C - sprintf() affecting array declaration

Question

Firstly I'm new to C but not programming; I do a lot of web development in PHP. To the question...

I've got a function that reads a series of binary files and outputs the content, however the use of sprintf() is affecting the memory allocation for the multi-dimensional array that follows. See below:

FILE *fp;

for(int i = 1; i <= 16; i++) {

    char format_1[] = "track_%d.bin";
    char filename_1[sizeof format_1+100];
    sprintf(filename_1,format_1,i);
    printf("Filename: %s \n", filename_1);
    fp = fopen(filename_1, "rb");

    if(fp) {
        fseek(fp, 0L, SEEK_END);
        int sz = (ftell(fp) / 3) - 1;
        fseek(fp, 0L, SEEK_SET);

        printf("File size: %d\n", sz);

        int nums[sz][3];

        fread(nums,sizeof(int),sz * 3, fp);

        for (int k = 0; k < sz; k++) {
            printf("Entry number %d = %d with extra values of %d and %d\n", k, nums[k][0], nums[k][1], nums[k][2]);
        }
    } else {
        printf("Skipping \n");
    }

    fclose(fp);
}

I know it's the sprintf() causing the problem because I simplified the function to:

printf("Please enter file number... \n");
scanf("%d%c", &num_file, &term);

printf("Please enter how many instances in this file... \n");
scanf("%d%c", &num_instance, &term);

int nums[num_instance][3];

char format_1[] = "track_%d.bin";
char filename_1[sizeof format_1+3];
sprintf(filename_1,format_1,num_file);
printf("Filename: %s \n", filename_1);

fp = fopen(filename_1, "rb");

fread(nums,sizeof(int),num_instance * 3, fp);
fclose(fp);

for (int k = 0; k < num_notes; k++) {
    printf("Entry number %d = %d with extra values of %d and %d\n", k, nums[k][0], nums[k][1], nums[k][2]);
}

When I move the declaration of nums above the sprintf() it works fine (even if it's just above it) as soon as I move the declaration of nums below sprintf() the output looks like below:

Entry number: 0 = 71 with extra values 116 and 158
Entry number: 1 = -1073743712 with extra values -1073744240 and -1073744304
Entry number: 2 = -1073743896 with extra values 59983 and -1073744005

As you can see from the top code snippet, I need to have the declaration of nums below the sprintf() because I need to know the number of entries in the file.

Edit: I suppose it might help if I mention that a file contains a multi-dimensional array: array[x][3].

Really stumped on this one, any help is greatly appreciated.

@FredrikPihl sizeof format_1+3 is a compile time constant. — Maxim Egorushkin
– Maxim Egorushkin, Commented Apr 3, 2013 at 21:20
You need to check for return values in C. This way you can find exactly where the code fails. — Maxim Egorushkin
– Maxim Egorushkin, Commented Apr 3, 2013 at 21:21
Your fclose() should be called only in the if (fp) branch of the code. — Jonathan Leffler
– Jonathan Leffler, Commented Apr 3, 2013 at 23:33

Chris Dodd · Accepted Answer · 2013-04-03 23:39:39Z

2

Your basic problem is that you're calculating the size of the file in bytes, and then trying to read that many integers from the file. Since integers are (on most machines) 4 bytes, this will fail, and you'll only read into the first quarter of your nums array. The rest will contain (and be printed as) random garbage, which is what you are seeing.

You should be checking the return value of fread to make sure it makes sense. If you change your code to

int numread = fread(nums,sizeof(int),sz * 3, fp);
printf("Read %d integers from file\n", numread);
for (int k = 0; k < sz; k++) {
    if (k < numread/3) {
        printf("Entry number %d = %d with extra values of %d and %d\n", k, nums[k][0], nums[k][1], nums[k][2]);
    } else {
        printf("Entry number %d doesn't exist in the file\n", k);
    }
}

you would see the problem immediately.

ALWAYS check your return values.

edited Apr 3, 2013 at 23:39

answered Apr 3, 2013 at 23:33

Chris Dodd

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1 Comment

Lyndon Cox Over a year ago

Thanks for the tip! I was wondering how to check the return values when @MaximYegorushkin mentioned it in the comments. This wasn't the problem I was running into though, the problem was actually occurring in the function where I write to the files, I think it was something to do with me not allocating enough room in the file. No idea why it was behaving that way with sprinf() though. I'll post the code when I'm on my other machine.

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