How can I manage to file rewrite

This is a discussion on How can I manage to file rewrite within the C Programming forums, part of the General Programming Boards category; I have a program that consists of a parent and a number of child processes. Basically parent writes to a ...

  1. #1
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    Join Date
    Mar 2011
    Posts
    2

    Unhappy How can I manage to file rewrite

    I have a program that consists of a parent and a number of child processes. Basically parent writes to a file, then first child reads and edits it, and writes to another file that is read and edited by second file. This goes on. Also last child writes to the file of first child. The idea is similar to a chain.

    However, I am having problem with rewriting. Generally the chain idea results in a number loops for children so that for each turn child needs to read and write. Reading is OK, but writing to same file again is problem. How can I solve that?

    This the part for child processes:

    Code:
    	// Run child processes
    	if(getpid() != pid) {
    		if(cid <= N%K)
    				turn++;
    		int myturn = 1;
    		
    		while(myturn <= turn) {
    			int in;
    			
    			// Open pipes
    			if(myturn == 1) {
    				mypipe[cid-1].streamr = fdopen (mypipe[cid-1].fd[READEND], "r");
    				mypipe[cid%K].streamw = fdopen (mypipe[cid%K].fd[WRITEEND], "w");
    			}
    
    			// First element is read
    			fscanf(mypipe[cid-1].streamr, "%d", &in);
    			
    			if(in == -1) {
    				myturn = turn + 1;
    			}
    			else {
    				printf("%d\n", in);
    				
    				int out = in;
    				while(out != -1) {
    					if(fscanf(mypipe[cid-1].streamr, "%d", &out))
    						printf("I am %d, read %d-%d from mypipe[%d]\n", getpid(), out, in, cid-1);
    
    					if(out % in != 0) {
    						// !!! - Problem is here - !!!
    						// Cannot write for the second time
    						fprintf(mypipe[cid%K].streamw, "%d\n", out);
    					}
    				}
    				
    				// Write -1 to the end
    				fprintf(mypipe[cid%K].streamw, "%d\n", -1);
    				fclose(mypipe[cid%K].streamw);
    
    				myturn++;
    			}
    		}
    		exit(0);
    	}
    	
    	return 0;

  2. #2
    Registered User
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    Mar 2011
    Posts
    2
    Code:
    		while(myturn <= turn) {
    			int in;
    			
    			close(mypipe[cid-1].fd[WRITEEND]);
    			close(mypipe[cid%K].fd[READEND]);
    			
    			FILE *streamw, *streamr;
    			
    			if(myturn < 2) {
    				streamw = fdopen (mypipe[cid%K].fd[WRITEEND], "w");
    				streamr = fdopen (mypipe[cid-1].fd[READEND], "r");
    			}
    
    			fscanf(streamr, "%d", &in);
    			
    			if(in == -1) {
    				myturn = turn + 1;
    			}
    			else {
    				int out = in;
    				while(out != -1) {
    					if(fscanf(streamr, "%d", &out))
    						printf("I am %d, read %d-%d from mypipe[%d]\n", getpid(), out, in, cid-1);
    
    					if(out % in != 0) {
    						printf("I am %d, write %d to mypipe[%d]\n", cid, fprintf(streamw, "%d\n", out), cid%K);
    					}
    				}
    				fprintf(streamw, "%d\n", -1);
    				fclose(streamw);
    
    				myturn++;
    			}
    		}
    		
    		printf("%d terminating\n", cid);
    		exit(0);

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