Dup2() help

This is a discussion on Dup2() help within the C Programming forums, part of the General Programming Boards category; I'm having issues with dup2(). I can write to a file, however it's classified as no read, no write. I ...

  1. #1
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    Solved Dup2() help

    I'm having issues with dup2(). I can write to a file, however it's classified as no read, no write. I want it to be read-only.

    I think I have what I need in this excerpt.
    graph[i] is an array holding structs
    graph[i].prog is a string of OS commands ie ls, echo.
    graph[i].output is the file to write to.

    Code:
    #include <errno.h>
    #include <stdlib.h>
    #include <string.h>
    #include <stdio.h>
    #include <sys/wait.h>
    #include <unistd.h>
    #include <fcntl.h>
    #define CREATE_FLAGS (O_RDWR | O_CREAT | O_APPEND)
    
     int fileOut=open(graph[i].output, CREATE_FLAGS);
          if(fileOut==-1) {
    	perror("Failed to open file");
    	return 1;
          }
          
          if(dup2(fileOut, STDOUT_FILENO)==-1) {
    	perror("Failed to redirect standard output");
    	return 1;
          }
    
    
         pid_t childpid;
             
         childpid=fork();
          
          //Fail to fork
          if (childpid==-1) {
    	perror("Failed to fork");
    	return 1;
          }
          
          //Entered Child
          if (childpid==0) {
    	int numArgs;
    	char **args;
    	numArgs=makeargv(graph[i].prog," ", &args);
    	execvp(args[0],args);
    	//	fileOut=close(*graph[i].output);
    	perror("Child failed to exec\n");
    	return 1;
          }


    Any help would be appreciated, thank you.
    Last edited by TIMBERings; 02-12-2010 at 01:20 PM. Reason: Solved

  2. #2
    and the hat of wrongness Salem's Avatar
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    So what actual message does your perror line print?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    It doesn't print anything, it never gets to it. The file gets written, but I just can't read it without changing permissions later.

  4. #4
    cas
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    If you supply O_CREAT to open(), you must also specify a mode as the third argument. open() is picking up junk and using that as the mode when creating the file, resulting in weird permissions.

  5. #5
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    Thank you for your help. Got it.

  6. #6
    Captain Crash brewbuck's Avatar
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    Also, the combination of O_RDWR with O_APPEND seem weird.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

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