write() - Unix, Linux System Call
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write() - Unix, Linux System Call

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write - write to a file descriptor


#include <unistd.h> 

ssize_t write(int fd, const void *buf, size_t count);


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.


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.


Error CodeDescription
EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and the write would block.
EBADF fd is not a valid file descriptor or is not open for writing.
EFAULT buf is outside your accessible address space.
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.
EINTR The call was interrupted by a signal before any data was written.
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.
EIO A low-level I/O error occurred while modifying the inode.
ENOSPC The device containing the file referred to by fd has no room for the data.
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.)

Other errors may occur, depending on the object connected to fd.


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.


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.


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