WRITE(2) — SYSTEM CALLS

NAME

write, writev − write on a file

SYNOPSIS

write(d, buf, nbytes)
int d;
char ∗buf;
int nbytes;

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

writev(d, iov, ioveclen)
int d;
struct iovec ∗iov;
int ioveclen;

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 iovlen buffers specified by the members of the iov array: iov[0], iov[1], etc.

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 actually 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.

RETURN VALUE

Upon successful completion the number of bytes actually writen is returned. Otherwise a −1 is returned and errno is set to indicate the error.

ERRORS

Write will fail and the file pointer will remain unchanged if one or more of the following are true:

[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 which 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.

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