fopen - open a stream
FILE *fopen(const char *filename, const char *mode);
The fopen() function opens the file whose pathname is the
string pointed to by filename, and associates a stream with
The argument mode points to a string beginning with one of
the following sequences:
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-
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
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.
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 interven-
ing 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
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 is w, a, 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
If mode is w or 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() func-
tion will allocate a file descriptor as open(2) does.
The largest value that can be represented correctly in an
object of type off_t will be established as the offset max-
imum in the open file description.
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.
The fopen() function will fail if:
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 direc-
tory of the file to be created.
EINTR A signal was caught during the execution of fopen().
The named file is a directory and mode requires write
ELOOP Too many symbolic links were encountered in resolving
There are OPEN_MAX file descriptors currently open in
the calling process.
The length of the filename exceeds PATH_MAX or a path-
name component is longer than NAME_MAX.
The maximum allowable number of files is currently
open in the system.
A component of filename does not name an existing file
or filename is an empty string.
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.
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.
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:
The value of the mode argument is not valid.
The number of streams currently open in the calling
process is either FOPEN_MAX or STREAM_MAX.
Pathname resolution of a symbolic link produced an
intermediate result whose length exceeds PATH_MAX.
Insufficient storage space is available.
The file is a pure procedure (shared text) file that
is being executed and mode requires write access.
The number of streams that a process can have open at one
time is STREAM_MAX. If defined, it has the same value as
The fopen() function has a transitional interface for 64-bit
file offsets. See lf64(5).
When a UFS file system is mounted with logging enabled, file
system transactions that free blocks from files might not
actually add those freed blocks to the file system's free
list until some unspecified time in the future. This
behavior improves file system performance but does not con-
form to the POSIX, Single UNIX Specification, SPARC Confor-
mance Definition, System V Application Binary Interface,
System V Interface Definition, and X/Open Portability Guide
Standards, which require that freed space be available
immediately. To enable standards conformance regarding file
deletions or to address the problem of not being able to
grow files on a relatively full UFS file system even after
files have been deleted, disable UFS logging (see
See attributes(5) for descriptions of the following attri-
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| MT-Level | MT-Safe |
mount_ufs(1M), fclose(3C), fdopen(3C), fflush(3C),
freopen(3C), fsetpos(3C), rewind(3C), attributes(5),
Man(1) output converted with