/* plot.c - example of unix pipe. Calls gnuplot graph drawing package to draw graphs from within a C program. Info is piped to gnuplot */ /* Creates 2 pipes one will draw graphs of y=0.5 and y = random 0-1.0 */ /* the other graphs of y = sin (1/x) and y = sin x */ /* Also user a plotter.c module */ /* compile: cc -o plot plot.c plotter.c */ #include "externals.h" #include  <file.h>#define DEG_TO_RAD(x) (x*180/M_PI) double drand48(); void quit(); FILE *fp1, *fp2, *fp3, *fp4, *fopen(); main() { float i; float y1,y2,y3,y4; /* open files which will store plot data */ if ( ((fp1 = fopen("plot11.dat","w")) == NULL) || ((fp2 = fopen("plot12.dat","w")) == NULL) || ((fp3 = fopen("plot21.dat","w")) == NULL) || ((fp4 = fopen("plot22.dat","w")) == NULL) ) { printf("Error can't open one or more data files\n"); exit(1); } signal(SIGINT,quit); /* trap ctrl-c call quit fn */ StartPlot(); y1 = 0.5; srand48(1); /* set seed */ for (i=0;;i+=0.01) /* increment i forever use ctrl-c to quit prog */ { y2 = (float) drand48(); if (i == 0.0) y3 = 0.0; else y3 = sin(DEG_TO_RAD(1.0/i)); y4 = sin(DEG_TO_RAD(i)); /* load files */ fprintf(fp1,"%f %f\n",i,y1); fprintf(fp2,"%f %f\n",i,y2); fprintf(fp3,"%f %f\n",i,y3); fprintf(fp4,"%f %f\n",i,y4); /* make sure buffers flushed so that gnuplot */ /* reads up to data file */  fflush(fp1); fflush(fp2); fflush(fp3); fflush(fp4); /* plot graph */ PlotOne(); usleep(250); /* sleep for short time */ } } void quit() { printf("\nctrl-c caught:\n Shutting down pipes\n"); StopPlot(); printf("closing data files\n"); fclose(fp1); fclose(fp2); fclose(fp3); fclose(fp4); printf("deleting data files\n"); RemoveDat(); } The plotter.c module is as follows:  /* plotter.c module */ /* contains routines to plot a data file produced by another program */ /* 2d data plotted in this version */ /**********************************************************************/ #include "externals.h" static FILE *plot1, *plot2, *ashell; static char *startplot1 = "plot [] [0:1.1]'plot11.dat' with lines,  'plot12.dat' with lines\n"; static char *startplot2 = "plot 'plot21.dat' with lines,  'plot22.dat' with lines\n"; static char *replot = "replot\n"; static char *command1= "/usr/local/bin/gnuplot> dump1"; static char *command2= "/usr/local/bin/gnuplot> dump2"; static char *deletefiles = "rm plot11.dat plot12.dat plot21.dat plot22.dat"; static char *set_term = "set terminal x11\n"; void StartPlot(void) { plot1 = popen(command1, "w"); fprintf(plot1, "%s", set_term); fflush(plot1); if (plot1 == NULL) exit(2); plot2 = popen(command2, "w"); fprintf(plot2, "%s", set_term); fflush(plot2); if (plot2 == NULL) exit(2); } void  RemoveDat(void) { ashell = popen(deletefiles, "w"); exit(0); } void StopPlot(void) { pclose(plot1); pclose(plot2); } void PlotOne(void) { fprintf(plot1, "%s", startplot1); fflush(plot1); fprintf(plot2, "%s", startplot2); fflush(plot2); } void RePlot(void) { fprintf(plot1, "%s", replot); fflush(plot1); } The header file externals.h contains the following:  /* externals.h */ #ifndef EXTERNALS #define EXTERNALS #include  #include  #include  /* prototypes */ void StartPlot(void); void RemoveDat(void); void StopPlot(void); void PlotOne(void); void RePlot(void); #endif