Why doesn't sleep work?

Question

Why does c_sleep return immediately in the following code?

{-# LANGUAGE ForeignFunctionInterface #-}
import Foreign.C.Types
import Data.Time.Clock
import Control.Concurrent

foreign import ccall unsafe "unistd.h sleep" 
    c_sleep :: CUInt -> IO CUInt

main :: IO ()
main = do
    getCurrentTime >>= print . utctDayTime
    c_sleep 10     >>= print                -- this doesn't sleep
    getCurrentTime >>= print . utctDayTime
    threadDelay $ 10 * 1000 * 1000          -- this does sleep
    getCurrentTime >>= print . utctDayTime

$ ghc --make Sleep.hs && ./Sleep
[1 of 1] Compiling Main             ( Sleep.hs, Sleep.o )
Linking Sleep ...
29448.191603s
10
29448.20158s
29458.211402s

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.3

$ cabal --version
cabal-install version 1.20.0.3
using version 1.20.0.0 of the Cabal library 

Note: Actually, I would like to use sleep in C code to simulate some heavy computation in a function func and call that function in Haskell, but that doesn't work either, probably for the same reasons.

Answer 1

GHC's RTS appears to use signals for its own purposes, which means it won't be long before a sleep gets interrupted by one of these signals. I don't think it's a bug either, the runtime does come with its own territory, so to speak. The Haskellian approach would be to use threadDelay but it's not easy for a C program to access that without some trickery.

The proper way is to repeatedly resume the sleep despite interruptions from other signals. I recommend using nanosleep since sleep only has a precision of seconds and the signals appear to occur much more frequently than that.

#include <errno.h>
#include <time.h>

/* same as 'sleep' except it doesn't get interrupted by signals */
int keep_sleeping(unsigned long sec) {
    struct timespec rem, req = { (time_t) sec, 0 }; /* warning: may overflow */
    while ((rem.tv_sec || rem.tv_nsec) && nanosleep(&req, &rem)) {
        if (errno != EINTR) /* this check is probably unnecessary */
            return -1;
        req = rem;
    }
    return 0;
}