using grace-library prog crashes using sytem()

[hanseldansel]

Dear ubuntu-users,

I use the grace-library grace_np.h for displaying and printing data from a running c-program;
my problem is:
As soon as I use a system command in this c-program, the grace-window closes and the running program outputs: "No grace subprocess4" for every xmgrace command.
OK so the grace-process stops due to a command sent to the shell. Is there a way to prevent the grace process being killed?

the function for displaying with grace:


1 #include <stdio.h>
2 #include <stdlib.h>
3 #include <math.h>
4 #include <unistd.h>
5 #include <grace_np.h> // including xmgrace library
6 void displaygrace( int Nel, int emr, double** fast, double** slow,
7 double** strain, double** Ta )
8 {
9 static int firsttimegrace = 1;
10 static int gracecount = 0;
11 int i, j;
12 if ( firsttimegrace == 1 )
13 {
14 firsttimegrace = 0;
15 if ( GraceOpen( 2048 ) == -1 )
16 {
17 fprintf( stderr, "Can't run Grace. \n" );
18 exit( 1 );
19 }
20 /*
21 .
22 .
23 */
24
25 }// if ( firsttimegrace == 1 )
26 /*
27 .
28 .
29 */
30 GracePrintf ("redraw");
31 if ( GraceIsOpen() ) // grace alive? then save and print otherwise crash!
32 {
33 GracePrintf("DEVICE \"PostScript\" OP \"color\" \n");
34 GracePrintf("HARDCOPY DEVICE \"PNG\" \n");
35 GracePrintf("print to \"output/PNGs/sample%.4d.png\" \n", gracecount );
36 GracePrintf("print \n");
37 GracePrintf("saveall \"output/grace/sample%.4d.agr\"", gracecount );
38 }
39 else
40 {
41 exit(EXIT_FAILURE);
42 }
43 gracecount++;
44 }// void displaygraceThank you very much in advance for suggestions!

Cheers!
Daniel