FREAD(3)                   POSIX Programmer's Manual                  FREAD(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
       fread -- binary input

SSYYNNOOPPSSIISS
       #include <stdio.h>

       size_t fread(void *restrict _p_t_r, size_t _s_i_z_e, size_t _n_i_t_e_m_s,
           FILE *restrict _s_t_r_e_a_m);

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_r_e_a_d() function shall read into the array pointed to by _p_t_r up to
       _n_i_t_e_m_s elements whose size is specified by _s_i_z_e in bytes, from the
       stream pointed to by _s_t_r_e_a_m.  For each object, _s_i_z_e calls shall be made
       to the _f_g_e_t_c() function and the results stored, in the order read, in
       an array of uunnssiiggnneedd cchhaarr exactly overlaying the object. The file
       position indicator for the stream (if defined) shall be advanced by the
       number of bytes successfully read. If an error occurs, the resulting
       value of the file position indicator for the stream is unspecified. If
       a partial element is read, its value is unspecified.

       The _f_r_e_a_d() function may mark the last data access timestamp of the
       file associated with _s_t_r_e_a_m for update. The last data access timestamp
       shall be marked for update by the first successful execution of
       _f_g_e_t_c(), _f_g_e_t_s(), _f_r_e_a_d(), _f_s_c_a_n_f(), _g_e_t_c(), _g_e_t_c_h_a_r(), _g_e_t_d_e_l_i_m(),
       _g_e_t_l_i_n_e(), _g_e_t_s(), or _s_c_a_n_f() using _s_t_r_e_a_m that returns data not
       supplied by a prior call to _u_n_g_e_t_c().

RREETTUURRNN VVAALLUUEE
       Upon successful completion, _f_r_e_a_d() shall return the number of elements
       successfully read which is less than _n_i_t_e_m_s only if a read error or
       end-of-file is encountered. If _s_i_z_e or _n_i_t_e_m_s is 0, _f_r_e_a_d() shall
       return 0 and the contents of the array and the state of the stream
       remain unchanged. Otherwise, if a read error occurs, the error
       indicator for the stream shall be set, and _e_r_r_n_o shall be set to
       indicate the error.

EERRRROORRSS
       Refer to _f_g_e_t_c().

       _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
   RReeaaddiinngg ffrroomm aa SSttrreeaamm
       The following example transfers a single 100-byte fixed length record
       from the _f_p stream into the array pointed to by _b_u_f.


           #include <stdio.h>
           ...
           size_t elements_read;
           char buf[100];
           FILE *fp;
           ...
           elements_read = fread(buf, sizeof(buf), 1, fp);
           ...

       If a read error occurs, _e_l_e_m_e_n_t_s___r_e_a_d will be zero but the number of
       bytes read from the stream could be anything from zero to
       _s_i_z_e_o_f(_b_u_f)-1.

       The following example reads multiple single-byte elements from the _f_p
       stream into the array pointed to by _b_u_f.


           #include <stdio.h>
           ...
           size_t bytes_read;
           char buf[100];
           FILE *fp;
           ...
           bytes_read = fread(buf, 1, sizeof(buf), fp);
           ...

       If a read error occurs, _b_y_t_e_s___r_e_a_d will contain the number of bytes
       read from the stream.

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       The _f_e_r_r_o_r() or _f_e_o_f() functions must be used to distinguish between an
       error condition and an end-of-file condition.

       Because of possible differences in element length and byte ordering,
       files written using _f_w_r_i_t_e() are application-dependent, and possibly
       cannot be read using _f_r_e_a_d() by a different application or by the same
       application on a different processor.

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_e_o_f(), _f_e_r_r_o_r(), _f_g_e_t_c(), _f_o_p_e_n(),
       _f_s_c_a_n_f(), _g_e_t_c(), _g_e_t_s()

       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                             FREAD(3)
