FSEEK(3)                   POSIX Programmer's Manual                  FSEEK(3)



PPRROOLLOOGG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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

SSYYNNOOPPSSIISS
       #include <stdio.h>

       int fseek(FILE *_s_t_r_e_a_m, long _o_f_f_s_e_t, int _w_h_e_n_c_e);
       int fseeko(FILE *_s_t_r_e_a_m, off_t _o_f_f_s_e_t, int _w_h_e_n_c_e);

DDEESSCCRRIIPPTTIIOONN
       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.

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

       The new position, measured in bytes from the beginning of the file,
       shall be obtained by adding _o_f_f_s_e_t to the position specified by _w_h_e_n_c_e.
       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 _o_f_f_s_e_t is either 0 or a value
       returned by an earlier call to _f_t_e_l_l() on the same stream and _w_h_e_n_c_e is
       SEEK_SET.

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

       If the most recent operation, other than _f_t_e_l_l(), on a given stream is
       _f_f_l_u_s_h(), the file offset in the underlying open file description shall
       be adjusted to reflect the location specified by _f_s_e_e_k().

       The _f_s_e_e_k() 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 _f_s_e_e_k() 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, _f_s_e_e_k() 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 _f_s_e_e_k() restores the
       stream's shift state is implementation-defined.

       The _f_s_e_e_k_o() function shall be equivalent to the _f_s_e_e_k() function
       except that the _o_f_f_s_e_t argument is of type ooffff__tt.

RREETTUURRNN VVAALLUUEE
       The _f_s_e_e_k() and _f_s_e_e_k_o() functions shall return 0 if they succeed.

       Otherwise, they shall return -1 and set _e_r_r_n_o to indicate the error.

EERRRROORRSS
       The _f_s_e_e_k() and _f_s_e_e_k_o() functions shall fail if, either the _s_t_r_e_a_m is
       unbuffered or the _s_t_r_e_a_m's buffer needed to be flushed, and the call to
       _f_s_e_e_k() or _f_s_e_e_k_o() causes an underlying _l_s_e_e_k() or _w_r_i_t_e() to be
       invoked, and:

       EEAAGGAAIINN The O_NONBLOCK flag is set for the file descriptor and the
              thread would be delayed in the write operation.

       EEBBAADDFF  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.

       EEFFBBIIGG  An attempt was made to write a file that exceeds the maximum
              file size.

       EEFFBBIIGG  An attempt was made to write a file that exceeds the file size
              limit of the process.

       EEFFBBIIGG  The file is a regular file and an attempt was made to write at
              or beyond the offset maximum associated with the corresponding
              stream.

       EEIINNTTRR  The write operation was terminated due to the receipt of a
              signal, and no data was transferred.

       EEIINNVVAALL The _w_h_e_n_c_e argument is invalid. The resulting file-position
              indicator would be set to a negative value.

       EEIIOO    A physical I/O error has occurred, or the process is a member of
              a background process group attempting to perform a _w_r_i_t_e() 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.

       EENNOOSSPPCC There was no free space remaining on the device containing the
              file.

       EEOOVVEERRFFLLOOWW
              For _f_s_e_e_k(), the resulting file offset would be a value which
              cannot be represented correctly in an object of type lloonngg.

       EEOOVVEERRFFLLOOWW
              For _f_s_e_e_k_o(), the resulting file offset would be a value which
              cannot be represented correctly in an object of type ooffff__tt.

       EEPPIIPPEE  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.

       EESSPPIIPPEE The file descriptor underlying _s_t_r_e_a_m is associated with a pipe,
              FIFO, or socket.

       The _f_s_e_e_k() and _f_s_e_e_k_o() functions may fail if:

       EENNXXIIOO  A request was made of a nonexistent device, or the request was
              outside the capabilities of the device.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
       None.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

RRAATTIIOONNAALLEE
       None.

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _S_e_c_t_i_o_n _2_._5, _S_t_a_n_d_a_r_d _I_/_O _S_t_r_e_a_m_s, _f_o_p_e_n(), _f_s_e_t_p_o_s(), _f_t_e_l_l(),
       _g_e_t_r_l_i_m_i_t(), _l_s_e_e_k(), _r_e_w_i_n_d(), _u_l_i_m_i_t(), _u_n_g_e_t_c(), _w_r_i_t_e()

       The Base Definitions volume of POSIX.1-2017, <<ssttddiioo..hh>>

CCOOPPYYRRIIGGHHTT
       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 .

       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 .



IEEE/The Open Group                  2017                             FSEEK(3)
