fopen(3p) — Linux manual page

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.1‐2017 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         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 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         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.1‐2017, stdio.h(0p)

COPYRIGHT         top

       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(3P)

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