confused about using ftell() to check if the file is empty - c

Question

I want to add a structure to a binary file but first i need to check whether the file has previous data stored in it, and if not i can add the structure,otherwise ill have to read all the stored data and stick the structure in its correct place, but i got confused about how to check if the file is empty ,i thought about trying something like this:
size = 0
if(fp!=NULL)
{
fseek (fp, 0, SEEK_END);
size = ftell (fp);
rewind(fp);
}
if (size==0)
{
// print your error message here
}
but if the file is empty or still not created how can the file pointer not be NULL ? whats the point of using ftell() if i can simply do something like this :
if(fp==NULL){fp=fopen("data.bin","wb");
fwrite(&struct,sizeof(struct),1,ptf);
fclose(fp);}
i know that NULL can be returned in other cases such as protected files but still i cant understand how using ftell() is effective when file pointers will always return NULL if the file is empty.any help will be appreciated :)

Answer 1

i need to check whether the file has previous data stored in it
There might be no portable and robust way to do that (that file might change during the check, because other processes are using it). For example, on Unix or Linux, that file might be opened by another process writing into it while your own program is running (and that might even happen between your ftell and your rewind). And your program might be running in several processes.
You could use operating system specific functions. For POSIX (including Linux and many Unixes like MacOSX or Android), you might use stat(2) to query the file status (including its size with st_size). But after that, some other process might still write data into that file.
You might consider advisory locking, e.g. with flock(2), but then you adopt the system-wide convention that every program using that file would lock it.
You could use some database with ACID properties. Look into sqlite or into RDBMS systems like PostGreSQL or MariaDB. Or indexed file library like gdbm.
You can continue coding with the implicit assumption (but be aware of it) that only your program is using that file, and that your program has at most one process running it.
if the file is empty [...] how can the file pointer not be NULL ?
As Increasingly Idiotic answered, fopen can fail, but usually don't fail on empty files. Of course, you need to handle fopen failure (see also this). So most of the time, your fp would be valid, and your code chunk (assuming no other process is changing that file simulateously) using ftell and rewind is an approximate way to check that the file is empty. BTW, if you read (e.g. with fread or fgetc) something from that file, that read would fail if your file was empty, so you probably don't need to check its emptiness before.
A POSIX specific way to query the status (including size) of some fopen-ed file is to use fileno(3) and fstat(2) together like fstat(fileno(fp), &mystat) after having declared struct stat mystat;

Answer 2

fopen() does not return NULL for empty files.
From the documentation:
If successful, returns a pointer to the object that controls the opened file stream ... On error, returns a null pointer.
NULL is returned only when the file could not be opened. The file could fail to open due to any number of reasons such as:
The file doesn't exist
You don't have permissions to read the file
The file cannot be opened multiple times simultaneously.
More possible reasons in this SO answer
In your case, if fp == NULL you'll need to figure out why fopen failed and handle each case accordingly. In most cases, an empty file will open just fine and return a non NULL file pointer.