lseek.3p (5192B)
- '\" et
- .TH LSEEK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- lseek
- \(em move the read/write file offset
- .SH SYNOPSIS
- .LP
- .nf
- #include <unistd.h>
- .P
- off_t lseek(int \fIfildes\fP, off_t \fIoffset\fP, int \fIwhence\fP);
- .fi
- .SH DESCRIPTION
- The
- \fIlseek\fR()
- function shall set the file offset for the open file description
- associated with the file descriptor
- .IR fildes,
- as follows:
- .IP " *" 4
- If
- .IR whence
- is SEEK_SET, the file offset shall be set to
- .IR offset
- bytes.
- .IP " *" 4
- If
- .IR whence
- is SEEK_CUR, the file offset shall be set to its current location plus
- .IR offset .
- .IP " *" 4
- If
- .IR whence
- is SEEK_END, the file offset shall be set to the size of the file plus
- .IR offset .
- .P
- The symbolic constants SEEK_SET, SEEK_CUR, and SEEK_END
- are defined in
- .IR <unistd.h> .
- .P
- The behavior of
- \fIlseek\fR()
- on devices which are incapable of seeking is implementation-defined.
- The value of the file offset associated with such a device is
- undefined.
- .P
- The
- \fIlseek\fR()
- function shall allow the file offset to be set beyond the end of the
- existing data in the file. If data is later written at this point,
- subsequent reads of data in the gap shall return bytes with the value 0
- until data is actually written into the gap.
- .P
- The
- \fIlseek\fR()
- function shall not, by itself, extend the size of a file.
- .P
- If
- .IR fildes
- refers to a shared memory object, the result of the
- \fIlseek\fR()
- function is unspecified.
- .P
- If
- .IR fildes
- refers to a typed memory object, the result of the
- \fIlseek\fR()
- function is unspecified.
- .SH "RETURN VALUE"
- Upon successful completion, the resulting offset, as measured in bytes
- from the beginning of the file, shall be returned. Otherwise, \-1
- shall be returned,
- .IR errno
- shall be set to indicate the error, and the file offset shall remain
- unchanged.
- .SH ERRORS
- The
- \fIlseek\fR()
- function shall fail if:
- .TP
- .BR EBADF
- The
- .IR fildes
- argument is not an open file descriptor.
- .TP
- .BR EINVAL
- The
- .IR whence
- argument is not a proper value, or the resulting file offset would be
- negative for a regular file, block special file, or directory.
- .TP
- .BR EOVERFLOW
- The resulting file offset would be a value which cannot be represented
- correctly in an object of type
- .BR off_t .
- .TP
- .BR ESPIPE
- The
- .IR fildes
- argument is associated with a pipe, FIFO, or socket.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- None.
- .SH RATIONALE
- The ISO\ C standard includes the functions
- \fIfgetpos\fR()
- and
- \fIfsetpos\fR(),
- which work on very large files by use of a special positioning type.
- .P
- Although
- \fIlseek\fR()
- may position the file offset beyond the end of the file, this function
- does not itself extend the size of the file. While the only function
- in POSIX.1\(hy2008 that may directly extend the size of the file is
- \fIwrite\fR(),
- \fItruncate\fR(),
- and
- \fIftruncate\fR(),
- several functions originally derived from the ISO\ C standard, such as
- \fIfwrite\fR(),
- \fIfprintf\fR(),
- and so on, may do so (by causing calls on
- \fIwrite\fR()).
- .P
- An invalid file offset that would cause
- .BR [EINVAL]
- to be returned may be both implementation-defined and
- device-dependent (for example, memory may have few invalid values). A
- negative file offset may be valid for some devices in some
- implementations.
- .P
- The POSIX.1\(hy1990 standard did not specifically prohibit
- \fIlseek\fR()
- from returning a negative offset. Therefore, an application was
- required to clear
- .IR errno
- prior to the call and check
- .IR errno
- upon return to determine whether a return value of (\c
- .BR off_t )\-1
- is a negative offset or an indication of an error condition. The
- standard developers did not wish to require this action on the part of
- a conforming application, and chose to require that
- .IR errno
- be set to
- .BR [EINVAL]
- when the resulting file offset would be negative for a regular file,
- block special file, or directory.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fIopen\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<sys_types.h>\fP",
- .IR "\fB<unistd.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .