What does a type followed by _t (underscore-t) represent?

c naming-conventions types
Kevin Griffin picture Kevin Griffin · Oct 24, 2008 · Viewed 88.9k times · Source

This seems like a simple question, but I can't find it with the Stack Overflow search or Google. What does a type followed by a _t mean? Such as

int_t anInt;

I see it a lot in C code meant to deal closely with hardware—I can't help but think that they're related.

Answer

Jonathan Leffler picture Jonathan Leffler · Oct 24, 2008

As Douglas Mayle noted, it basically denotes a type name. Consequently, you would be ill-advised to end variable or function names with '_t' since it could cause some confusion. As well as size_t, the C89 standard defines wchar_t, off_t, ptrdiff_t, and probably some others I've forgotten. The C99 standard defines a lot of extra types, such as uintptr_t, intmax_t, int8_t, uint_least16_t, uint_fast32_t, and so on. These new types are formally defined in <stdint.h> but most often you will use <inttypes.h> which (unusually for standard C headers) includes <stdint.h>. It (<inttypes.h>) also defines macros for use with the printf() and scanf().

As Matt Curtis noted, there is no significance to the compiler in the suffix; it is a human-oriented convention.

However, you should also note that POSIX defines a lot of extra type names ending in '_t', and reserves the suffix for the implementation. That means that if you are working on POSIX-related systems, defining your own type names with the convention is ill-advised. The system I work on has done it (for more than 20 years); we regularly get tripped up by systems defining types with the same name as we define.

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