August 2, 2007 READ 2

### NAME

read, readv, pread, preadv - read input

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```ssize_t` read(```int d``, void *buf``, size_t nbytes`)
```ssize_t` pread(```int d``, void *buf``, size_t nbytes``, off_t offset`)
```ssize_t` readv(```int d``, const struct iovec *iov``, int iovcnt`)
```ssize_t` preadv(```int d``, const struct iovec *iov``, int iovcnt``, off_t offset`)

### DESCRIPTION

read(``) attempts to read ```nbytes` of data from the object referenced by the descriptor ```d` into the buffer pointed to by ```buf`. readv(``) performs the same action, but scatters the input data into the ```iovcnt` buffers specified by the members of the ```iov` array: iov[0], iov[1], ..., iov[iovcnt-1]. pread(``) and preadv(``) perform the same functions, but read from the specified position in the file without modifying the file pointer.

For readv(``) and preadv(``), the ```iovec` structure is defined as:

  `

      struct iovec {
              void *iov_base;
              size_t iov_len;
      };

  `

Each ```iovec` entry specifies the base address and length of an area in memory where data should be placed. readv(``) will always fill an area completely before proceeding to the next.

On objects capable of seeking, the read(``) starts at a position given by the file pointer associated with ```d` (see lseek(2)). Upon return from read(``), the file 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 file pointer associated with such an object is undefined.

Upon successful completion, read(``), readv(``), pread(``), and preadv(``) 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.

### RETURN VALUES

If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.

### ERRORS

read(``), readv(``), pread(``), and preadv(``) will succeed unless:

[```EBADF`]
  ```d` is not a valid file or socket descriptor open for reading.

[```EFAULT`]
  ```buf` points outside the allocated address space.

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

[```EINTR`]
  A read from a slow device (i.e. one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data arrived. See sigaction(2) for more information on the interaction between signals and system calls.

[```EINVAL`]
  The file pointer associated with ```d` was negative.

[```EINVAL`]
  The total length of the I/O is more than can be expressed by the ssize_t return value.

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

In addition, readv(``) and preadv(``) may return one of the following errors:

[```EINVAL`]
  ```iovcnt` was less than or equal to 0, or greater than ```{IOV_MAX}`.

[```EINVAL`]
  One of the ```iov_len` values in the ```iov` array was negative.

[```EINVAL`]
  The sum of the ```iov_len` values in the ```iov` array overflowed a 32-bit integer.

[```EFAULT`]
  Part of the ```iov` points outside the process's allocated address space.

The pread(``) and preadv(``) calls may also return the following errors:

[```EINVAL`]
  The specified file offset is invalid.

[```ESPIPE`]
  The file descriptor is associated with a pipe, socket, or FIFO.

### SEE ALSO

dup(2), fcntl(2), open(2), pipe(2), poll(2), select(2), sigaction(2), socket(2), socketpair(2)

### STANDARDS

The read(``) function conforms to ISO/IEC 9945-1:1990 (``POSIX.1'') . The readv(``) and pread(``) functions conform to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'') .

### HISTORY

The preadv(``) function call appeared in NetBSD1.4. The pread(``) function call appeared in `AT&T` System V.4 `UNIX`. The readv(``) function call appeared in 4.2BSD. The read(``) function call appeared in Version 6 `AT&T UNIX`.

### CAVEATS

Error checks should explicitly test for -1. Code such as `

            while ((nr = read(fd, buf, sizeof(buf))) > 0)

`

is not maximally portable, as some platforms allow for nbytes to range between ```SSIZE_MAX` and ```SIZE_MAX` - 2, in which case the return value of an error-free read(``) may appear as a negative number distinct from -1. Proper loops should use ``

            while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)