c - Why does the compiler ignore OpenMP pragmas? -

in following c code using openmp in nested loop. since race condition occurs, want perform atomic operations @ end:

double mysumallatomic() {    double s2 = 0.;   #pragma omp parallel shared(s2)   for(int a=0; a<128; a++){     for(int b=0; b<128;b++){       double myterm = (double)a*b;       #pragma omp atomic       s2 += myterm;     }   }   return s2; } 

the thing #pragma omp atomic has no effect on program behaviour, if remove it, nothing happens. if change #pragma oh_my_god, no error!

i wonder going wrong here, whether can tell compiler more strict when checking omp pragmas or why not error when make last change

ps: compilation use:

gcc-4.2 -fopenmp main.c functions.c -o main_elec_gcc.exe 

ps2: new code gives me same problem , based on gillespie idea:

#include <stdio.h> #include <string.h> #include <stdlib.h> #include <omp.h> #include <math.h>  #define nrack 64 #define nstars 1024  double mysumallatomic_serial(float rocks[nrack][3], float moon[nstars][3],                              float qr[nrack],float ql[nstars]) {   int j,i;   float temp_div=0.,temp_sqrt=0.;   float difx,dify,difz;   float mod2x, mod2y, mod2z;   double s2 = 0.;    for(j=0; j<nrack; j++){     for(i=0; i<nstars;i++){            difx=rocks[j][0]-moon[i][0];       dify=rocks[j][1]-moon[i][1];       difz=rocks[j][2]-moon[i][2];       mod2x=difx*difx;       mod2y=dify*dify;       mod2z=difz*difz;       temp_sqrt=sqrt(mod2x+mod2y+mod2z);       temp_div=1/temp_sqrt;       s2 += ql[i]*temp_div*qr[j];            }   }   return s2; }  double mysumallatomic(float rocks[nrack][3], float moon[nstars][3],                        float qr[nrack],float ql[nstars]) {   float temp_div=0.,temp_sqrt=0.;   float difx,dify,difz;   float mod2x, mod2y, mod2z;   double s2 = 0.;    #pragma omp parallel shared(s2)   for(int j=0; j<nrack; j++){     for(int i=0; i<nstars;i++){       difx=rocks[j][0]-moon[i][0];       dify=rocks[j][1]-moon[i][1];       difz=rocks[j][2]-moon[i][2];       mod2x=difx*difx;       mod2y=dify*dify;       mod2z=difz*difz;       temp_sqrt=sqrt(mod2x+mod2y+mod2z);       temp_div=1/temp_sqrt;       float myterm=ql[i]*temp_div*qr[j];           #pragma omp atomic       s2 += myterm;     }   }   return s2; } int main(int argc, char *argv[]) {   float rocks[nrack][3], moon[nstars][3];   float qr[nrack], ql[nstars];   int i,j;    for(j=0;j<nrack;j++){     rocks[j][0]=j;     rocks[j][1]=j+1;     rocks[j][2]=j+2;     qr[j] = j*1e-4+1e-3;     //qr[j] = 1;   }    for(i=0;i<nstars;i++){     moon[i][0]=12000+i;     moon[i][1]=12000+i+1;     moon[i][2]=12000+i+2;     ql[i] = i*1e-3 +1e-2 ;     //ql[i] = 1 ;   }   printf(" serial: %f\n", mysumallatomic_serial(rocks,moon,qr,ql));   printf(" openmp: %f\n", mysumallatomic(rocks,moon,qr,ql));   return(0); } 

1. using flag -wall highlights pragma errors. example, when misspell atomic following warning.

   main.c:15: warning: ignoring #pragma omp atomic1

2. i'm sure know, in case, example should handled reduction

3. when use omp parallel, default variables shared. not want in case. example, each thread have different value difx. instead, loop should be:

   #pragma omp parallel default(none),\ private(difx, dify, difz, mod2x, mod2y, mod2z, temp_sqrt, temp_div, i, j),\ shared(rocks, moon, ql, qr), reduction(+:s2) for(j=0; j<nrack; j++){   for(i=0; i<nstars;i++){     difx=rocks[j][0]-moon[i][0];     dify=rocks[j][1]-moon[i][1];     difz=rocks[j][2]-moon[i][2];     mod2x=difx*difx;     mod2y=dify*dify;     mod2z=difz*difz;     temp_sqrt=sqrt(mod2x+mod2y+mod2z);     temp_div=1/temp_sqrt;     s2 += ql[i]*temp_div*qr[j];     } }