write(2) System Calls Manual write(2)
Name
write, writev - write on a file
Syntax
write (fd, buf, nbytes)
int fd;
char *buf;
int nbytes;
#include <sys/types.h>
#include <sys/uio.h>
writev (fd, iov, ioveclen)
int fd;
struct iovec *iov;
int ioveclen;
Arguments
fd Descriptor returned by a or system call.
buf Points to the buffer containing the data to be written.
nbytes Positive integer defining the number of bytes to be written from the buffer.
iov Points to a data structure of type which defines the starting location of the set of vectors forming the array and the length
of each individual vector in the array to be written.
This structure is defined in as follows:
struct iovec {
caddr_t iov_base ;
int iov_len ;
} ;
The data type is defined in and is the recommended way to define an address for a character value. In any case, the address is
the starting address of the set of vectors. The integer value is the length of each individual vector, in bytes.
ioveclen Defines the number of vectors in the array of data to be written. Note that the numbering of the vectors begins with 0 and
proceeds through ioveclen -1.
Description
The system call attempts to write a buffer of data to a file. The system call attempts to write an array of buffers of data to a file.
When a file is opened to a device capable of seeking (such as a disk or tape), the write starts at the position given by the file pointer
associated with the file descriptor, fd. This file pointer is the offset, in bytes, from the beginning of the file where the write is to
begin. When the file is first opened, the file pointer is set at 0. It can be modified by the and system calls. When the call returns,
the file pointer is incremented by the number of bytes actually written.
When the file is opened to a device not capable of seeking (such as sockets, pipes, or terminals), the write starts at the current posi-
tion. The value of the pointer associated with such an object is undefined.
By default, does asynchronous writes. That is, after the data is written to a buffer cache, control returns to the program. The actual
write to a device takes place after control returns. However, if you use an or call to open a file for synchronous writes, control does
not return to the program until after the buffer cache has been written to the device.
If a program is using to a remote file over NFS, and an asynchronous write error occurs, then all subsequent requests will return -1 and
errno will be set to the asynchronous error code. Also, a subsequent or will likewise fail. The return code from should be inspected by any
program that can over NFS.
Write requests to a pipe (or FIFO) are handled the same as a regular file, with the following exceptions:
o A file offset is not associated with a pipe. Therefore, each request appends to the end of the pipe.
o Write requests less than or equivalent to {PIPE_BUF} bytes are not interleaved with data from other processes doing writes on the same
pipe. Write requests greater than {PIPE_BUF} bytes can interleave on arbitrary boundaries with writes by other processes.
o If the O_NDELAY and O_NONBLOCK flags are clear, a write can cause the process to block, but, under normal completion, it returns
nbytes.
o If the O_NDELAY or O_NONBLOCK flag is set, the function does not block the process. Write requests less than or equal to {PIPE_BUF}
bytes either succeed and return nbytes or -1, and errno is set to [EWOULDBLOCK]. Write requests that exceed {PIPE_BUF} bytes can
return complete success, partial write, or no success, and errno is to [EWOULDBLOCK].
Environment
SYSTEM V
When your program is compiled using the System V environment, and the file was opened with the O_NDELAY flag set, a to a full pipe (or
FIFO) returns a zero (0), rather than an error, as for the ULTRIX non-System V environment.
Differs from the System V definition in that the value nbytes is int, rather than unsigned.
When your program is compiled using POSIX environment, EAGAIN is returned
in errno, in place of EWOULDBLOCK.
Return Values
Upon successful completion, the number of bytes actually written is returned. Otherwise, a -1 is returned, and errno is set to indicate
the error.
Diagnostics
The system call fails and the file pointer will remain unchanged, if any of the following is true:
[EACCESS] The file does not permit writing. NFS only.
[EBADF] The fd argument is not a valid descriptor open for writing.
[EPIPE] An attempt was made to write to a pipe that is not open for reading by any process.
[EPIPE] An attempt was 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, set by or the maximum file size (approxi-
mately 2 Gigabytes).
[EFAULT] Part of the array pointed to by iov or data to be written to the file points outside the process's allocated address space.
[EWOULDBLOCK] The O_NDELAY or O_NONBLOCK flag is set for the file descriptor and the process would be delayed in the write operation.
[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] The write operation was interrupted, no data was transferred.
[EINVAL] The nbytes argument is negative.
[EROFS] The file is on a read-only file system. NFS only.
[ESTALE] The fd argument is invalid because the file referred to by that file handle no longer exists or has been revoked. NFS only.
[ETIMEDOUT] A write operation failed because the server did not properly respond after a period of time that is dependent on the
options. NFS only.
See Also
close(2), creat(2), dup(2), fcntl(2), fsync(2), lseek(2), open(2), pipe(2), socket(2)
write(2)