PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       fopen — open a stream

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

       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() 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         top

       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         top

       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         top

   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         top

       None.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.5, Standard I/O Streams, creat(3p), fclose(3p), fdopen(3p),
       fmemopen(3p), freopen(3p), open_memstream(3p)

       The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)

COPYRIGHT         top

       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 .

IEEE/The Open Group                 2013                           FOPEN(3P)

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