FOPEN(3C) Standard C Library Functions FOPEN(3C)


NAME


fopen - open a stream

SYNOPSIS


#include <stdio.h>

FILE *fopen(const char *filename, const char *mode);


DESCRIPTION


The fopen() function opens the file whose pathname is the string pointed
to by filename, and associates a stream with it.


The argument mode points to a string beginning with one of the following
base sequences:

r
Open file for reading.


w
Truncate to zero length or create file for writing.


a
Append; open or create file for writing at end-of-
file.


r+
Open file for update (reading and writing).


w+
Truncate to zero length or create file for update.


a+
Append; open or create file for update, writing at
end-of-file.


In addition to the base sequences for the mode argument above, three
additional flags are supported via the b character, the e character, and
the x character. Order of these additional flags (including the +) does
not matter.


The character b has no effect, but is allowed for ISO C standard
conformance (see standards(5)). Opening a file with read mode (r as the
first character in the mode argument) fails if the file does not exist or
cannot be read.


The character e will cause the underlying file descriptor to be opened
with the O_CLOEXEC flag as described in open(2).


The character x will attempt to open the specified file exclusively. This
is the same behavior as opening the underlying file with the O_EXCL flag
as described in open(2).


Opening a file with append mode (a as the first character in the mode
argument) causes all subsequent writes to the file to be forced to the
then current end-of-file, regardless of intervening calls to fseek(3C).
If two separate processes open the same file for append, each process may
write freely to the file without fear of destroying output being written
by the other. The output from the two processes will be intermixed in
the file in the order in which it is written.


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, output must not be directly followed
by input without an intervening call to fflush(3C) or to a file
positioning function ( fseek(3C), fsetpos(3C) or rewind(3C)), and input
must not be 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 are cleared.


If mode begins with w or a and the file did not previously exist, upon
successful completion, fopen() function will mark for update the
st_atime, st_ctime and st_mtime fields of the file and the st_ctime and
st_mtime fields of the parent directory.


If mode begins with w and the file did previously exist, upon successful
completion, fopen() will mark for update the st_ctime and st_mtime fields
of the file. The fopen() function will allocate a file descriptor as
open(2) does.


Normally, 32-bit applications return an EMFILE error when attempting to
associate a stream with a file accessed by a file descriptor with a value
greater than 255. If the last character of mode is F, 32-bit applications
will be allowed to associate a stream with a file accessed by a file
descriptor with a value greater than 255. A FILE pointer obtained in this
way must never be used by any code that might directly access fields in
the FILE structure. If the fields in the FILE structure are used directly
by 32-bit applications when the last character of mode is F, data
corruption could occur. See the USAGE section of this manual page and the
enable_extended_FILE_stdio(3C) manual page for other options for enabling
the extended FILE facility.


In 64-bit applications, the last character of mode is silently ignored if
it is F. 64-bit applications are always allowed to associate a stream
with a file accessed by a file descriptor with any value.


The largest value that can be represented correctly in an object of type
off_t will be established as the offset maximum in the open file
description.

RETURN VALUES


Upon successful completion, fopen() returns a pointer to the object
controlling the stream. Otherwise, a null pointer is returned and errno
is set to indicate the error.


The fopen() function may fail and not set errno if there are no free
stdio streams.

ERRORS


The fopen() function will 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 the execution of fopen().


EISDIR
The named file is a directory and mode requires write
access.


ELOOP
Too many symbolic links were encountered in resolving
path.


EMFILE
There are {OPEN_MAX} file descriptors currently open in
the calling process.


ENAMETOOLONG
The length of the filename exceeds PATH_MAX or a pathname
component is longer than NAME_MAX.


ENFILE
The maximum allowable number of files is currently open
in the system.


ENOENT
A component of filename does not name an existing file or
filename is an empty string.


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 is not 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 current value of the file position cannot be
represented correctly in an object of type fpos_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.


EMFILE
{FOPEN_MAX} streams are currently open in the calling
process.

{STREAM_MAX} streams are currently open in the calling
process.


ENAMETOOLONG
Pathname resolution of a symbolic link produced an
intermediate result whose length exceeds {PATH_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.


USAGE


A process is allowed to have at least {FOPEN_MAX} stdio streams open at a
time. For 32-bit applications, however, the underlying ABIs formerly
required that no file descriptor used to access the file underlying a
stdio stream have a value greater than 255. To maintain binary
compatibility with earlier Solaris releases, this limit still constrains
32-bit applications. However, when a 32-bit application is aware that no
code that has access to the FILE pointer returned by fopen() will use the
FILE pointer to directly access any fields in the FILE structure, the F
character can be used as the last character in the mode argument to
circumvent this limit. Because it could lead to data corruption, the F
character in mode must never be used when the FILE pointer might later be
used by binary code unknown to the user. The F character in mode is
intended to be used by library functions that need a FILE pointer to
access data to process a user request, but do not need to pass the FILE
pointer back to the user. 32-bit applications that have been inspected
can use the extended FILE facility to circumvent this limit if the
inspection shows that no FILE pointers will be used to directly access
FILE structure contents.


The fopen() function has a transitional interface for 64-bit file
offsets. See lf64(5).

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | See below. |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+


The F character in the mode argument is Evolving. In all other respects
this function is Standard.

SEE ALSO


enable_extended_FILE_stdio(3C), fclose(3C), fcloseall(3C), fdopen(3C),
fflush(3C), freopen(3C), fsetpos(3C), rewind(3C), open(2), attributes(5),
lf64(5), standards(5)


November 24, 2014 FOPEN(3C)