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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  following,  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 standard conformance.
       Opening a file with read mode (r as the first character 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 operation encounters end-of-file.

       When opened, a stream is fully buffered if and only if it can be determined 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 conversion state.

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

                             ┌─────────────────┬───────────────────────────┐
                             │  fopen() Modeopen() 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 resolution 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 existing file, or pathname is an empty string.

       ENOENT or ENOTDIR
              The pathname argument contains at least one non-<slash> character 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(), fmemopen(), 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  Electrical  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.kernel.org/doc/man-pages/reporting_bugs.html .