I want to use ftruncate to create a file with fixed size, but how can I deal with off_t type?

Question

SYNOPSIS

    #include <unistd.h>
    #include <sys/types.h>
    int ftruncate(int fd, off_t length);

Someone says that offset types are usually long integers. So I use %ld to scanf it as follows:

    off_t size;
    scanf("%ld",&size);
    ftruncate(fout,size);

But the compiler warn :expected "int" but argumant is of type "struct FILE *" What can I deal with it?

The error isn't about off_t at all, it's about your _first_ parameter to ftruncate. — Mat, Jan 09 '16 at 08:40
Not a `char*` either, but a filedescriptor, which is an `int`. In summary, you just fail to call `ftruncate()` correctly. — Ulrich Eckhardt, Jan 09 '16 at 08:42
For your reference, the official Linux man-page: http://man7.org/linux/man-pages/man2/truncate.2.html — alk, Jan 09 '16 at 09:21

score 2 · Answer 1 · answered Jan 09 '16 at 08:47

2

The ftruncate function expects its first argument to be a file descriptor, while you pass struct FILE* instead, apparently. The correct way is:

ftruncate(fileno(fout),size);

answered Jan 09 '16 at 08:47

Oleg Andriyanov

I was about to write this - but here is the link to the relevant manual page http://linux.die.net/man/3/fileno – Ed Heal Jan 09 '16 at 08:48

score 2 · Answer 2 · edited May 23 '17 at 11:59

OP has 2 issues. The first, wrong use of ftruncate() is well answered by @Oleg Andriyanov

The 2nd issue if how to use scanf() with off_t. off_t is a signed type: ref.

If off_t was always long, then there would be no reason to define a new type. So assuming off_t is long is a not portable.

To read an integer that is signed with unknown width, suggest reading it using the maximum width.

intmax_t wide_int;
scanf("%jd", &wide_int);
off_t size = (off_t) wide_int;

The above omits error checking. Robust error checking code would use fgets(), strtoimax(), etc.

2 Answers2