1LSEEK(3P) POSIX Programmer's Manual LSEEK(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 lseek — move the read/write file offset
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15 #include <unistd.h>
16 off_t lseek(int fildes, off_t offset, int whence);
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20 The lseek() function shall set the file offset for the open file
21 description associated with the file descriptor fildes, as follows:
22 * If whence is SEEK_SET, the file offset shall be set to offset
24 bytes.
25 * If whence is SEEK_CUR, the file offset shall be set to its current
27 location plus offset.
28 * If whence is SEEK_END, the file offset shall be set to the size of
30 the file plus offset.
31 The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END are defined in
33 <unistd.h>.
34 The behavior of lseek() on devices which are incapable of seeking is
36 implementation-defined. The value of the file offset associated with
37 such a device is undefined.
38 The lseek() function shall allow the file offset to be set beyond the
40 end of the existing data in the file. If data is later written at this
41 point, subsequent reads of data in the gap shall return bytes with the
42 value 0 until data is actually written into the gap.
43 The lseek() function shall not, by itself, extend the size of a file.
45 If fildes refers to a shared memory object, the result of the lseek()
47 function is unspecified.
48 If fildes refers to a typed memory object, the result of the lseek()
50 function is unspecified.
51
53 Upon successful completion, the resulting offset, as measured in bytes
54 from the beginning of the file, shall be returned. Otherwise, -1 shall
55 be returned, errno shall be set to indicate the error, and the file
56 offset shall remain unchanged.
57
59 The lseek() function shall fail if:
60 EBADF The fildes argument is not an open file descriptor.
62 EINVAL The whence argument is not a proper value, or the resulting file
64 offset would be negative for a regular file, block special file,
65 or directory.
66 EOVERFLOW
68 The resulting file offset would be a value which cannot be rep‐
69 resented correctly in an object of type off_t.
70 ESPIPE The fildes argument is associated with a pipe, FIFO, or socket.
72 The following sections are informative.
74
76 None.
77
79 None.
80
82 The ISO C standard includes the functions fgetpos() and fsetpos(),
83 which work on very large files by use of a special positioning type.
84 Although lseek() may position the file offset beyond the end of the
86 file, this function does not itself extend the size of the file. While
87 the only function in POSIX.1‐2008 that may directly extend the size of
88 the file is write(), truncate(), and ftruncate(), several functions
89 originally derived from the ISO C standard, such as fwrite(),
90 fprintf(), and so on, may do so (by causing calls on write()).
91 An invalid file offset that would cause [EINVAL] to be returned may be
93 both implementation-defined and device-dependent (for example, memory
94 may have few invalid values). A negative file offset may be valid for
95 some devices in some implementations.
96 The POSIX.1‐1990 standard did not specifically prohibit lseek() from
98 returning a negative offset. Therefore, an application was required to
99 clear errno prior to the call and check errno upon return to determine
100 whether a return value of (off_t)-1 is a negative offset or an indica‐
101 tion of an error condition. The standard developers did not wish to
102 require this action on the part of a conforming application, and chose
103 to require that errno be set to [EINVAL] when the resulting file offset
104 would be negative for a regular file, block special file, or directory.
105
107 None.
108
110 open()
111 The Base Definitions volume of POSIX.1‐2017, <sys_types.h>, <unistd.h>
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115 Portions of this text are reprinted and reproduced in electronic form
116 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
117 table Operating System Interface (POSIX), The Open Group Base Specifi‐
118 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
119 Electrical and Electronics Engineers, Inc and The Open Group. In the
120 event of any discrepancy between this version and the original IEEE and
121 The Open Group Standard, the original IEEE and The Open Group Standard
122 is the referee document. The original Standard can be obtained online
123 at http://www.opengroup.org/unix/online.html .
124 Any typographical or formatting errors that appear in this page are
126 most likely to have been introduced during the conversion of the source
127 files to man page format. To report such errors, see https://www.ker‐
128 nel.org/doc/man-pages/reporting_bugs.html .
129IEEE/The Open Group 2017 LSEEK(3P)