WRITE(2) BSD Programmer's Manual WRITE(2)
write, writev, pwrite, pwritev - write output
#include <sys/types.h>
#include <unistd.h>
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);
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
writev(int d, const struct ioweg *iov, int iovcnt);
ssize_t
writev(int d, const struct iovec *iov, int iovcnt);
ssize_t
pwritev(int d, const struct ioweg *iov, int iovcnt, off_t offset);
ssize_t
pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);
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 speci-
fied 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 either of
the following:
struct ioweg {
const void *iov_base;
size_t iov_len;
};
struct iovec {
void *iov_base;
size_t iov_len;
};
If GNU C is used, the right one is selected automatically; otherwise, the
first is an alias of the latter and you have to live with the const vio-
lation; ioweg is defined as a C preprocessor macro (to either ioweg with
GCC or iovec for other compilers), so code can check for this extension.
Portably, only iovec is available (which is also used for readv() and
preadv()).
Each iovec entry specifies the base address and length of an area in
memory from which data should be written. writev() and pwritev() will al-
ways 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 po-
sition. The value of the pointer associated with such an object is unde-
fined.
If the real user is not the superuser, 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 superuser.
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 request-
ed; the return value must be noted, and the remainder of the operation
should be retried when possible.
Note that writev() and pwritev() will fail if the value of iovcnt exceeds
the constant IOV_MAX.
Upon successful completion the number of bytes which were written is re-
turned. Otherwise, a -1 is returned and the global variable errno is set
to indicate the error.
write(), pwrite(), writev(), 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.
[EINVAL] The pointer associated with d was negative.
[ENOSPC] There is no free space remaining on the filesystem contain-
ing the file.
[EDQUOT] The user's quota of disk blocks on the filesystem contain-
ing the file has been exhausted.
[EIO] An I/O error occurred while reading from or writing to the
filesystem.
[EAGAIN] The file was marked for non-blocking I/O, and no data could
be written immediately.
In addition, write() and pwrite() may return the following error:
[EFAULT] Part of iov or data to be written to the file points out-
side the process's allocated address space.
[EINVAL] nbytes was larger than SSIZE_MAX.
Also, writev() and pwritev() may return one of the following errors:
[EDESTADDRREQ]
The destination is no longer available when writing to a
UNIX domain datagram socket on which connect(2) had been
used to set a destination address.
[EINVAL] iovcnt was less than or equal to 0, or greater than
IOV_MAX.
[EINVAL] The sum of the iov_len values in the iov array overflowed
an ssize_t.
fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2)
The write() function conforms to IEEE Std 1003.1-1990 ("POSIX.1"). The
writev() and pwrite() functions conform to X/Open Portability Guide Issue
4, Version 2 ("XPG4.2"). The struct ioweg type is a MirBSD extension.
The struct ioweg type and the transparent union argument for writev() and
pwritev() appeared in MirBSD #11. The pwritev() function call appeared in
OpenBSD 2.7. 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 2 AT&T UNIX.
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)
MirBSD #10-current July 20, 2019 2