READ(2) System Calls Manual READ(2) NAME read, readv -- read input SYNOPSIS #include <unistd.h> #include <sys/types.h> #include <sys/uio.h> ssize_t read(int d, char *buf, size_t nbytes); int readv(int d, struct iovec *iov, int iovcnt); DESCRIPTION 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; 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 posi- tion. The value of the pointer associated with such an object is unde- fined. 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. RETURN VALUES If successful, the number of bytes actually read is returned. Upon reading end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error. ERRORS Read() and readv() will succeed unless: [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 sys- tem. [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 over- flowed a 32-bit integer. [EFAULT] Part of the 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) STANDARDS Read() is expected to conform to IEEE Std 1003.1-1988 ("POSIX"). HISTORY The readv() function call appeared in 4.2BSD. A read function call ap- peared in Version 6 AT&T UNIX. 4th Berkeley Distribution March 10, 1991 READ(2)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY
Want to link to this manual page? Use this URL:
<https://man.FreeBSD.org/cgi/man.cgi?query=read&sektion=2&manpath=4.3BSD+NET%2f2>