Nanosleep Error: Storage Size Isn't Known

Compiling a C program that used nanosleep turned out to be quite the thorn in my side. I’m documenting this here for anyone else facing the same issues.

TL;DR — Try compiling your program with a GNU standard e.g. gcc -std=gnu11 ...

Debugging

Here’s a simplified version of what I was trying to compile.

// Compiled via: gcc -std=c99 main.c
#include <time.h>

int main() {
    struct timespec sleep;
    sleep.tv_sec = 0;
    sleep.tv_nsec = 50000000;
    nanosleep(&sleep, NULL);
}

I kept getting the following error and warning.

main.c: In function ‘main’:
main.c:4:21: error: storage size of ‘sleep’ isn’t known
     struct timespec sleep;
                     ^~~~~
main.c:7:5: warning: implicit declaration of function ‘nanosleep’ [-Wimplicit-function-declaration]
     nanosleep(&sleep, NULL);
     ^~~~~~~~~

My initial googling only turned up the suggestion to define a POSIX source macro¹ at the start of the file. This didn’t work for me.

Inspired by some obscure forum posts, I noticed that the program compiled when I removed the -std=c99 GCC option. Peculiar, why was that making it fail? To dig in I inspected the preprocessor output.

$ gcc -std=c99 -E main.c | grep timespec
    struct timespec sleep;
$ gcc -E main.c | grep timespec
struct timespec
    struct timespec it_interval;
    struct timespec it_value;
extern int nanosleep (const struct timespec *__requested_time,
        struct timespec *__remaining);
...

A grep of the C99 standard output only turned up the instance of timespec used in main.c. The default standard output had a timespec struct, a nanosleep declaration, and even more I’ve left out. These declarations resolved both the error and warning I was encountering.

Going Further

A fix, hooray! But, this left me with more questions than answers. First off, what standard does GCC use by default when we don’t select one? My machine is running GCC v7.5.0 and thanks to this helpful Stackoverflow answer we can see that GCC is defaulting to the GNU11 standard. So apparently time.h from the GNU11 standard’s gives us what we need where as the time.h from the C99 standard does not.

This begs the question of what other standards work. I tested a few of the common options and have placed the results below.

GNU89 / GNU99 / GNU11

No issues here. The nanosleep function works flawlessly with all these standards.

C89 / C99

Both fail to compile with the all too familiar “storage size isn’t known error”. Neither standards include struct timespec or nanosleep. A fun tidbit is that the C89 standard does not trigger a warning for the implicit declaration of a function — this wasn’t part of the C language yet.

C11

C11 is something of an anomaly. It compiles successfully but still throws the implicit declaration error. However, we can quench this error by defining the POSIX source macro¹ I mentioned earlier.