Implicit declaration of function 'min'

Question

I am using Matlab R2013b and Xcode 5.0.2 on my MacBook to mex a file called sfun_rttime.c using Matlab's command: mex sfun_rrtime.c

This results in the following error message:

sfun_rttime.c:108:26: warning: implicit declaration of function 'min' is invalid in
      C99 [-Wimplicit-function-declaration]
   while (t_diff < (dt - min(dt,t_execution))) {
                         ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_min", referenced from:
      _mdlOutputs in sfun_rttime.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

    mex: link of ' "sfun_rttime.mexmaci64"' failed.

Unable to complete successfully.

The .c file looks as follows:

#define S_FUNCTION_LEVEL 2
#define S_FUNCTION_NAME  sfun_rttime

#define TIME_SCALE_FACTOR(S) ssGetSFcnParam(S,0)

/* Need to include simstruc.h for the definition of the SimStruct and
 * its associated macro definitions. */
#include <simstruc.h>

#if defined(_WIN32)
/* Include the windows SDK header for handling time functions. */
#include <windows.h>
#include <math.h>

/* Function of the high performance counter (in seconds). */
__inline double hightimer()
{
    HANDLE hCurrentProcess = GetCurrentProcess();
    DWORD dwProcessAffinity;
    DWORD dwSystemAffinity;    
    LARGE_INTEGER frequency, counter;
    double sec_per_tick, total_ticks;

    /* force thread on first cpu */
    GetProcessAffinityMask(hCurrentProcess,&dwProcessAffinity,&dwSystemAffinity);
    SetProcessAffinityMask(hCurrentProcess, 1);

    /* retrieves the frequency of the high-resolution performance counter */
    QueryPerformanceFrequency(&frequency);

    /* retrieves the current value of the high-resolution performance counter */
    QueryPerformanceCounter(&counter);

     /* reset thread */
    SetProcessAffinityMask(hCurrentProcess,dwProcessAffinity);

    /* time in seconds */
    sec_per_tick = (double)1/(double)frequency.QuadPart;
    total_ticks = (double)counter.QuadPart;    
    return sec_per_tick*total_ticks;
}   /* end hightimer */
#else
/* Include the standard ANSI C header for handling time functions. */
#include <time.h>

/* Function of the high performance counter (in seconds). */
__inline double hightimer()
{    
    return (double)clock()/CLOCKS_PER_SEC;
}   /* end hightimer */
#endif

static void mdlInitializeSizes(SimStruct *S)
{
   ssSetNumSFcnParams(S, 1);  /* Number of expected parameters */
   if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) return;
    ssSetNumContStates(S, 0);
    ssSetNumDiscStates(S, 1);
   if (!ssSetNumInputPorts(S, 0)) return;
   if (!ssSetNumOutputPorts(S, 1)) return;
   ssSetOutputPortWidth(S, 0, 1);
   ssSetNumSampleTimes(S, 1);
   ssSetNumRWork(S, 1);
   ssSetNumIWork(S, 0);
   ssSetNumPWork(S, 0);
   ssSetNumModes(S, 0);
   ssSetNumNonsampledZCs(S, 0);
   ssSetOptions(S, 0);
}

#define MDL_INITIALIZE_SAMPLE_TIMES
static void mdlInitializeSampleTimes(SimStruct *S)
{
   ssSetSampleTime(S, 0, CONTINUOUS_SAMPLE_TIME);
   ssSetOffsetTime(S, 0, 0.0);
}

#define MDL_START
static void mdlStart(SimStruct *S)
{
   ssSetRWorkValue(S,0,ssGetTStart(S));
}

static void mdlOutputs(SimStruct *S, int_T tid)
{
   double            *t_x = ssGetDiscStates(S);
   double            *t_y = ssGetOutputPortRealSignal(S,0);
   double             t_previousSimTime = ssGetRWorkValue(S,0);
   const double      *scaleFactor = mxGetPr(TIME_SCALE_FACTOR(S));
   time_T             t_SimTime = ssGetT(S);
   double             t_diff = 0.0;
   double             dt;
   double             t_current;
   double             t_0;
   double             t_previous;
   double             t_elapsed;
   double             t_execution;

   /* Desired Delta time */
   dt = (t_SimTime - t_previousSimTime)*(scaleFactor[0]);

   /* Get clock time at the beginning of this step*/   
   t_previous = hightimer();
   t_0 = t_previous;

   /* Wait to reach the desired time */
   t_execution = t_0-t_x[0];
   while (t_diff < (dt - min(dt,t_execution))) {
       t_current = hightimer();
       /* Look for wrapup */
       if (t_current<t_previous){
           t_elapsed = t_previous - t_0;
           t_0 = hightimer() - t_elapsed;
       }
       t_diff = t_current - t_0;
       t_previous = t_current;
   }

   /* Store current time to be used in next time step*/
   t_y[0] = dt - t_execution;
   t_x[0] = t_previous;
   ssSetRWorkValue(S,0,t_SimTime);
}

static void mdlTerminate(SimStruct *S)
{
    UNUSED_ARG(S); /* unused input argument */
}

/* Required S-function trailer */
#ifdef  MATLAB_MEX_FILE    /* Is this file being compiled as a MEX-file? */
#include "simulink.c"      /* MEX-file interface mechanism */
#else
#include "cg_sfun.h"       /* Code generation registration function */
#endif

I didn't write this file myself, and neither I have a clue what's wrong with it.

Answer 1

On a non-Windows machine, you don't #include <windows.h> (due to the #ifdef and obvious other reasons). Windows.h pulls in WinDef.h, and WinDef.h contains:

#ifndef min
#define min(a,b)            (((a) < (b)) ? (a) : (b))
#endif

That's where he went!

You can implement the macro, use fmin() as mentioned (note the current math.h include is #ifdef'd out too), or if you're feeling really kind and want to give the linker exactly what it was looking for:

double min(double a, double b) {
    return a<b ? a : b;
}

Answer 2

How about using "double fmin(double x, double y)" instead of min as the following?

...
/* Include the standard ANSI C header for handling time functions. */
#include <time.h>

#include <math.h>
#define min(a,b) fmin(a,b)
...