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POWhy does the compiler ignore OpenMP pragmas?
 

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    <p>In the following C code I am using OpenMP in a nested loop. Since race condition occurs, I want to perform atomic operations at the end:</p> <pre><code>double mysumallatomic() { double S2 = 0.; #pragma omp parallel for shared(S2) for(int a=0; a&lt;128; a++){ for(int b=0; b&lt;128;b++){ double myterm = (double)a*b; #pragma omp atomic S2 += myterm; } } return S2; } </code></pre> <p>The thing is that <code>#pragma omp atomic</code> has no effect on the program behaviour, even if I remove it, nothing happens. Even if I change it to <code>#pragma oh_my_god</code>, I get no error!</p> <p>I wonder what is going wrong here, whether I can tell the compiler to be more strict when checking omp pragmas or why I do not get an error when I make the last change</p> <p>PS: For compilation I use:</p> <pre><code>gcc-4.2 -fopenmp main.c functions.c -o main_elec_gcc.exe </code></pre> <p>PS2: New code that gives me the same problem and based on gillespie idea:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;stdlib.h&gt; #include &lt;omp.h&gt; #include &lt;math.h&gt; #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&lt;NRACK; j++){ for(i=0; i&lt;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 for shared(S2) for(int j=0; j&lt;NRACK; j++){ for(int i=0; i&lt;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&lt;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&lt;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); } </code></pre>
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