Effect of usleep(0) in C++ on Linux

Question

The documentation for usleep states that calling usleep(0) has no effect. However, on my system (RHEL 5.2) running the small snippets of C++ code below, I find that it actually appears to have the same effect as usleep(1). Is this to be expected, and if so, why is there the discrepancy between the documentation and what I see in real life?

Exhibit A

Code:

#include <unistd.h>

int main()
{
    for( int i = 0; i < 10000; i++ )
    {
        usleep(1);
    }
}

Output:

$ time ./test
real   0m10.124s
user   0m0.001s
sys    0m0.000s

Exhibit B

Code:

#include <unistd.h>

int main()
{
    for( int i = 0; i < 10000; i++ )
    {
        usleep(1);
        usleep(0);
    }
}

Output:

$ time ./test
real   0m20.770s
user   0m0.002s
sys    0m0.001s

Answer 1

Technically it should have no effect. But you must remember that the value passed is used as a minimum, and not an absolute, therefore the system is free to use the smallest possible interval instead.

Answer 2

I just wanted to point out about the time command used here. You should use /usr/bin/time instead of only time command if you want to check your program memory,cpu,time stat. When you call time without full path then built-in time command is called. Look at the difference.

without full path:

# time -v ./a.out
-bash: -v: command not found

real    0m0.001s
user    0m0.000s
sys     0m0.001s

with full path:

# /usr/bin/time -v ./a.out
Command being timed: "./a.out"
User time (seconds): 0.00
System time (seconds): 0.00
Percent of CPU this job got: 0%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:10.87
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 0
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 220
Voluntary context switches: 10001
Involuntary context switches: 1
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

use man time for /usr/bin/time manual and use help time for built in time information.

Answer 3

usleep() and sleep() are translated to nanosleep() system calls. Try strace your program and you'll see it. From nanosleep() manual:

   nanosleep() suspends the execution of the calling thread until either
   at least the time specified in *req has elapsed, or the delivery of a
   signal that triggers the invocation of a handler in the calling
   thread or that terminates the process.

So I think ulseep(0) will generate an interrupt and a context switch.