C: Access violation when declaring array

Question

Inside main, i have the following procedure to get numbers from a file:

FILE *f = fopen("numbers.txt", "r");
if(f != NULL) {
    char line[BUFFER_SIZE];
    while(fgets(line, sizeof(line), f) != NULL) {
        char *start = line;
        int field;
        int n;
        while(sscanf(start, "%d", &field, &n) == 1) {
            printf("%d \n", field);
            start += n;
        }
    }
    fclose(f);
}

If I add an integer array above this, e.g. int num[100], I get an access violation.

It seems that this somehow causes problem with the file-reading, but I can't see how at the moment.

must be UB. you have a bug in your code, the extra array just makes it visible. — Karoly Horvath
– Karoly Horvath, Commented Jun 30, 2013 at 0:11
Show all your code and be explicit about the error that you get back, please. — Alex Reynolds
– Alex Reynolds, Commented Jun 30, 2013 at 0:12
@mbratch: sizeof(line) is guaranteed to be the same as BUFFER_SIZE, not 4 or 8. Where did "4 or 8" come from? Arrays don't decay to pointers under sizeof. — AnT stands with Russia
– AnT stands with Russia, Commented Jun 30, 2013 at 0:20

ouah · Accepted Answer · 2013-06-30 00:15:19Z

2

sscanf(start, "%d", &field, &n)

You have too many arguments for your function call.

answered Jun 30, 2013 at 0:15

ouah

147k16 gold badges287 silver badges338 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Keith Thompson Over a year ago

True, and that should be corrected, but I believe excess arguments are ignored.

helm Over a year ago

Thanks, I somehow deleted that. It should be while(sscanf(start, "%d%n", &field, &n) == 1) {

AnT stands with Russia Over a year ago

@Helm Hammerhand: So, does it crash in this form?

helm Over a year ago

Nope, that solved it appararently. So simple, and yet I've been looking at the code for minutes and didn't see it.

ouah Over a year ago

@HelmHammerhand You should always turn on all warnings in your compiler options and fix them all. This will save you a lot of time.

LoztInSpace · Accepted Answer · 2013-06-30 00:20:51Z

0

while(sscanf(start, "%d", &field, &n)

It's been a while since I've done C but I don't think n is filled in as the length of the chars.

Do fscanf straight from your file and check the return to see if it got anything.

answered Jun 30, 2013 at 0:20

LoztInSpace

5,6971 gold badge17 silver badges29 bronze badges

Comments

Baratong · Accepted Answer · 2013-06-30 00:23:09Z

0

Your 'access violation' is no doubt coming from the fact that your variable 'n' is not getting initialized. Your sscanf(start, "%d", &field, &n) has, probably, not enough format specifiers so only the variable 'field' is getting set from that sscanf. This sscanf is not touching variable n so it is still uninitialized.

When you then add this uninitialized 'n' to your pointer 'start', you end up with an invalid address, and ultimately your access violation error.

answered Jun 30, 2013 at 0:23

Baratong

1014 bronze badges

1 Comment

Baratong Over a year ago

BTW: Anytime you are doing pointer arithmetic, you really, really should do bounds testing.. i.e. make sure adding 'n' to your pointer doesn't push it past the boundary of your target data...

Collectives™ on Stack Overflow

C: Access violation when declaring array

3 Answers 3

5 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

5 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related