There seems to be no perfect way. The root of the problem is that clock_t
can be either integer or floating point.
clock_t can be a floating point type
As Bastien Léonard mentions for POSIX (go upvote him), C99 N1256 draft 7.23.1/3 also says that:
[clock_t is] arithmetic types capable of representing times
and 6.2.5/18:
Integer and floating types are collectively called arithmetic types.
and the standard defines arithmetic type as either integers or floating point types.
If you will divide by CLOCKS_PER_SEC, use long double
The return value of clock()
is implementation defined, and the only way to get standard meaning out of it is to divide by CLOCKS_PER_SEC
to find the number of seconds:
clock_t t0 = clock();
/* Work. */
clock_t t1 = clock();
printf("%Lf", (long double)(t1  t0));
This is good enough, although not perfect, for the two following reasons:

there seems to be no analogue to
intmax_t
for floating point types: How to get the largest precision floating point data type of implemenation and its printf specifier? So if a larger floating point type comes out tomorrow, it could be used and break your implementation. 
if
clock_t
is an integer, the cast to float is well defined to use the nearest float possible. You may lose precision, but it would not matter much compared to the absolute value, and would only happen for huge amounts of time, e.g.long int
in x86 is the 80bit float with 64bit significant, which is millions of years in seconds.
Go upvote lemonad who said something similar.
If you suppose it is an integer, use %ju and uintmax_t
Although unsigned long long
is currently the largest standard integer type possible:
 a larger one could come out in the future
 the standard already explicitly allows larger implementation defined types (kudos to @FUZxxl) and
clock_t
could be one of them
so it is best to typecast to the largest unsigned integer type possible:
#include <stdint.h>
printf("%ju", (uintmax_t)(clock_t)1);
uintmax_t
is guaranteed to have the size of the largest possible integer size on the machine.
uintmax_t
and its printf specifier %ju
were introduced in c99 and gcc for example implements them.
As a bonus, this solves once and for all the question of how to reliably printf
integer types (which is unfortunately not the necessarily the case for clock_t
).
What could go wrong if it was a double:
 if too large to fit into the integer, undefined behavior
 much smaller than 1, will get rounded to 0 and you won’t see anything
Since those consequences are much harsher than the integer to float conversion, using float is likely a better idea.
On glibc 2.21 it is an integer
The manual says that using double
is a better idea:
On GNU/Linux and GNU/Hurd systems, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the macro CLOCKS_PER_SEC can be either integer or floatingpoint types. Casting CPU time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.
In glibc 2.21:

clock_t
islong int
: time/time.h sets it to
__clock_t
 bits/types.h sets it to
__CLOCK_T_TYPE
 bits/typesizes.h sets it to
__SLONGWORD_TYPE
 bits/types.h sets it to
long int
 time/time.h sets it to

clock()
in Linux is implemented withsys_clock_gettime
: sysdeps/unix/sysv/linux/clock.c calls
__clock_gettime
 sysdeps/unix/clock_gettime.c calls
SYSDEP_GETTIME_CPU
 sysdeps/unix/sysv/linux/clock_gettime.c calls
SYSCALL_GETTIME
which finally makes an inline system call
man clock_gettime
, tells us that it returns astruct timespec
which in GCC containslong int
fields.So the underlying implementation really returns integers.
 sysdeps/unix/sysv/linux/clock.c calls
See also
 How to print types of unknown size like ino_t?
 How to use printf to display off_t, nlink_t, size_t and other special types?