The SUSv3 namespace excludes all withdrawn headers, functions, external variables, and constants. The specification further targets additional symbols for removal in a future version of the standard.
As noted earlier, Single UNIX Specification, Version 3 aligns with ISO/IEC 9899:1999, is commonly referred to as the C99 language standard. In some cases, SUSv3 extends the C99 definition, although in the case of a conflict, it always defers to the C99 standard. For this reason, applications compiled for SUSv3 are also implicitly C99, and you do not need to define feature test macro _ISOC99_SOURCE when _XOPEN_SOURCE 600 or _POSIX_C_SOURCE 200112L is defined.
Beyond these obvious differences in the library, there are other behavioral differences. One of the more notable differences is the change in the return value of most threads functions. The POSIX.4a, draft 6 threads behavior indicates a return of -1 on failure with the error code set in errno. In SUSv3, the majority of these functions now return the error code on failure rather than a value of -1. With the exception of pthread_getspecific(), the z/OS implementation will continue to set errno in addition to returning the error code.
For complete details about syntactical differences and special behavior of functions, see the individual function descriptions in z/OS XL C/C++ Runtime Library Reference.