In modern
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
compliant
operating systems, a program that needs to access data from a
file
File or filing may refer to:
Mechanical tools and processes
* File (tool), a tool used to ''remove'' fine amounts of material from a workpiece
**Filing (metalworking), a material removal process in manufacturing
** Nail file, a tool used to gent ...
stored in a
file system uses the read system call. The file is identified by a
file descriptor that is normally obtained from a previous call to
open
Open or OPEN may refer to:
Music
* Open (band), Australian pop/rock band
* The Open (band), English indie rock band
* ''Open'' (Blues Image album), 1969
* ''Open'' (Gotthard album), 1999
* ''Open'' (Cowboy Junkies album), 2001
* ''Open'' ( ...
. This system call reads in data in
byte
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s, the number of which is specified by the caller, from the file and stores then into a buffer supplied by the calling process.
The read system call takes three arguments:
# The file descriptor of the file.
# the buffer where the read data is to be stored and
# the number of bytes to be read from the file.
POSIX usage
The read system call interface is standardized by the POSIX specification. Data from a file is read by calling the read function:
ssize_t read(int fd, void *buf, size_t count);
The value returned is the number of bytes read (zero indicates
end of file
In computing, end-of-file (EOF) is a condition in a computer operating system where no more data can be read from a data source. The data source is usually called a file or stream.
Details
In the C standard library, the character reading funct ...
) and the file position is advanced by this number. It is not an error if this number is smaller than the number of bytes requested; this may happen for example because fewer bytes are actually available right now (maybe because we were close to end-of-file, or because we are reading from a
pipe
Pipe(s), PIPE(S) or piping may refer to:
Objects
* Pipe (fluid conveyance), a hollow cylinder following certain dimension rules
** Piping, the use of pipes in industry
* Smoking pipe
** Tobacco pipe
* Half-pipe and quarter pipe, semi-circular ...
, or from a
terminal
Terminal may refer to:
Computing Hardware
* Terminal (electronics), a device for joining electrical circuits together
* Terminal (telecommunication), a device communicating over a line
* Computer terminal, a set of primary input and output dev ...
), or because the system call was interrupted by a
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...
.
Alternatively, -1 is returned when an error occurs, in such a case
errno
errno.h is a header file in the standard library of the C programming language. It defines macros for reporting and retrieving error conditions using the symbol errno (short for "error number").International Standard for Programming Language C ( ...
is set appropriately and further it is left unspecified whether the file position (if any) changes.
See also
*
write (system call)
References
POSIX read
External links
*
{{Computer files
C POSIX library
System calls