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BSD 2.11 - man page for read (bsd section 2)

READ(2) 							System Calls Manual							   READ(2)

read, readv - read input
cc = read(d, buf, nbytes) int cc, d; char *buf; unsigned short nbytes; #include <sys/types.h> #include <sys/uio.h> cc = readv(d, iov, iovcnt) int cc, d; struct iovec *iov; int iovcnt;
Read attempts to read nbytes of data from the object referenced 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; u_short 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 that has that many bytes left before the end-of-file, but in no other case. If the returned value is 0, then end-of-file has been reached.
If successful, the number of bytes actually read is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.
Read and readv will fail if one or more of the following are true: [EBADF] D is not a valid file or socket descriptor open for reading. [EFAULT] Buf points outside the allocated address space. [EIO] An I/O error occurred while reading from 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] The sum of the iov_len values in the iov array overflowed a short. [EFAULT] Part of the iov points outside the process's allocated address space.
dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2) 4th Berkeley Distribution August 1, 1987 READ(2)
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