Naming conventions in UNIX C functions (_t and _st)

Question

I notice that there are some function return types named *****_t or ******_st. What do "_st" and "_t" mean?

Answer 1

POSIX reserves names ending _t for types. Although it is quite common to see code that invents its own type names ending _t, doing so is dangerous - you can run into POSIX systems that define a (different) type with the same name.

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In the libmemcached source, it looks like the _st suffix is used to indicate a structure type:

types.h:typedef struct memcached_st memcached_st;
types.h:typedef struct memcached_stat_st memcached_stat_st;
types.h:typedef struct memcached_analysis_st memcached_analysis_st;
types.h:typedef struct memcached_result_st memcached_result_st;
types.h:// All of the flavors of memcache_server_st
types.h:typedef struct memcached_server_st memcached_server_st;
types.h:typedef const struct memcached_server_st *memcached_server_instance_st;
types.h:typedef struct memcached_server_st *memcached_server_list_st;

I didn't find a single instance of a function ending _st (but I may not have looked hard enough).