Writing to single file in multiple functions

This is a discussion on Writing to single file in multiple functions within the C Programming forums, part of the General Programming Boards category; Hi all from a beginner, I'm trying to print all my output from a program to a single text file ...

  1. #1
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    Unhappy Writing to single file in multiple functions

    Hi all from a beginner,

    I'm trying to print all my output from a program to a single text file (running in unix) (I tried the redirection ">" but didn't work)

    I opened the file in main(), and written some output to it using fprintf(), then I called a function, opened the file again with
    fopen("output.txt","a")
    and tried to use fprintf again to append output to the same file, but when I run the program, only the output from main() were printed to the file, nothing was printed after the call to the second function.

    How do I do this?
    Cheers.

  2. #2
    Im a Capricorn vsriharsha's Avatar
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    Are you checking the return status of "fopen()" command in the function you are calling to see if it is really opening the file or not?

    -Harsha.
    Help everyone you can

  3. #3
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    Yes I am, sorry I didn't make that clear.

  4. #4
    Im a Capricorn vsriharsha's Avatar
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    Is it not feasible for you to declare a global file pointer for that file and open it first in the main function (before you start redirecting out put to it) and then use that same filepointer everywhere. You can open the file in the append mode, so you are sure the pointer will always be at the end and what ever you write into it will be appended at the end...

    -Harsha.
    Help everyone you can

  5. #5
    Im a Capricorn vsriharsha's Avatar
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    Like this...
    Code:
    /* File myfile.c */
    /* Global Variable */
    FILE *outputfile;
    ...
    ...
    int main()
    {
      ...
      ...
      outputfile = fopen("OutPutFile","a");
      if(outputfile == NULL)
      { 
          printf("Error Opening Output file\n");
          .........
      }
      ....
      ....
      fprintf(outputfile,<<format>>...);
      ... 
      ....
      some_function_that_prints_to_the_file();
      another_function_that_prints_to_the_file();
      ...
      return 0;
    }
    ...
    ...
    Help everyone you can

  6. #6
    Obsessed with C chrismiceli's Avatar
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    Why didn't the redirectional operator > not work? How did you use it?
    Help populate a c/c++ help irc channel
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  7. #7
    & the hat of GPL slaying Thantos's Avatar
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    Quote Originally Posted by vsriharsha
    Is it not feasible for you to declare a global file pointer for that file and open it first in the main function (before you start redirecting out put to it) and then use that same filepointer everywhere.
    Except that you are using global variables when you can easily just pass it around and have things be a lot safer.

    Example:
    Code:
    #include <stdio.h>
    
    void func(FILE *);
    
    int main(void)
    {
      FILE *out = stdout;
    
      if ( out == NULL )
        return 1;
    
      fprintf(out, "%s", "Hello world\n");
      func(out);
      return 0;
    }
    
    void func (FILE *out)
    {
      fprintf(out, "Hello my name is bob\n");
    }
    Now just replace
    Code:
    FILE *out = stdout;
    with an fopen() call.
    Last edited by Thantos; 08-21-2004 at 12:18 PM.

  8. #8
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    Thanks all for helping out...
    When I used the redirection operator >, the output are not in the correct order.
    The structure of my program is like this:
    main() prints to output.txt, then calls a function solver(),
    solver() append output to output.txt, then use system("./generate > dev/null") to call another program generate,
    Is it because I didn't declare the file globally?
    or because I opened the file in main() (in write mode) and then opened it again in solver() (in append mode)?

  9. #9
    End Of Line Hammer's Avatar
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    Either
    - open the file once, and use it until you're finished.
    or
    - open it, use it, close it, open it, use it, close it, open it, use it, close it, etc etc

    Always make sure you close it before opening it again, and before ending the program.
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  10. #10
    & the hat of GPL slaying Thantos's Avatar
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    When I used the redirection operator >, the output are not in the correct order.
    Um hate to tell you but using redirection doesn't magically change the order of the output, all it does is takes the output to stdout and sends it to the file. If there is something wrong with the order of the output then I'd suggest you look at your program.

  11. #11
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    I don't know why, but when I run the program without the redirection operator, the output are in the correct order, but when I call the program with a redirection to a text file AND within the program a system call to another program also with a redirection to the same text file, the order changed...

  12. #12
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    If there is something wrong with the order of the output then I'd suggest you look at your program.
    Post your current code so we can take a look at it.

  13. #13
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    How about using fflush() on that stream after every fwrite() ?

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