What is time_t ultimately a typedef to?

The time_t Wikipedia article article sheds some light on this. The bottom line is that the type of time_t is not guaranteed in the C specification.

The time_t datatype is a data type in the ISO C library defined for storing system time values. Such values are returned from the standard time() library function. This type is a typedef defined in the standard header. ISO C defines time_t as an arithmetic type, but does not specify any particular type, range, resolution, or encoding for it. Also unspecified are the meanings of arithmetic operations applied to time values.

Unix and POSIX-compliant systems implement the time_t type as a signed integer (typically 32 or 64 bits wide) which represents the number of seconds since the start of the Unix epoch: midnight UTC of January 1, 1970 (not counting leap seconds). Some systems correctly handle negative time values, while others do not. Systems using a 32-bit time_t type are susceptible to the Year 2038 problem.

[root]# cat time.c

#include <time.h>

int main(int argc, char** argv)
{
        time_t test;
        return 0;
}

[root]# gcc -E time.c | grep __time_t

typedef long int __time_t;

It's defined in $INCDIR/bits/types.h through:

# 131 "/usr/include/bits/types.h" 3 4
# 1 "/usr/include/bits/typesizes.h" 1 3 4
# 132 "/usr/include/bits/types.h" 2 3 4

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