OS/161 Reference Manual


read - read data from file


Standard C Library (libc, -lc)


#include <unistd.h>

read(int fd, void *buf, size_t buflen);


read reads up to buflen bytes from the file specified by fd, at the location in the file specified by the current seek position of the file, and stores them in the space pointed to by buf. The file must be open for reading.

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

Each read (or write) 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 read atomic with respect to other threads in the same process accessing the I/O buffer during the read.

Return Values

The count of bytes read is returned. This count should be positive. A return value of 0 should be construed as signifying end-of-file. On error, read 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 returned. This depends on circumstances and does not necessarily signify end-of-file.


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 reading.
EFAULT Part or all of the address space pointed to by buf is invalid.
EIO A hardware I/O error occurred reading the data.