What is the difference between _LARGEFILE_SOURCE and _FILE_OFFSET_BITS=64?

c linux linux-from-scratch
panzi picture panzi · Jan 6, 2013 · Viewed 9.7k times · Source

I understand that -D_FILE_OFFSET_BITS=64 causes off_t to be 64bits. So what does -D_LARGEFILE_SOURCE do that isn't already done by -D_FILE_OFFSET_BITS=64? What do these definitions do exactly?

Answer

Mat picture Mat · Jan 6, 2013

The GLIBC Feature test macros documentation states:

_LARGEFILE_SOURCE
If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions fseeko and ftello are available. Without these functions the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).

So that macro specifically makes fseeko and ftello available. _FILE_OFFSET_BITS settings alone don't make these functions available.

(Note that if you're using a GNU dialect of C, the default with GCC, you might not need to explicitly define _LARGEFILE_SOURCE. You do if you use -std=c99 for instance.)

Related questions

Undefined reference to pthread_create in Linux

I picked up the following demo off the web from https://computing.llnl.gov/tutorials/pthreads/ #include <pthread.h> #include <stdio.h> #define NUM_THREADS 5 void *PrintHello(void *threadid) { long tid; tid = (long)threadid; printf("Hello …

Read More
Write to .txt file?

How can I write a little piece of text into a .txt file? I've been Googling for over 3-4 hours, but can't find out how to do it. fwrite(); has so many arguments, and I don't know how to use …

Read More
Easily measure elapsed time

I am trying to use time() to measure various points of my program. What I don't understand is why the values in the before and after are the same? I understand this is not the best way to profile my …

Read More