What is time_t ultimately a typedef to?

Question

I searched my Linux box and saw this typedef:

typedef __time_t time_t;

But I could not find the __time_t definition.

Answer 1

Under Visual Studio 2008, it defaults to an __int64 unless you define _USE_32BIT_TIME_T. You're better off just pretending that you don't know what it's defined as, since it can (and will) change from platform to platform.

Answer 2

What is ultimately a time_t typedef to?

Robust code does not care what the type is.

C species time_t to be a real type like double, long long, int64_t, int, etc.

It even could be unsigned as the return values from many time function indicating error is not -1, but (time_t)(-1) - This implementation choice is uncommon.

The point is that the "need-to-know" the type is rare. Code should be written to avoid the need.

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Yet a common "need-to-know" occurs when code wants to print the raw time_t. Casting to the widest integer type will accommodate most modern cases.

time_t now = 0;
time(&now);
printf("%jd", (intmax_t) now);
// or 
printf("%lld", (long long) now);

Casting to a double or long double will work too, yet could provide inexact decimal output

printf("%.16e", (double) now);

Answer 3

Standards

William Brendel quoted Wikipedia, but I prefer it from the horse's mouth.

C99 N1256 standard draft 7.23.1/3 "Components of time" says:

The types declared are size_t (described in 7.17) clock_t and time_t which are arithmetic types capable of representing times

and 6.2.5/18 "Types" says:

Integer and floating types are collectively called arithmetic types.

POSIX 7 sys_types.h says:

[CX] time_t shall be an integer type.

where [CX] is defined as:

[CX] Extension to the ISO C standard.

It is an extension because it makes a stronger guarantee: floating points are out.

gcc one-liner

No need to create a file as mentioned by Quassnoi:

echo | gcc -E -xc -include 'time.h' - | grep time_t

On Ubuntu 15.10 GCC 5.2 the top two lines are:

typedef long int __time_t;
typedef __time_t time_t;

Command breakdown with some quotes from man gcc:

• -E: "Stop after the preprocessing stage; do not run the compiler proper."
• -xc: Specify C language, since input comes from stdin which has no file extension.
• -include file: "Process file as if "#include "file"" appeared as the first line of the primary source file."
• -: input from stdin

Answer 4

It's a 32-bit signed integer type on most legacy platforms. However, that causes your code to suffer from the year 2038 bug. So modern C libraries should be defining it to be a signed 64-bit int instead, which is safe for a few billion years.