问题
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?
回答1:
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.
来源:https://stackoverflow.com/questions/18293310/how-can-i-portably-turn-on-large-file-support