The
fopen() function opens the file whose name is the string pointed to by
path and associates a stream with it.
The argument
mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):
“r+”
Open for reading and writing.
“w”
Open for writing. Truncate file to zero length or create file.
“w+”
Open for reading and writing. Truncate file to zero length or create file.
“a”
Append; open for writing. The file is created if it does not exist.
“a+”
Append; open for reading and writing. The file is created if it does not exist.
The
mode string can also include the letter ``b'' either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with ANSI X3.159-1989 (“ANSI C”) and has no effect; the ``b'' is ignored.
The letter ``f'' in the mode string restricts fopen to regular files; if the file opened is not a regular file,
fopen() will fail. This is a non ANSI X3.159-1989 (“ANSI C”) extension.
Any created files will have mode "
S_IRUSR |
S_IWUSR |
S_IRGRP |
S_IWGRP |
S_IROTH |
S_IWOTH" (
0666), as modified by the process' umask value (see
umask(2)).
Opening a file with append mode causes all subsequent writes to it to be forced to the then current end of file, regardless of intervening repositioning of the stream.
The
fopen() and
freopen() functions initially position the stream at the start of the file unless the file is opened with append mode, in which case the stream is initially positioned at the end of the file.
The
fdopen() function associates a stream with the existing file descriptor,
fildes. The
mode of the stream must be compatible with the mode of the file descriptor. The stream is positioned at the file offset of the file descriptor.
The
freopen() function opens the file whose name is the string pointed to by
path and associates the stream pointed to by
stream with it. The original stream (if it exists) is closed. The
mode argument is used just as in the
fopen() function. The primary use of the
freopen() function is to change the file associated with a standard text stream (
stderr,
stdin, or
stdout).