Segmentation Fault after reading a file into a struct array

Question

I am trying to write a program that reads data from a file and puts it into a struct array. I have succeeded in putting it into the array but I then get a segmentation fault. I believe I have to use malloc to initialize the struct array but I am new to programming and don't really understand how to do this. Thanks for your help! I have copied pieces of my code so you can see what I've done. I have not included my functions in this code.

struct Weather
{
    char location;
    int daynum;
    double temp;
    double precip;
};

int main (void)
{
    FILE*openFile;
    char buffer[COLS][ROWS];
    int i = 0;
    struct Weather loc1; //initialize here?

    for (i = 0; i <200; i++)
    {
        fgets (buffer[i], ROWS, openFile);
        parseLine(buffer[i], &loc1);
        printf ("%d %c %d %.2lf %.2lf\n",i, loc1.location, loc1.daynum, loc1.temp, loc1.precip);

    }
}

first problem: you need to open the file. Take a look at fopen — yano
– yano, Commented Nov 30, 2016 at 0:36
@Mason: you can accept one of the answers by clicking on the grey checkmark below its score. — chqrlie
– chqrlie, Commented Dec 12, 2016 at 2:37

Community · Accepted Answer · 2017-05-23 12:08:38Z

1

Your file stream (openFile) is not initialized; it does not actually point to a file. As yano said in his comment, use fopen() in order to properly initialize the file pointer.

edited May 23, 2017 at 12:08

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answered Nov 30, 2016 at 0:40

Alex P

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Comments

kgbook · Accepted Answer · 2016-11-30 05:25:01Z

1

You must initialize the file stream with fopen() before any I/O operations!

int main()
{
    char filename[64];
    FILE *fp;

    snprintf(filename, sizeof(filename), "hello1234.txt");
    if(NULL == (fp = fopen(filename, "r")))
    {
        printf("err, failed when fopen(), %s %s[%d].\n", __FILE__, __FUNCTION__, __LINE__);
        return -1;
    }
    //your code here

    return 0;
}

Initialize the struct

Note that malloc() cannot initialize the struct.

two methods：

M0:

struct Weather loc1; memset(&loc1, 0, sizeof(struct Weather));

M1:

struct Weather loc1 = {0};
man malloc or click the link for a malloc manual.

edited Nov 30, 2016 at 5:25

answered Nov 30, 2016 at 0:54

kgbook

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Comments

chqrlie · Accepted Answer · 2016-11-30 05:33:15Z

Multiple problems in your code:

The stream pointer openFile is uninitialized, calling fgets() for it invokes undefined behavior. You want to open a file for fopen() or set the value of openFile to the standard input stream stdin.
The 2D char array should be defined in the other order:
```
char buffer[ROWS][COLS];
```
you should use the same constant for the loop counter and the 2D array definition: ROWS might be defined to something less than 200.
the size of the line buffer is COLS, pass that to fgets().
you should test the return value of fgets(): it returns NULL at end of file and the contents of the destination array is indeterminate in this case.
whether or not to initialize loc1 depends on what the parseLine() function does. It would make sense that parseLine() make no assumptions about the contents of the destination structure, but the source has not been posted, so we cannot know for sure.
the printf format specifier for type double is %f, the extra l is simply ignored.

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