realloc inside a routine

This is a discussion on realloc inside a routine within the C Programming forums, part of the General Programming Boards category; I wish to increase the size of an array inside a function, since I don't know the size of the ...

  1. #1
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    realloc inside a routine

    I wish to increase the size of an array inside a function, since I don't know the size of the array before I call the function.

    I would think I could do (for example):

    Code:
    char log_filename[100];
    double *buf = NULL;
    int buf_len;
    
    sprintf(log_filename, "log.txt");
    
    fill_array(log_filename, buf, &buf_len);
    fprintf(stderr, "buf[0]=%f\n", buf[0]);
    
    void fill_array(char *log_filename, double *buf, int *buf_len) {
      int i = 0;
      FILE *fd;
      double time;
    
      fd = fopen(log_filename, "r");
    
      while (fscanf(fd_cam, "%lf\n", &time) != EOF) {
        times = (double *) realloc(times, (i + 1) * sizeof(double));
        times[i] = time;
        i++;
      }
    
    }
    But I get a seg fault, cause the memory is only accessible inside the fill_array() routine. How can I write such a routine, so it is accessible outside the routine. Thx.
    Last edited by coletek; 06-04-2009 at 08:56 PM. Reason: set log_filename to a path
    "What comes around, goes around"

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Are those statements in main? If so, realloc is going to fail hard when you pass it the garbage pointer buf; make sure it's NULL before you send it off.

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    Quote Originally Posted by tabstop View Post
    Are those statements in main? If so, realloc is going to fail hard when you pass it the garbage pointer buf; make sure it's NULL before you send it off.
    The routine is in another .c file and the calls to it are in a main() routine set. Except the buf variable is a global variable.

    And yes the buf is set to NULL first. Sorry forgot to add that in my example.
    "What comes around, goes around"

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    The function can only change its local copy of buf (presumably = "times" in your example). If you want to be able to change the value of buf, you must pass a pointer to it.

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    Quote Originally Posted by tabstop View Post
    The function can only change its local copy of buf (presumably = "times" in your example). If you want to be able to change the value of buf, you must pass a pointer to it.
    But I have passed it a pointer.

    Or do you mean doing?:

    Code:
    char log_filename[100];
    double *buf = NULL;
    int buf_len;
    
    sprintf(log_filename, "log.txt");
    
    fill_array(log_filename, &buf, &buf_len);
    fprintf(stderr, "buf[0]=%f\n", buf[0]);
    
    void fill_array(char *log_filename, double **buf, int *buf_len) {
      int i = 0;
      FILE *fd;
      double time;
    
      fd = fopen(log_filename, "r");
    
      while (fscanf(fd_cam, "%lf\n", &time) != EOF) {
        *times = (double *) realloc(*times, (i + 1) * sizeof(double));
        (*times)[i] = time;
        i++;
      }
    
    }
    That works. Thanks...
    Last edited by coletek; 06-04-2009 at 09:04 PM. Reason: Fixed bug, it doesn't seg fault now.
    "What comes around, goes around"

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