Truncate file on Linux with c99

Question

I would like to open a file and set its size so that I can then use mmap to write to it.

I found that I can use function truncate or ftruncate. Unfortunately, when I include <unistd.h> I got error:

error: implicit declaration of function ‘truncate’

I read on Internet that I should use gnu or something like that but this is for school project and we have to compile with -std=c99.

Are there any alternatives?

Answer 1

When you use -std=c99 the C library makes sure that the headers do not declare any symbols that are not reserved/are not defined in the C standard library. Since ftruncate does not belong to the C standard library, being a POSIX extension instead, it is not defined by default.

A POSIX program must, for maximal compatibility, define the _POSIX_C_SOURCE feature test macro, or _XOPEN_SOURCE, with appropriate values, before including the headers.

The feature test macros are listed conveniently on for example Linux manual pages; for ftruncate these would be:

   ftruncate():
       _XOPEN_SOURCE >= 500
           || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
           || /* Glibc versions <= 2.19: */ _BSD_SOURCE

i.e. use

#define _XOPEN_SOURCE 500 // (or greater; current is 700)
#include <unistd.h>

or

#define _POSIX_C_SOURCE 200112L // (or greater) 
#include <unistd.h>