Copyright (c) 1980 Regents of the University of California.
All rights reserved. The Berkeley software License Agreement
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@(#)read.2 6.6 (Berkeley) 5/23/86

C 4 read, pread - read input

#include <sys/types.h>
#include <unistd.h>

ssize_t read(int d, void *buf, size_t nbytes)
ssize_t pread(int d, void *buf, size_t nbytes, off_t offset)

Read attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf .

On objects capable of seeking, the read starts at a position given by the pointer associated with d (see lseek (2)). Upon return from read , the pointer is incremented by the number of bytes actually read.

Objects that are not capable of seeking always read from the current position. The value of the pointer associated with such an object is undefined.

Upon successful completion, read return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file that has that many bytes left before the end-of-file, but in no other case.

If the returned value is 0, then end-of-file has been reached.

The pread system call performs the same functions, but reads from the specified position in the file without modifying the file pointer.

If successful, the number of bytes actually read is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error. Read and pread will fail if one or more of the following are true:

15 [EBADF] D is not a valid descriptor open for reading.

15 [EFAULT] Buf points outside the allocated address space.

15 [EIO] An I/O error occurred while reading from the file system.

15 [EINTR] A read from a slow device was interrupted before any data arrived by the delivery of a signal.

15 [EAGAIN] The file was marked for non-blocking I/O, and no data were ready to be read.

The pread system call may also return any of the lseek errors.

dup (2), fcntl (2), open (2), pipe (2), lseek (2), write (2). pread is currently implemented as a library function instead of a system call.