The Linux Programming Interface

106 Chapter 5

if (argc != 3 || strcmp(argv[1], "--help") == 0)

usageErr("%s pathname offset\n", argv[0]);

fd = open64(argv[1], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);

if (fd == -1)

errExit("open64");

off = atoll(argv[2]);

if (lseek64(fd, off, SEEK_SET) == -1)

errExit("lseek64");

if (write(fd, "test", 4) == -1)

errExit("write");

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––– fileio/large_file.c

The _FILE_OFFSET_BITS macro

The recommended method of obtaining LFS functionality is to define the macro

_FILE_OFFSET_BITS with the value 64 when compiling a program. One way to do this

is via a command-line option to the C compiler:

$ cc -D_FILE_OFFSET_BITS=64 prog.c

Alternatively, we can define this macro in the C source before including any header

files:

#define _FILE_OFFSET_BITS 64

This automatically converts all of the relevant 32-bit functions and data types into

their 64-bit counterparts. Thus, for example, calls to open() are actually converted

into calls to open64(), and the off_t data type is defined to be 64 bits long. In other

words, we can recompile an existing program to handle large files without needing

to make any changes to the source code.

Using _FILE_OFFSET_BITS is clearly simpler than using the transitional LFS API,

but this approach relies on applications being cleanly written (e.g., correctly using

off_t to declare variables holding file offsets, rather than using a native C integer

type).

The _FILE_OFFSET_BITS macro is not required by the LFS specification, which

merely mentions this macro as an optional method of specifying the size of the off_t

data type. Some UNIX implementations use a different feature test macro to

obtain this functionality.

If we attempt to access a large file using 32-bit functions (i.e., from a program

compiled without setting _FILE_OFFSET_BITS to 64), then we may encounter the

error EOVERFLOW. For example, this error can occur if we attempt to use the 32-bit

version of stat() (Section 15.1) to retrieve information about a file whose size

exceeds 2 GB.