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We have now began to see how multiple processes may be running on a machine and maybe be controlled (spawned by fork() by one of our programs.
In numerous applications there is clearly a need for these processes to communicate with each exchanging data or control information. There are a few methods which can accomplish this task. We will consider:
In this chapter, we will study the piping of two processes. We will study the others in turn in subsequent chapters.
Piping is a process where the input of one process is made the input of
another. We have seen examples of this from the UNIX command line using .
We will now see how we do this from C programs.
We will have two (or more) forked processes and will communicate
between them.
We must first open a pipe
UNIX allows two ways of opening a pipe.
FILE *popen(char *command, char *type) -- opens a pipe for I/O
where the command is the process that will be connected to the calling process
thus creating the pipe. The type is either ``r'' - for reading, or ``w''
for writing.
popen() returns is a stream pointer or NULL for any errors.
A pipe opened by popen() should always be closed by pclose(FILE
*stream).
We use fprintf() and fscanf() to communicate with the pipe's stream.
int pipe(int fd[2]) -- creates a pipe and returns two file
descriptors, fd[0], fd[1]. fd[0] is opened for reading, fd[1]
for writing.
pipe() returns 0 on success, -1 on failure and sets errno
accordingly.
The standard programming model is that after the pipe has been set up, two (or
more) cooperative processes will be created by a fork and data will be
passed using read() and write().
Pipes opened with pipe() should be closed with close(int fd).
Example: Parent writes to a child
int pdes[2];
pipe(pdes);
if ( fork() == 0 )
{ /* child */
close(pdes[1]); /* not required */
read( pdes[0]); /* read from parent */
.....
}
else
{ close(pdes[0]); /* not required */
write( pdes[1]); /* write to child */
.....
}
An futher example of piping in a C program is plot.c and subroutines and it performs
as follows:
The code listing for plot.c is:
/* 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 <signal.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 <stdio.h> #include <stdlib.h> #include <math.h> /* prototypes */ void StartPlot(void); void RemoveDat(void); void StopPlot(void); void PlotOne(void); void RePlot(void); #endif
Exercise 12733
Setup a two-way pipe between parent and child processes in a C program. i.e. both can send and receive signals.