用c99定义off_t

Question

I'm trying to port some code from windows to linux, but I'm having difficulty with support for large files. off_t seems to be defined when gcc is run with -std=c89 but not for -std=c99. Even a trivial test case will not compile:

#define _LARGEFILE_SOURCE
#define _LARGEFILE64_SOURCE
#define _FILE_OFFSET_BITS 64
#include <stdio.h>

int main()
{
    off_t x = 0;
    return 0;
}

It really doesn't seem like this should be difficult (in fact, it's not on all other operating systems). Anyone have any idea what is happening?

Answer 1

The type off_t is not defined by ISO C; it's defined by POSIX.

I get

error: unknown type name ‘off_t’

if I compile with either -std=c90 , -std=c99 , or -std=c11 . That's to be expected, since those options specify conformance to the relevant C standard. Since you're compiling C code that doesn't conform to any of those C standards, you shouldn't use those options.

I find that off_t is defined if I compile with -std=gnu90 , -std=gnu99 , or -std=gnu11 .

Also, off_t is the return type of the lseek function, whose man page on my system says it requires:

#include <sys/types.h>
 #include <unistd.h>

You should add those.