# freopen

## 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



           freopen — open a stream
    


## SYNOPSIS



           #include <stdio.h>
    
           FILE *freopen(const char *restrict _pathname_ , const char *restrict _mode_ ,
               FILE *restrict _stream_);
    


## 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 _freopen_() function shall first attempt to flush the stream
           associated with _stream_ as if by a call to _fflush_(_stream_).  Failure
           to flush the stream successfully shall be ignored. If _pathname_ is
           not a null pointer, _freopen_() shall close any file descriptor
           associated with _stream_.  Failure to close the file descriptor
           successfully shall be ignored.  The error and end-of-file
           indicators for the stream shall be cleared.
    
           The _freopen_() function shall open the file whose pathname is the
           string pointed to by _pathname_ and associate the stream pointed to
           by _stream_ with it. The _mode_ argument shall be used just as in
           _fopen_().
    
           The original stream shall be closed regardless of whether the
           subsequent open succeeds.
    
           If _pathname_ is a null pointer, the _freopen_() function shall
           attempt to change the mode of the stream to that specified by
           _mode_ , as if the name of the file currently associated with the
           stream had been used. In this case, the file descriptor associated
           with the stream need not be closed if the call to _freopen_()
           succeeds. It is implementation-defined which changes of mode are
           permitted (if any), and under what circumstances.
    
           After a successful call to the _freopen_() 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.
    
           If _pathname_ is not a null pointer, or if _pathname_ is a null
           pointer and the specified mode change necessitates the file
           descriptor associated with the stream to be closed and reopened,
           the file descriptor associated with the reopened stream shall be
           allocated and opened as if by a call to _open_() with the following
           flags:
                   ┌──────────────────┬───────────────────────────┐
                   │  _freopen_**() 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, _freopen_() shall return the value of
           _stream_.  Otherwise, a null pointer shall be returned, and _errno_
           shall be set to indicate the error.
    


## ERRORS



           The _freopen_() 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.
    
           **EBADF** The file descriptor underlying the stream is not a valid
                  file descriptor when _pathname_ is a null pointer.
    
           **EINTR** A signal was caught during _freopen_().
    
           **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.
    
           **ENAMETOOLONG**
                  The length of a component of a pathname is longer than
                  {NAME_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 it
                  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 _freopen_() function may fail if:
    
           **EBADF** The mode with which the file descriptor underlying the
                  stream was opened does not support the requested mode when
                  _pathname_ is a null pointer.
    
           **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.
    
           **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}.
    
           **ENOMEM** Insufficient storage space is available.
    
           **ENXIO** A request was made of a nonexistent device, or the request
                  was outside the capabilities of the device.
    
           **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



       **Directing Standard Output to a File**
           The following example logs all standard output to the **/tmp/logfile**
           file.
    
               #include <stdio.h>
               ...
               FILE *fp;
               ...
               fp = freopen ("/tmp/logfile", "a+", stdout);
               ...
    


## APPLICATION USAGE



           The _freopen_() function is typically used to attach the pre-opened
           _streams_ associated with _stdin_ , _stdout_ , and _stderr_ to other files.
    
           Since implementations are not required to support any stream mode
           changes when the _pathname_ argument is NULL, portable applications
           cannot rely on the use of _freopen_() to change the stream mode, and
           use of this feature is discouraged. The feature was originally
           added to the ISO C standard in order to facilitate changing _stdin_
           and _stdout_ to binary mode. Since a **'b'** character in the mode has
           no effect on POSIX systems, this use of the feature is unnecessary
           in POSIX applications. However, even though the **'b'** is ignored, a
           successful call to _freopen_(NULL, "_wb_ ", _stdout_) does have an
           effect. In particular, for regular files it truncates the file and
           sets the file-position indicator for the stream to the start of
           the file. It is possible that these side-effects are an unintended
           consequence of the way the feature is specified in the
           ISO/IEC 9899:1999 standard, but unless or until the ISO C standard
           is changed, applications which successfully call _freopen_(NULL,
           "_wb_ ", _stdout_) will behave in unexpected ways on conforming systems
           in situations such as:
    
               { appl file1; appl file2; } > file3
    
           which will result in **file3** containing only the output from the
           second invocation of _appl_.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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