1lseek64(3) Library Functions Manual lseek64(3)
2
6 lseek64 - reposition 64-bit read/write file offset
7
9 Standard C library (libc, -lc)
10
12 #define _LARGEFILE64_SOURCE /* See feature_test_macros(7) */
13 #include <sys/types.h>
14 #include <unistd.h>
15 off64_t lseek64(int fd, off64_t offset, int whence);
17
19 The lseek() family of functions reposition the offset of the open file
20 associated with the file descriptor fd to offset bytes relative to the
21 start, current position, or end of the file, when whence has the value
22 SEEK_SET, SEEK_CUR, or SEEK_END, respectively.
23 For more details, return value, and errors, see lseek(2).
25 Four interfaces are available: lseek(), lseek64(), llseek(), and
27 _llseek().
28 lseek()
30 Prototype:
31 off_t lseek(int fd, off_t offset, int whence);
33 The C library's lseek() wrapper function uses the type off_t. This is
35 a 32-bit signed type on 32-bit architectures, unless one compiles with
36 #define _FILE_OFFSET_BITS 64
38 in which case it is a 64-bit signed type.
40 lseek64()
42 Prototype:
43 off64_t lseek64(int fd, off64_t offset, int whence);
45 The lseek64() library function uses a 64-bit type even when off_t is a
47 32-bit type. Its prototype (and the type off64_t) is available only
48 when one compiles with
49 #define _LARGEFILE64_SOURCE
51 The function lseek64() is available since glibc 2.1.
53 llseek()
55 Prototype:
56 loff_t llseek(int fd, loff_t offset, int whence);
58 The type loff_t is a 64-bit signed type. The llseek() library function
60 is available in glibc and works without special defines. However, the
61 glibc headers do not provide a prototype. Users should add the above
62 prototype, or something equivalent, to their own source. When users
63 complained about data loss caused by a miscompilation of e2fsck(8),
64 glibc 2.1.3 added the link-time warning
65 "the `llseek´ function may be dangerous; use `lseek64´ instead."
67 This makes this function unusable if one desires a warning-free compi‐
69 lation.
70 Since glibc 2.28, this function symbol is no longer available to newly
72 linked applications.
73 _llseek()
75 On 32-bit architectures, this is the system call that is used (by the C
76 library wrapper functions) to implement all of the above functions.
77 The prototype is:
78 int _llseek(int fd, off_t offset_hi, off_t offset_lo,
80 loff_t *result, int whence);
81 For more details, see llseek(2).
83 64-bit systems don't need an _llseek() system call. Instead, they have
85 an lseek(2) system call that supports 64-bit file offsets.
86
88 For an explanation of the terms used in this section, see at‐
89 tributes(7).
90 ┌────────────────────────────────────────────┬───────────────┬─────────┐
92 │Interface │ Attribute │ Value │
93 ├────────────────────────────────────────────┼───────────────┼─────────┤
94 │lseek64() │ Thread safety │ MT-Safe │
95 └────────────────────────────────────────────┴───────────────┴─────────┘
96
98 lseek64() is one of the functions that was specified in the Large File
99 Summit (LFS) specification that was completed in 1996. The purpose of
100 the specification was to provide transitional support that allowed
101 applications on 32-bit systems to access files whose size exceeds that
102 which can be represented with a 32-bit off_t type. As noted above,
103 this symbol is exposed by header files if the _LARGEFILE64_SOURCE
104 feature test macro is defined. ALternatively, on a 32-bit system, the
105 symbol lseek is aliased to lseek64 if the macro _FILE_OFFSET_BITS is
106 defined with the value 64.
107
109 llseek(2), lseek(2)
110Linux man-pages 6.05 2023-07-20 lseek64(3)