OS/161 Reference Manual


write - write data to file


Standard C Library (libc, -lc)


#include <unistd.h>

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


write writes up to buflen bytes to the file specified by fd, at the location in the file specified by the current seek position of the file, taking the data from the space pointed to by buf. The file must be open for writing.

The current seek position of the file is advanced by the number of bytes written.

Each write (or read) operation is atomic relative to other I/O to the same file. Note that the kernel is not obliged to (and generally cannot) make the write atomic with respect to other threads in the same process accessing the I/O buffer during the write.

Return Values

The count of bytes written is returned. This count should be positive. A return value of 0 means that nothing could be written, but that no error occurred; this only occurs at end-of-file on fixed-size objects. On error, write returns -1 and sets errno to a suitable error code for the error condition encountered.

Note that in some cases, particularly on devices, fewer than buflen (but greater than zero) bytes may be written. This depends on circumstances and does not necessarily signify end-of-file. In most cases, one should loop to make sure that all output has actually been written.


The following error codes should be returned under the conditions given. Other error codes may be returned for other cases not mentioned here.
  EBADF fd is not a valid file descriptor, or was not opened for writing.
EFAULT Part or all of the address space pointed to by buf is invalid.
ENOSPC There is no free space remaining on the filesystem containing the file.
EIO A hardware I/O error occurred writing the data.