CODEWITHSUNDEEP
cpointers
Aman
Aman

Reputation: 783

Why is this change causing seg fault?

I have the following method:

 17 //returns size in bytes of binary file stream
 18 //leaves the file at original position
 19 long getFileSize(FILE * fp)
 20 {
 21    long pos = -1;
 22    fpos_t *curPos;
 23    if(fgetpos(fp, curPos) == 0)
 24    {
 25       fseek(fp, 0L, SEEK_END);
 26       pos = ftell(fp);
 27       fsetpos(fp, curPos);
 28    }
 29
 30    return pos;
 31 }

This will compile and work but compiler will give me a warning because curPos is not a initialized variable.

If I change that line to (which the compiler recommends)

 22    fpos_t *curPos = NULL;

I receive a seg fault.

Why is this the case?

Upvotes: 0

Views: 295

Answers (3)

Axalo
Axalo

Reputation: 2953

You haven't allocated curPos. So either allocate it on the heap (and be sure to free it later!)

fpos_t *curPos = malloc(sizeof(fpos_t));

Or on the stack

fpos_t curPos;

In case you allocate it on the stack you need to pass the reference to fgetpos:

fgetpos(fp, &curPos)

Upvotes: 5

Scott Hunter
Scott Hunter

Reputation: 49893

curPos needs to point to a fpos_t object, so fgetpos has a place to store the position.

Upvotes: 0

dsh
dsh

Reputation: 12214

You declared curPos as a pointer (fpos_t *), but never filled in the address where the data is to be found. That is why you get a segmentation fault when you try to store a value in the location indicated by the address (when you changed it to fpos_t *curPos = NULL;)

As I look at the signature for fgetpos(), I realize that what you want is to declare an fpos_t and pass the address of the storage you have allocated so that fgetpos() will write the value.

fpos_t curPos;
fgetpos(fp, &curPos);

Upvotes: 1

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