The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


fseek, fseeko - reposition a file-position indicator in a stream


#include <stdio.h>

int fseek(FILE *
stream, long offset, int whence);

[CX] [Option Start] int fseeko(FILE *stream, off_t offset, int whence); [Option End]


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard. [Option End]

The fseek() function shall set the file-position indicator for the stream pointed to by stream. If a read or write error occurs, the error indicator for the stream shall be set and fseek() fails.

The new position, measured in bytes from the beginning of the file, shall be obtained by adding offset to the position specified by whence. The specified point is the beginning of the file for SEEK_SET, the current value of the file-position indicator for SEEK_CUR, or end-of-file for SEEK_END.

If the stream is to be used with wide-character input/output functions, the application shall ensure that offset is either 0 or a value returned by an earlier call to ftell() on the same stream and whence is SEEK_SET.

A successful call to fseek() shall clear the end-of-file indicator for the stream and undo any effects of ungetc() and ungetwc() on the same stream. After an fseek() call, the next operation on an update stream may be either input or output.

[CX] [Option Start] If the most recent operation, other than ftell(), on a given stream is fflush(), the file offset in the underlying open file description shall be adjusted to reflect the location specified by fseek().

The fseek() function shall allow the file-position indicator to be set beyond the end of 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.

The behavior of fseek() on devices which are incapable of seeking is implementation-defined. The value of the file offset associated with such a device is undefined.

If the stream is writable and buffered data had not been written to the underlying file, fseek() shall cause the unwritten data to be written to the file and shall mark the last data modification and last file status change timestamps of the file for update.

In a locale with state-dependent encoding, whether fseek() restores the stream's shift state is implementation-defined.

The fseeko() function shall be equivalent to the fseek() function except that the offset argument is of type off_t. [Option End]


The fseek() [CX] [Option Start]  and fseeko() [Option End] functions shall return 0 if they succeed.

[CX] [Option Start] Otherwise, they shall return -1 and set errno to indicate the error. [Option End]


The fseek() [CX] [Option Start]  and fseeko() [Option End]  functions shall fail if, [CX] [Option Start]  either the stream is unbuffered or the stream's buffer needed to be flushed, and the call to fseek() or fseeko() causes an underlying lseek() or write() to be invoked, and: [Option End]

[CX] [Option Start] The O_NONBLOCK flag is set for the file descriptor and the thread would be delayed in the write operation. [Option End]
[CX] [Option Start] The file descriptor underlying the stream file is not open for writing or the stream's buffer needed to be flushed and the file is not open. [Option End]
[CX] [Option Start] An attempt was made to write a file that exceeds the maximum file size. [Option End]
[XSI] [Option Start] An attempt was made to write a file that exceeds the file size limit of the process. [Option End]
[CX] [Option Start] The file is a regular file and an attempt was made to write at or beyond the offset maximum associated with the corresponding stream. [Option End]
[CX] [Option Start] The write operation was terminated due to the receipt of a signal, and no data was transferred. [Option End]
[CX] [Option Start] The whence argument is invalid. The resulting file-position indicator would be set to a negative value. [Option End]
[CX] [Option Start] A physical I/O error has occurred, or the process is a member of a background process group attempting to perform a write() to its controlling terminal, TOSTOP is set, the calling thread is not blocking SIGTTOU, the process is not ignoring SIGTTOU, and the process group of the process is orphaned. This error may also be returned under implementation-defined conditions. [Option End]
[CX] [Option Start] There was no free space remaining on the device containing the file. [Option End]
[CX] [Option Start] For fseek(), the resulting file offset would be a value which cannot be represented correctly in an object of type long. [Option End]
[CX] [Option Start] For fseeko(), the resulting file offset would be a value which cannot be represented correctly in an object of type off_t. [Option End]
[CX] [Option Start] An attempt was made to write to a pipe or FIFO that is not open for reading by any process; a SIGPIPE signal shall also be sent to the thread. [Option End]
[CX] [Option Start] The file descriptor underlying stream is associated with a pipe, FIFO, or socket. [Option End]

The fseek() [CX] [Option Start]  and fseeko() [Option End]  functions may fail if:

[CX] [Option Start] A request was made of a nonexistent device, or the request was outside the capabilities of the device. [Option End]

The following sections are informative.










Standard I/O Streams, fopen, fsetpos, ftell, getrlimit, lseek, rewind, ulimit, ungetc, write

XBD <stdio.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the DESCRIPTION.

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following change is incorporated for alignment with the FIPS requirements:

The normative text is updated to avoid use of the term "must" for application requirements.

The DESCRIPTION is updated to explicitly state that fseek() sets the file-position indicator, and then on error the error indicator is set and fseek() fails. This is for alignment with the ISO/IEC 9899:1999 standard.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/42 is applied, updating the [EAGAIN] error in the ERRORS section from "the process would be delayed" to "the thread would be delayed".

Issue 7

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0188 [79], XSH/TC1-2008/0189 [122], XSH/TC1-2008/0190 [225], and XSH/TC1-2008/0191 [14] are applied.

End of informative text.


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