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FOPEN(3P)		   POSIX Programmer's Manual		     FOPEN(3P)

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.

NAME
       fopen — open a stream

SYNOPSIS
       #include <stdio.h>

       FILE *fopen(const char *restrict pathname, const char *restrict mode);

DESCRIPTION
       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.1‐2008
       defers to the ISO C standard.

       The  fopen()  function shall open the file whose pathname is the string
       pointed to by pathname, and associates a stream with it.

       The mode argument points to a string. If the string is one of the  fol‐
       lowing,	the file shall be opened in the indicated mode. Otherwise, the
       behavior is undefined.

       r or rb	     Open file for reading.

       w or wb	     Truncate to zero length or create file for writing.

       a or ab	     Append; open or create file for writing at end-of-file.

       r+ or rb+ or r+b
		     Open file for update (reading and writing).

       w+ or wb+ or w+b
		     Truncate to zero length or create file for update.

       a+ or ab+ or a+b
		     Append; open or create file for update, writing  at  end-
		     of-file.

       The  character 'b' shall have no effect, but is allowed for ISO C stan‐
       dard conformance.  Opening a file with read mode (r as the first	 char‐
       acter  in  the  mode argument) shall fail if the file does not exist or
       cannot be read.

       Opening a file with append mode (a as the first character in  the  mode
       argument) shall cause all subsequent writes to the file to be forced to
       the then	 current  end-of-file,	regardless  of	intervening  calls  to
       fseek().

       When  a	file  is  opened  with update mode ('+' as the second or third
       character in the mode argument), both input and output may be performed
       on  the	associated  stream. However, the application shall ensure that
       output is not directly followed by input without an intervening call to
       fflush()	 or  to	 a  file  positioning function (fseek(), fsetpos(), or
       rewind()), and input is not directly  followed  by  output  without  an
       intervening call to a file positioning function, unless the input oper‐
       ation encounters end-of-file.

       When opened, a stream is fully buffered if and only if it can be deter‐
       mined  not to refer to an interactive device. The error and end-of-file
       indicators for the stream shall be cleared.

       If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b,	and  the  file
       did  not	 previously  exist,  upon successful completion, fopen() shall
       mark for update the last data access, last data modification, and  last
       file  status  change  timestamps	 of  the file and the last file status
       change and last data modification timestamps of the parent directory.

       If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b,	and  the  file
       did  not	 previously exist, the fopen() function shall create a file as
       if it called the creat() function with a value appropriate for the path
       argument	 interpreted  from pathname and a value of S_IRUSR | S_IWUSR |
       S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.

       If mode is w, wb, w+, wb+, or w+b, and the file did  previously	exist,
       upon successful completion, fopen() shall mark for update the last data
       modification and last file status change timestamps of the file.

       After a successful call to the fopen() function, the orientation of the
       stream  shall  be  cleared, the encoding rule shall be cleared, and the
       associated mbstate_t object shall be set to describe an initial conver‐
       sion state.

       The  file  descriptor  associated with the opened stream shall be allo‐
       cated and opened as if by a call to open() with the following flags:

		   ┌─────────────────┬───────────────────────────┐
		   │  fopen() Mode   │	     open() Flags	 │
		   ├─────────────────┼───────────────────────────┤
		   │r or rb	     │ O_RDONLY			 │
		   │w or wb	     │ O_WRONLY|O_CREAT|O_TRUNC	 │
		   │a or ab	     │ O_WRONLY|O_CREAT|O_APPEND │
		   │r+ or rb+ or r+b │ O_RDWR			 │
		   │w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC	 │
		   │a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND	 │
		   └─────────────────┴───────────────────────────┘
RETURN VALUE
       Upon successful completion, fopen()  shall  return  a  pointer  to  the
       object  controlling  the	 stream.  Otherwise,  a	 null pointer shall be
       returned, and errno shall be set to indicate the error.

ERRORS
       The fopen() function shall fail if:

       EACCES Search permission is denied on a component of the	 path  prefix,
	      or  the  file  exists  and the permissions specified by mode are
	      denied, or the file does	not  exist  and	 write	permission  is
	      denied for the parent directory of the file to be created.

       EINTR  A signal was caught during fopen().

       EISDIR The named file is a directory and mode requires write access.

       ELOOP  A loop exists in symbolic links encountered during resolution of
	      the path argument.

       EMFILE All file descriptors available  to  the  process	are  currently
	      open.

       EMFILE {STREAM_MAX} streams are currently open in the calling process.

       ENAMETOOLONG
	      The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
	      tion of a symbolic link produced an intermediate result  with  a
	      length that exceeds {PATH_MAX}.

       ENFILE The  maximum  allowable number of files is currently open in the
	      system.

       ENOENT The mode string begins with 'r' and a component of pathname does
	      not  name an existing file, or mode begins with 'w' or 'a' and a
	      component of the path prefix of pathname does not name an exist‐
	      ing file, or pathname is an empty string.

       ENOENT or ENOTDIR
	      The  pathname argument contains at least one non-<slash> charac‐
	      ter and ends with one or more trailing  <slash>  characters.  If
	      pathname	names  an  existing  file, an [ENOENT] error shall not
	      occur.

       ENOSPC The directory or file system that would  contain	the  new  file
	      cannot be expanded, the file does not exist, and the file was to
	      be created.

       ENOTDIR
	      A component of the path prefix names an existing	file  that  is
	      neither  a  directory nor a symbolic link to a directory, or the
	      pathname argument contains at least  one	non-<slash>  character
	      and  ends	 with  one or more trailing <slash> characters and the
	      last pathname component names an existing file that is neither a
	      directory nor a symbolic link to a directory.

       ENXIO  The named file is a character special or block special file, and
	      the device associated with this special file does not exist.

       EOVERFLOW
	      The named file is a regular file and the size of the file cannot
	      be represented correctly in an object of type off_t.

       EROFS  The  named  file	resides	 on  a	read-only file system and mode
	      requires write access.

       The fopen() function may fail if:

       EINVAL The value of the mode argument is not valid.

       ELOOP  More than {SYMLOOP_MAX} symbolic links were  encountered	during
	      resolution of the path argument.

       EMFILE {FOPEN_MAX} streams are currently open in the calling process.

       ENAMETOOLONG
	      The  length  of  a  component  of	 a  pathname  is  longer  than
	      {NAME_MAX}.

       ENOMEM Insufficient storage space is available.

       ETXTBSY
	      The file is a pure procedure (shared text) file  that  is	 being
	      executed and mode requires write access.

       The following sections are informative.

EXAMPLES
   Opening a File
       The  following  example	tries to open the file named file for reading.
       The fopen() function returns a file pointer that is used in  subsequent
       fgets()	and  fclose()  calls.  If the program cannot open the file, it
       just ignores it.

	   #include <stdio.h>
	   ...
	   FILE *fp;
	   ...
	   void rgrep(const char *file)
	   {
	   ...
	       if ((fp = fopen(file, "r")) == NULL)
		   return;
	   ...
	   }

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.5, Standard I/O Streams, creat(), fclose(), fdopen(),	fmemo‐
       pen(), freopen(), open_memstream()

       The Base Definitions volume of POSIX.1‐2008, <stdio.h>

COPYRIGHT
       Portions	 of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating	System	Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
       cal  and	 Electronics  Engineers,  Inc  and  The	 Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum	 1  applied.)  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.unix.org/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.ker‐
       nel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group		     2013			     FOPEN(3P)
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