December 30, 2002 WRITE 2

### NAME

write, writev, pwrite, pwritev - write output

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```ssize_t` write(```int d``, const void *buf``, size_t nbytes`)
```ssize_t` pwrite(```int d``, const void *buf``, size_t nbytes``, off_t offset`)
```ssize_t` writev(```int d``, const struct iovec *iov``, int iovcnt`)
```ssize_t` pwritev(```int d``, const struct iovec *iov``, int iovcnt``, off_t offset`)

### DESCRIPTION

write(``) attempts to write ```nbytes` of data to the object referenced by the descriptor ```d` from the buffer pointed to by ```buf`. writev(``) performs the same action, but gathers the output data from the ```iovcnt` buffers specified by the members of the ```iov` array: iov[0], iov[1], ..., iov[iovcnt-1]. pwrite(``) and pwritev(``) perform the same functions, but write to the specified position in the file without modifying the file pointer.

For writev(``) and pwritev(``), 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 from which data should be written. writev(``) and pwritev(``) will always write a complete area before proceeding to the next.

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

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

If the real user is not the super-user, then write(``) clears the set-user-id bit on a file. This prevents penetration of system security by a user who ``captures'' a writable set-user-id file owned by the super-user.

If write(``) succeeds it will update the st_ctime and st_mtime fields of the file's meta-data (see stat(2)).

When using non-blocking I/O on objects such as sockets that are subject to flow control, write(``) and writev(``) may write fewer bytes than requested; the return value must be noted, and the remainder of the operation should be retried when possible.

### RETURN VALUES

Upon successful completion the number of bytes which were written is returned. Otherwise a -1 is returned and the global variable errno is set to indicate the error.

### ERRORS

write(``), writev(``), pwrite(``), and pwritev(``) will fail and the file pointer will remain unchanged if:

[```EBADF`]
  ```d` is not a valid descriptor open for writing.

[```EPIPE`]
  An attempt is made to write to a pipe that is not open for reading by any process.

[```EPIPE`]
  An attempt is made to write to a socket of type ```SOCK_STREAM` that is not connected to a peer socket.

[```EFBIG`]
  An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.

[```EFAULT`]
  Part of ```iov` or data to be written to the file points outside the process's allocated address space.

[```EINVAL`]
  The 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.

[```ENOSPC`]
  There is no free space remaining on the file system containing the file.

[```EDQUOT`]
  The user's quota of disk blocks on the file system containing the file has been exhausted.

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

[```EINTR`]
  A signal was received before any data could be written to a slow device. See sigaction(2) for more information on the interaction between signals and system calls.

[```EAGAIN`]
  The file was marked for non-blocking I/O, and no data could be written immediately.

In addition, writev(``) and pwritev(``) 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.

pwrite(``) and pwritev(``) 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

fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), sigaction(2)

### STANDARDS

The write(``) function is expected to conform to IEEE Std 1003.1-1988 (``POSIX.1'') . The writev(``) and pwrite(``) functions conform to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'') .

### HISTORY

The pwritev(``) function call appeared in NetBSD1.4. The pwrite(``) function call appeared in `AT&T` System V.4 `UNIX`. The writev(``) function call appeared in 4.2BSD. The write(``) function call appeared in Version 6 `AT&T UNIX`.

### CAVEATS

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

            while ((nr = write(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 write(``) may appear as a negative number distinct from -1. Proper loops should use ``

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