USC: A Universal Stub Compiler

base types in any byte order. USC requires stub parameters that give the lengths of any variable length array arguments. The code below copies one variable length character array to another. len gives the current length of the arrays. void foo (len, str1, str2) int len; /* Must be a native type */ char str1[len]; /* May be native or not */ char str2(1, 4, 4, <0>)[len];/* May be native or not */ { str1 = str2; } Any USC stub that contains a variable length array definition as a parameter must have a native integer parameter whose name matches the name given in the array definition. USC supports the inline qualifier found in many C compilers. A USC stub declared as inline will generate an inline function. In addition USC supports the qualifier macro which directs USC to produce a C macro implementation of the specified stub. Currently the qualifier macro may only be used on stubs returning type void. The stub tcphdr defined in Figure 1 shows how to define a stub to copy a TCP header from network format to DecStation 5000 native format. The generated C code swaps bytes and realigns the data. Less traditional stubs can also be generated. It is often useful to read and write fields into a network header stored in network format. A stub that peeks into a TCP header in network format and returns the offset field in a four byte, big-endian, integer would be defined as follows: int(4,4,4,<3,2,1,0>) tcpgetoff (net_hdr *hdr) { return hdr->off; } USC also provides for in-place modification of a data value. The pragma alias can be used to inform USC that two parameters will be aliased every time the procedure is called. To generate a stub which is intended to do an in-place translation of a tcp header one would use the following USC stub definition: void tcphdr(net_hdr *src, native_hdr *dest) { #pragma alias(src,dest) *src = *dest; } The stub generated assumes that the parameter dest is aliased with the parameter src. On machines where the layout of network_hdr and native_hdr are the same no code will be generated. USC will generate correct code if parameters to a stub are aliased regardless of the use of the alias pragma. However, such code will not be optimal. Note that only pointers may be aliased in this way and inplace conversion between two types with different lengths …