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



PROLOG
       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.

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.1-2017
       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(()) 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
              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 without the trailing <slash> characters would name
              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 aa 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.1-2017, <stdio.h>

COPYRIGHT
       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                             FOPEN(3)
