WRITE(2V) — SYSTEM CALLS

NAME

write, writev − write output

SYNOPSIS

int write(fd, buf, nbyte)
int fd;
char ∗buf;
int nbyte;

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

int writev(fd, iov, iovcnt)
int fd;
struct iovec ∗iov;
int iovcnt;

SYSTEM V SYNOPSIS

int write(fd, buf, nbyte)
int fd;
char ∗buf;
unsigned nbyte;

DESCRIPTION

write() attempts to write nbyte bytes of data to the object referenced by the descriptor fd 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]. If nbyte is zero, write() takes no action and returns 0. writev(), however, returns −1 and sets the global variable errno (see ERRORS below).

For writev(), the iovec structure is defined as

struct iovec {
caddr_tiov_base;
intiov_len;
};

Each iovec entry specifies the base address and length of an area in memory from which data should be written. writev() always writes a complete area before proceeding to the next.

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

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

If the O_APPEND flag of the file status flags is set, the seek pointer is set to the end of the file prior to each write.

If the process calling write() or writev() receives a signal before any data are written, the system call is restarted, unless the process explicitly set the signal to interrupt the call using sigvec() or sigaction() (see the discussions of SV_INTERRUPT on sigvec(2) and SA_INTERRUPT on sigaction(3V)). If write() or writev() is interrupted by a signal after successfully writing some data, it returns the number of bytes written.

For regular files, if the O_SYNC flag of the file status flags is set, write() does not return until both the file data and file status have been physically updated. This function is for special applications that require extra reliability at the cost of performance. For block special files, if O_SYNC is set, the write() does not return until the data has been physically updated.

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.

For STREAMS (see intro(2)) files, the operation of write() and writev() are determined by the values of the minimum and maximum packet sizes accepted by the stream. These values are contained in the topmost stream module. Unless the user pushes (see I_PUSH in streamio(4)) the topmost module, these values can not be set or tested from user level. If the total number of bytes to be written falls within the packet size range, that many bytes are written. If the total number of bytes to be written does not fall within the range and the minimum packet size value is zero, write() and writev() break the data to be written into maximum packet size segments prior to sending the data downstream (the last segment may contain less than the maximum packet size). If the total number of bytes to be written does not fall within the range and the minimum value is non-zero, write() and writev() fail and set errno to ERANGE. Writing a zero-length buffer (the total number of bytes to be written is zero) sends zero bytes with zero returned.

When a descriptor or the object it refers to is marked for non-blocking I/O, and the descriptor refers to an object subject to flow control, such as a socket, a pipe (or FIFO), or a stream, 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. If such an object’s buffers are full, so that it cannot accept any data, then:

• If the object to which the descriptor refers is marked for non-blocking I/O using the FIONBIO request to ioctl(2), or by using fcntl(2V) to set the FNDELAY or O_NDELAY flag (defined in <sys/fcntl.h>), write() returns −1 and sets errno to EWOULDBLOCK.

Upon successful completion, write() marks for update the st_ctime and st_mtime fields of the file.

SYSTEM V DESCRIPTION

write() and writev() behave as described above, except:

• If the descriptor is marked for non-blocking I/O by using fcntl() to set the FNBIO or O_NDELAY flag (defined in <sys/fcntl.h>), and does not refer to a stream, the write() returns 0. If the descriptor is marked for non-blocking I/O, and refers to a stream, write() returns −1 and sets errno to EAGAIN.

• If the descriptor is marked for non-blocking I/O using fcntl() to set the FNONBLOCK or O_NONBLOCK flag (defined in <sys/fcntl.h>), write() requests for {PIPE_BUF} (see pathconf(2V)) or fewer bytes either succeed completely and return nbyte, or return −1 and set errno to EAGAIN. A write() request for greater than {PIPE_BUF} bytes either transfers what it can and returns the number of bytes written, or transfers no data and returns −1 and sets errno to EAGAIN. If a write() request is greater than {PIPE_BUF} bytes and all data previously written to the pipe has been read, write() transfers at least {PIPE_BUF} bytes.

RETURN VALUES

write() and writev() return the number of bytes actually written on success. On failure, they return −1 and set errno to indicate the error.

ERRORS

write() and writev() fail and the seek pointer remains unchanged if one or more of the following are true:

EBADF fd is not a valid descriptor open for writing.

EDQUOT The user’s quota of disk blocks on the file system containing the file has been exhausted.

EFAULT Part of iov or data to be written to the file points outside the process’s allocated address space.

EFBIG An attempt was made to write a file that exceeds the process’s file size limit or the maximum file size.

EINTR The process performing a write received a signal before any data were written, and the signal was set to interrupt the system call.

EINVAL The stream is linked below a multiplexor.

The seek pointer associated with fd was negative.

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

The process is in a background process group and is attempting to write to its controlling terminal, TOSTOP is set, the process is neither ignoring nor blocking SIGTTOU, and the process group of the process is orphaned.

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

ENXIO A hangup occurred on the stream being written to.

EPIPE An attempt is made to write to a pipe that is not open for reading by any process (or to a socket of type SOCK_STREAM that is connected to a peer socket.) Note: an attempted write of this kind also causes you to receive a SIGPIPE signal from the kernel. If you’ve not made a special provision to catch or ignore this signal, then your process dies.

ERANGE fd refers to a stream, the total number of bytes to be written is outside the minimum and maximum write range, and the minimum value is non-zero.

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

In addition to the above, writev() may set errno to:

EINVAL iovcnt was less than or equal to 0, or greater than 16.

One of the iov_len values in the iov array was negative.

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

A write to a STREAMS file can fail if an error message has been received at the stream head. In this case, errno is set to the value included in the error message.

SYSTEM V ERRORS

write() fails and sets errno as described above, except:

EAGAIN The descriptor referred to a stream, was marked for non-blocking I/O, and no data could be written immediately.

The O_NONBLOCK flag is set for the file descriptor and write() would block.

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