READ(2V) SYSTEM CALLS READ(2V) NAME read, readv - read input SYNOPSIS cc = read(d, buf, nbytes) int cc, d; char *buf; int nbytes; #include #include cc = readv(d, iov, iovcnt) int cc, d; struct iovec *iov; int iovcnt; DESCRIPTION read attempts to read nbytes of data from the object refer- enced by the descriptor d into the buffer pointed to by buf. readv performs the same action, but scatters the input data into the iovcnt buffers specified by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1]. For readv, the iovec structure is defined as struct iovec { caddr_t iov_base; int iov_len; }; Each iovec entry specifies the base address and length of an area in memory where data should be placed. readv will always fill an area completely before proceeding to the next. On objects capable of seeking, the read starts at a position given by the pointer associated with d (see lseek(2)). Upon return from read, the pointer is incremented by the number of bytes actually read. Objects that are not capable of seeking always read from the current position. The value of the pointer associated with such an object is undefined. Upon successful completion, read and readv return the number of bytes actually read and placed in the buffer. The system guarantees to read the number of bytes requested if the descriptor references a normal file which has that many bytes left before the end-of-file, but in no other case. Sun Release 3.4 Last change: 25 July 1986 1 READ(2V) SYSTEM CALLS READ(2V) If the returned value is 0, then end-of-file has been reached. When attempting to read from a descriptor associated with an empty pipe, socket, or FIFO: If O_NDELAY is set, the read will return a -1 and errno will be set to EWOULDBLOCK. If O_NDELAY is clear, the read will block until data is written to the pipe or the file is no longer open for writing. When attempting to read from a descriptor associated with a tty that has no data currently available: If O_NDELAY is set, the read will return a -1 and errno will be set to EWOULDBLOCK. If O_NDELAY is clear, the read will block until data becomes available. If O_NDELAY is set, and less data are available than are requested by the read or readv, only the data that are available are returned, and the count indicates how many bytes of data were actually read. SYSTEM V DESCRIPTION When an attempt is made to read a descriptor which is in no-delay mode, and there is no data currently available, read will return a 0 instead of returning a -1 and setting errno to EWOULDBLOCK. Note that this is indistinguishable from end-of-file. RETURN VALUE If successful, the number of bytes actually read is returned. Otherwise, a -1 is returned and the global vari- able errno is set to indicate the error. ERRORS read and readv will fail if one or more of the following are true: EBADF d is not a valid file descriptor open for reading. EISDIR d refers to a directory which is on a file system mounted using the NFS. EFAULT buf points outside the allocated address space. Sun Release 3.4 Last change: 25 July 1986 2 READ(2V) SYSTEM CALLS READ(2V) EIO An I/O error occurred while reading from or writing to the file system. EINTR A read from a slow device was interrupted before any data arrived by the delivery of a signal. EINVAL The pointer associated with d was negative. EWOULDBLOCK The file was marked for non-blocking I/O, and no data were ready to be read. In addition, readv may return one of the following errors: EINVAL Iovcnt was less than or equal to 0, or greater than 16. EINVAL One of the iov_len values in the iov array was negative. EINVAL The sum of the iov_len values in the iov array overflowed a 32-bit integer. EFAULT Part of iov points outside the process's allocated address space. SEE ALSO dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2) Sun Release 3.4 Last change: 25 July 1986 3