write() - Unix, Linux System Call
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NAME
write - write to a file descriptor
SYNOPSIS
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);
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DESCRIPTION
write() writes up to count bytes to the file referenced by the file descriptor
fd from the buffer starting at
buf. POSIX requires that a read() which can be proved to occur after a
write() has returned returns the new data. Note that not all file
systems are POSIX conforming.
RETURN VALUE
On success, the number of bytes written are returned (zero indicates
nothing was written). On error, -1 is returned, and errno is set
appropriately.
If count is zero and the file descriptor refers to
a regular file, 0 may be returned, or an error could be detected.
For a special file, the results are not portable.
ERRORS
| Error Code | Description |
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EAGAIN |
Non-blocking I/O has been selected using
O_NONBLOCK and the write would block.
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EBADF |
fd is not a valid file descriptor or is not open for writing.
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EFAULT |
buf is outside your accessible address space.
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EFBIG |
An attempt was made to write a file that exceeds the implementation-defined
maximum file size or the process’ file size limit, or to write at a position
past the maximum allowed offset.
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EINTR |
The call was interrupted by a signal before any data was written.
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EINVAL |
fd is attached to an object which is unsuitable for writing;
or the file was opened with the
O_DIRECT flag, and either the address specified in
buf, the value specified in
count, or the current file offset is not suitably aligned.
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EIO |
A low-level I/O error occurred while modifying the inode.
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ENOSPC |
The device containing the file referred to by
fd has no room for the data.
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EPIPE |
fd is connected to a pipe or socket whose reading end is closed. When this
happens the writing process will also receive a
SIGPIPE signal.
(Thus, the write return value is seen only if the program
catches, blocks or ignores this signal.)
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Other errors may occur, depending on the object connected to fd.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. Under SVr4 a write may be interrupted and return EINTR at any point,
not just before any data is written.
NOTES
A successful return from write() does not make any guarantee that data has been committed to disk.
In fact, on some buggy implementations, it does not even guarantee
that space has successfully been reserved for the data.
The only way to be sure is to call fsync(2)
after you are done writing all your data.
SEE ALSO
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