Unix, C, and C++
Function Reference
Unix Input and Output
int open(char * filename, int flags)
int open(char * filename, int flags, int mode)
include: <fcntl.h>
flags = bitwise | or of any of the following:
O_RDONLY Only read operations permitted
O_WRONLY Only write operations permitted
O_RDWR Read and Write operations both permitted
O_NONBLOCK Non-blocking, applies to open operation only
O_APPEND All writes go to end of file
O_CREAT Create file if it doesn't already exist
O_TRUNC Delete existing contents of file
O_EXCL Open fails if file already exists
O_SHLOCK Get a "shared lock" on the file
O_EXLOCK Get an "exclusive lock" on the file
O_DIRECT Try to avoid all caching of operations
O_FSYNC All writes immediately effective, no buffering
O_NOFOLLOW If file is symbolic link, open it, don't follow it
mode required if file is created, ignored otherwise.
mode specifies the protection bits, e.g. 0644 = rw-r--r--
returns <0 for error, or integer file descriptor.
int close(int fd)
include: <unistd.h>
fd = file descriptor as returned by open
returns <0 for error, 0 for success.
int read(int fd, void * ptr, int numbytes)
include: <unistd.h>
fd = file descriptor as returned by open
ptr = address in memory where data is to be stored;
may be a pointer of any type.
numbytes = number of bytes to attempt to read
returns <0 for error, 0 for end-of-file,
or number of bytes successfully read.
(for a non-blocking interactive file, that may be zero).
int write(int fd, void * ptr, int numbytes)
include: <unistd.h>
fd = file descriptor as returned by open
ptr = address in memory where data already is;
may be a pointer of any type.
numbytes = number of bytes to attempt to write
returns <=0 for error,
or number of bytes successfully written.
(for a non-blocking file, that may be less than the number attempted
without any errors having occurred).
int lseek(int fd, int position, int startpoint)
include: <unistd.h>
Sets the file position effective for the next read or write operation.
fd = file descriptor as returned by open
position = position within file:
number of bytes between startpoint and the desired position, may be negative.
startpoint = location in file that position is relative to, one of:
SEEK_SET Position is number of bytes from beginning of file
SEEK_END Position is number of bytes after end of file
SEEK_CUR Position is number of bytes after current position
returns <0 for error,
or resulting file position relative to beginning of file.
Moving beyond the end of the file is permitted; the file is extended in length
only if a write occurs beyond the current end of file. Moveing before the
beginning of a file is not permitted. A write operation after a move only
overwrites the number of bytes actually written.
int stat(char * file, struct stat * info)
include: <sys/types.h>
include: <sys/stat.h>
Finds information about a file.
file = The name (or path) for the file.
info = A pointer to an uninitialised stat structure.
Even in C++, the type must be "struct stat *" because of the name clash.
Information about the file is stored in the info object.
returns <0 for error (including file does not exist),
or 0 for success.
The useful fields of a stat object are as follows. All have types that behave like
ints, although they may be 16, 32, or 64 bits long.
st_size size in bytes
st_ino inode number
st_mode protection or mode (see below)
st_nlink number of hard links
st_uid file's owner's identification number
st_gid file's group identification number
st_birthtime date and time of file creation (see below)
st_mtime date and time of last modification (see below)
st_atime date and time of last access (see below)
All times are stored as time_t values, as described here.
Mode and Protection. st_mode is the logical-or of a number of bits each representing
different properties. The named constants for these bits, and their values in octal are:
S_ISUID 0004000 set user id on execution
S_IRUSR 0000400 protection: readable by owner
S_IWUSR 0000200 writable by owner
S_IXUSR 0000100 executable by owner
S_IRGRP 0000040 readable by group
S_IWGRP 0000020 writable by group
S_IXGRP 0000010 executable by group
S_IROTH 0000004 readable by all
S_IWOTH 0000002 writable by all
S_IXOTH 0000001 executable by all
Four bits of the mode give the file type. The mask for type is S_IFMT = 0170000
if (mode & S_IFMT) is S_IFREG = 0100000, type = perfectly ordinary file
if (mode & S_IFMT) is S_IFDIR = 0040000, type = directory
if (mode & S_IFMT) is S_IFLNK = 0120000, type = symbolic link
if (mode & S_IFMT) is S_IFIFO = 0010000, type = named pipe
if (mode & S_IFMT) is S_IFSOCK = 0140000, type = named socket
