SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
ssize_t pread64(int fildes, void *buf, size_t nbyte, off64_t offset);
ssize_t read(int fildes, void *buf, size_t nbyte);
DESCRIPTION
The
If the number of bytes is 0,
then
On file that support seeking,
such as regular files, the
On files that do not support seeking, such as terminals, reads always occur from the current position. The value of a file offset associated with such a file is undefined.
No data transfer occurs
past the current end-of-file. If the starting position is at or
after the end-of-file, 0 is returned. If the file refers to
a device or special file, the result of subsequent
Reads larger than SSIZE_MAX are unsupported.
When attempting to read from an empty pipe or FIFO:
- If no process has the pipe open for writing,
read() returns 0 and indicates end-of-file. - If some process has the pipe open for writing, and
O_NONBLOCK is set,
read() returns -1 and sets errno to EAGAIN. - If some process has the pipe open for writing, and
O_NONBLOCK is clear,
read() blocks the calling thread until some data is written or the pipe is closed by all processes that had the pipe open for writing.
When attempting to read a file (other than a pipe or FIFO) that supports non-blocking reads and has no data currently available:
- If O_NONBLOCK is set,
read() returns -1 and sets errno to EWOULDBLOCK. - If O_NONBLOCK is clear,
read() blocks the calling thread until some data becomes available. - The use of the O_NONBLOCK flag has no effect if there is some data available.
The
Upon successful completion, if the number of bytes requested is greater than 0,
The
The
PARAMETERS
- fildes
-
Is the file descriptor that references an open file.
- buf
-
Points to the buffer to place the read information into.
- nbyte
-
Specifies the maximum number of bytes to attempt to read.
- offset
-
Specifies the point in the file at which
pread() orpread64() begins reading.
RETURN VALUES
If successful, these functions return a non-negative integer that indicates the number of bytes read. The number of bytes read may be less than the number of bytes requested in any of the following conditions:
- The number of bytes left in the file is less than the requested length.
-
The
read() was interrupted by a signal. - The file is a pipe or FIFO or special device and has fewer bytes than requested immediately available for reading.
On error, these functions return -1, and set errno to one of the following values:
- EAGAIN
-
The O_NONBLOCK flag is set for the file descriptor and the process would be delayed.
- EBADF
-
The fildes parameter is not a valid file descriptor open for reading.
- EFAULT
-
The buf parameter is not a valid pointer, or the buffer was overrun during the
read() request. - EINTR
-
The
read() ,pread() , orpread64() request was interrupted by a signal. - EIO
-
A physical I/O error occurred.
- EISDIR
-
The fildes parameter refers to a directory. Use
readdir() to read from directories. - ENXIO
-
A request was made of a non-existent device, or the request was outside the capabilities of the device.
- EOVERFLOW
-
For all functions, the file is a regular file, nbyte is greater than 0, the starting position is before the end-of-file, and the starting position is greater than or equal to the offset maximum established in the open file description associated with fildes.
For
pread() andpread64() , the specified offset would cause a read beyond the 2 GB boundary. - EWOULDBLOCK
-
The O_NONBLOCK flag is set for the file descriptor and the process would be delayed.
CONFORMANCE
POSIX.1 (1996), with exceptions. UNIX 03, with exceptions.
MULTITHREAD SAFETY LEVEL
Async-signal-safe.
PORTING ISSUES
Refer to File Management in the Windows Concepts chapter of the PTC MKS Toolkit UNIX to Windows Porting Guide for a detailed discussion of file handling, including a discussion of text mode and binary mode for files.
The test which causes the EOVERFLOW error condition
to be returned can be disabled by using the
While the UNIX 03 specification states that
Additionally, although the UNIX 03 specification states that using
AVAILABILITY
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition
SEE ALSO
- Functions:
creat() ,dup() ,dup2() ,fcntl() ,ioctl() ,lseek() ,open() ,pipe() ,readdir() ,readv() ,socket() ,write() ,writev()
- Miscellaneous:
- lf64
PTC MKS Toolkit 10.5 Documentation Build 40.