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 application programming interfaces (APIs), along with comm ...
compliant
operating systems
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
, a program that needs to access data from a
file stored in a
file system uses the read system call. The file is identified by a
file descriptor
In Unix and Unix-like computer operating systems, a file descriptor (FD, less frequently fildes) is a process-unique identifier (handle) for a file or other input/output resource, such as a pipe or network socket.
File descriptors typically h ...
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'' (Gerd Dudek, Buschi Niebergall, and Edward Vesala album), 1979
* ''Open'' (Go ...
. 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 un ...
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.
# 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 functi ...
) 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, or from a
terminal), or because the system call was interrupted by a
signal
A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In ...
.
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 form for "error number").International Standard for Programming Langu ...
is set appropriately and further it is left unspecified whether the file position (if any) changes.
See also
*
write (system call)
The write is one of the most basic routines provided by a Unix-like operating system kernel. It writes data from a buffer declared by the user to a given device, such as a file. This is the primary way to output data from a program by directly ...
References
POSIX read
External links
*
{{Computer files
C POSIX library
System calls