associated with the primary entry point. It is also defined with an ALIAS control statement, but is not an external label in the module.
Alternate primary. MVS™ places certain restrictions on the lengths of member names and aliases. If you specify a name on the NAME control statement that exceeds the 8-byte limitation for member names, the binder will generate an 8-byte primary name and store the specified name as a true alias. This alias is referred to as the alternate primary and flagged in the directory entry. The primary entry is also referred to as the generated primary.

The linkage editor does not support alternate primaries or any entry point name longer than eight bytes.

The way entry points are represented in the system depends on the type of file in which the module is stored:

PDSE program libraries support all of the entry point types listed above as directory entries. The primary or generated primary name becomes the member name and is limited to eight bytes. Alternate entry points, true aliases and the alternate primary are stored as aliases and are limited in length to 1024 bytes.
Partitioned data set (PDS) program libraries support primary entry point, alternate entry point and true alias names up to a maximum of eight bytes. The primary entry point appears as the primary directory entry; aliases and alternate entry points appear as alias directory entries. Alternate primaries are not supported in a PDS.
z/OS UNIX-resident program objects can contain primary names and true aliases only. All names are limited to 255 bytes, not including the path name. Alternate entry points and alternate primary entry points are not supported. As far as UNIX System Services is concerned, there is no difference between primary names and alias names.