fopen(3C) Standard C Library Functions fopen(3C)NAMEfopen - 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 follow‐
ing sequences:
r or rb Open file for reading.
w or wb Truncate to zero length or create file for writ‐
ing.
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 has no effect, but is allowed for ISO C standard con‐
formance (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.
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 inter‐
mixed in the file in the order in which it is written.
When a file is opened with update mode (+ as the second or third char‐
acter 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 position‐
ing 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 deter‐
mined 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 success‐
ful completion, fopen() will mark for update the st_ctime and st_mtime
fields of the file. The fopen() function will allocate a file descrip‐
tor 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 struc‐
ture 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 speci‐
fied 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 path‐
name 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 repre‐
sented 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 compati‐
bility 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 struc‐
ture, the F character can be used as the last character in the mode
argument to circumvent this limit. Because it could lead to data cor‐
ruption, 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 circum‐
vent 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 off‐
sets. 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 ALSOenable_extended_FILE_stdio(3C), fclose(3C), fdopen(3C), fflush(3C),
freopen(3C), fsetpos(3C), rewind(3C), attributes(5), lf64(5), stan‐
dards(5)SunOS 5.10 18 Apr 2006 fopen(3C)
