Question

I started studying POSIX timers, so I started also doing some exercises, but I immediately had some problems with the compiler.
When compiling this code, I get some strange messages about macros like CLOCK_MONOTONIC. Those are defined in various libraries like time.h etc. but the compiler gives me errors as if they are not defined.

It is strange because I am using a Fedora 16, and some of my friends with Ubuntu get less compiler errors than I :-O

I am compiling with gcc -O0 -g3 -Wall -c -fmessage-length=0 -std=c99 -lrt

Here the errors I get:

• struct sigevent sigeventStruct gives:

  storage size of ‘sigeventStruct’ isn’t known
  unused variable ‘sigeventStruct’ [-Wunused-variable]
  Type 'sigevent' could not be resolved
  unknown type name ‘sigevent’
  

• sigeventStruct.sigev_notify = SIGEV_SIGNAL gives:

  ‘SIGEV_SIGNAL’ undeclared (first use in this function)
  request for member ‘sigev_notify’ in something not a structure or union
  Field 'sigev_notify' could not be resolved
  

• if(timer_create(CLOCK_MONOTONIC, sigeventStruct, numero1) == -1) gives:

  implicit declaration of function ‘timer_create’ [-Wimplicit-function- declaration]
  ‘CLOCK_MONOTONIC’ undeclared (first use in this function)
  Symbol 'CLOCK_MONOTONIC' could not be resolved
  

Here is the code:

#include <stdio.h>
#include <fcntl.h>
#include <time.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>
#include <signal.h>

int main()
{
        timer_t numero1;
        struct sigevent sigeventStruct;
        sigeventStruct.sigev_notify = SIGEV_SIGNAL;
        if(timer_create(CLOCK_MONOTONIC, sigeventStruct, numero1) == -1)
        {
                printf( "Errore: %s\n", strerror( errno ) );
        }
        return 0;
}

Answer 1

Firstly, you can compile your code with -std=gnu99 instead of -std=c99 if you want to have the identifiers SIGEV_SIGNAL, sigeventStruct, and CLOCK_MONOTONIC available.

As noted by @adwoodland these identifiers are declared when _POSIX_C_SOURCE is set to a value >= 199309L, which is the case with -std=gnu99. You can also use -D_POSIX_C_SOURCE=199309L -std=c99 or have the macro defined in source code.

Secondly, see the timer_create prototype, you have to pass pointers as the second and the third argument to the function:

timer_create(CLOCK_MONOTONIC, &sigeventStruct, &numero1)
                              ^                ^

Also you have to include the standard header string.h for strerror function declaration.