I am currently writing a C program that reads and writes files that might be over 2 GiB in size. On linux feature_test_macros(7) specifies:
_LARGEFILE64_SOURCE Expose definitions for the alternative API specified by the LFS (Large File Summit) as a "tran‐ sitional extension" to the Single UNIX Specification. (See ⟨http://opengroup.org/platform /lfs.html⟩) The alternative API consists of a set of new objects (i.e., functions and types) whose names are suffixed with "64" (e.g., off64_t versus off_t, lseek64() versus lseek(), etc.). New programs should not employ this interface; instead _FILE_OFFSET_BITS=64 should be employed. _FILE_OFFSET_BITS Defining this macro with the value 64 automatically converts references to 32-bit functions and data types related to file I/O and file system operations into references to their 64-bit coun‐ terparts. This is useful for performing I/O on large files (> 2 Gigabytes) on 32-bit systems. (Defining this macro permits correctly written programs to use large files with only a recompi‐ lation being required.) 64-bit systems naturally permit file sizes greater than 2 Gigabytes, and on those systems this macro has no effect.
Is this guaranteed to be portable?
Solaris suggests in lfcompile(5):
Applications can be compiled in the large file compilation environment by using the following methods: o Use the getconf(1) utility with one or more of the arguments listed in the table below. This method is recommended for portable applications. ____________________________________________________________ | argument | purpose | |__________________|________________________________________| | LFS_CFLAGS | obtain compilation flags necessary to| | | enable the large file compilation| | | environment | | LFS_LDFLAGS | obtain link editor options | | LFS_LIBS | obtain link library names | | LFS_LINTFLAGS | obtain lint options | |__________________|________________________________________| o Set the compile-time flag _FILE_OFFSET_BITS to 64 before including any headers. Applications may com- bine objects produced in the large file compilation environment with objects produced in the transi- tional 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 com- pilation environments.
Is this portable? On my linux machine, passing these keys to getconf will output nothing. (Which means the keys exist).
Is there any portable approach?
Use the confstr interface or the corresponding getconf program and try the various _CS_POSIX_V7_ILP32_OFFBIG_CFLAGS, etc. options until you find one that works. This will give you CFLAGS, LDFLAGS, and LIBS needed for an appropriate environment.