All 64-bit applications can manipulate large files by default. The methods described on this page allow 32-bit applications to manipulate large files.
In the large file compilation environment, source interfaces are bound to appropriate 64-bit functions, structures, and types. Compiling in this environment allows 32-bit applications to access files whose size is greater than or equal to 2 Gbyte ( 231 bytes).
Each interface named xxx() that needs to access 64-bit entities to access large files maps to a xxx64() call in the resulting binary. All relevant data types are defined to be of correct size (for example, off_t has a typedef definition for a 64-bit entity).
An application compiled in this environment is able to use the xxx() source interfaces to access both large and small files, rather than having to explicitly utilize the transitional xxx64() interface calls to access large files. See the lfcompile64(5) manual page for information regarding the transitional compilation environment.
Applications can be compiled in the large file compilation environment by using the following methods:
Use the getconf(1) utility with one or more of the arguments listed in the table below. This method is recommended for portable applications.
Set the compile-time flag _FILE_OFFSET_BITS to 64 before including any headers. Applications may combine objects produced in the large file compilation environment with objects produced in the transitional compilation environment, but must be careful with respect to interoperability between those objects. Applications should not declare global variables of types whose sizes change between compilation environments.
The fseek() and ftell() functions do not map to functions named fseek64() and ftell64(); rather, the large file additions fseeko() and ftello(), have functionality identical to fseek() and ftell() and do map to the 64-bit functions fseeko64() and ftello64(). Applications wishing to access large files should use fseeko() and ftello() in place of fseek() and ftell(). See the fseek(3C) and ftell(3C) manual pages for information about fseeko() and ftello().
Applications wishing to access fseeko() and ftello() as well as the POSIX and X/Open specification-conforming interfaces should define the macro _LARGEFILE_SOURCE to be 1 and set whichever feature test macros are appropriate to obtain the desired environment (see standards(5)).
In the following examples, the large file compilation environment is accessed by invoking the getconf utility with one of the arguments listed in the table above. The additional large file interfaces are accessed by specifying –D_LARGEFILE_SOURCE.
The examples that use the form of command substitution specifying the command within parentheses preceded by a dollar sign can be executed only in a POSIX-conforming shell such as the Korn Shell (see ksh (1)). In a shell that is not POSIX-conforming, such as the Bourne Shell (see sh(1)) and the C Shell (see csh(1)), the getconf calls must be enclosed within grave accent marks, as shown in the second example.Example 1 Compile a program with a “large” off_t that uses fseeko(), ftello(), and yacc.
The following example compiles a program with a “large” off_t and uses fseeko(), ftello(), and yacc(1) .
$ c89 -D_LARGEFILE_SOURCE \ -D_FILE_OFFSET_BITS=64 -o foo \ $(getconf LFS_CFLAGS) y.tab.c b.o \ $(getconf LFS_LDFLAGS) \ -ly $(getconf LFS_LIBS)Example 2 Compile a program with a “large” off_t that does not use fseeko() and ftello() and has no application specific libraries.
% c89 -D_FILE_OFFSET_BITS=64 \ `getconf LFS_CFLAGS` a.c \ `getconf LFS_LDFLAGS` \ `getconf LFS_LIBS` \Example 3 Compile a program with a “default” off_t that uses fseeko() and ftello().
$ c89 –D_LARGEFILE_SOURCE a.c
Certain system-specific or non-portable interfaces are not usable in the large file compilation environment. Known cases are:
Kernel data structures read from /dev/kmem.
Interfaces in the kernel virtual memory library, –lkvm.
Interfaces in the ELF access library, –lelf.
Interfaces to /proc defined in <procfs.h>.
The ustat(2) system call.
Programs that use these interfaces should not be compiled in the large file compilation environment. As a partial safeguard against making this mistake, including either of the <libelf.h> or <sys/procfs.h> header files will induce a compilation error when the large file compilation environment is enabled.
In general, caution should be exercised when using any separately-compiled library whose interfaces include data items of type off_t or the other redefined types either directly or indirectly, such as with 'struct stat'. (The redefined types are off_t , rlim_t, ino_t, blkcnt_t, fsblkcnt_t, and fsfilcnt_t.) For the large file compilation environment to work correctly with such a library, the library interfaces must include the appropriate xxx64() binary entry points and must have them mapped to the corresponding primary functions when _FILE_OFFSET_BITS is set to 64.
Care should be exercised using any of the printf() or scanf() routines on variables of the types mentioned above. In the large file compilation environment, these variables should be printed or scanned using long long formats.
Symbolic formats analogous to those found in <sys/int_fmtio.h> do not exist for printing or scanning variables of the types that are redefined in the large file compilation environment.