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READ(2) BSD System Calls Manual READ(2) NAME read, readv, pread -- read input LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <sys/types.h> #include <sys/uio.h> #include <unistd.h> ssize_t read(int d, void *buf, size_t nbytes); ssize_t readv(int d, const struct iovec *iov, int iovcnt); ssize_t pread(int d, void *buf, size_t nbytes, off_t offset); DESCRIPTION The read() system call attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf. The readv() system call 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]. The pread() system call performs the same function, but reads from the specified position in the file without modifying the file pointer. For readv(), the iovec structure is defined as: struct iovec { void *iov_base; /* Base address. */ size_t iov_len; /* Length. */ }; Each iovec entry specifies the base address and length of an area in mem- ory where data should be placed. The readv() system call 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 po- sition. The value of the pointer associated with such an object is unde- fined. Upon successful completion, read(), readv(), and pread() return the num- ber of bytes actually read and placed in the buffer. The system guaran- tees 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 read- ing end-of-file, zero is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error. ERRORS The read(), readv(), and pread() system calls will succeed unless: [EBADF] The d argument is not a valid file or socket descrip- tor open for reading. [EFAULT] The buf argument 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. [EAGAIN] The file was marked for non-blocking I/O, and no data were ready to be read. [EISDIR] The file descriptor is associated with a directory re- siding on a filesystem that does not allow regular read operations on directories (e.g. NFS). [EOPNOTSUPP] The file descriptor is associated with a filesystem and file type that do not allow regular read opera- tions on it. In addition, readv() may return one of the following errors: [EINVAL] The iovcnt argument was less than or equal to 0, or greater than 16. [EINVAL] One of the iov_len values in the iov array was nega- tive. [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. The pread() system call may also return the following errors: [EINVAL] The specified file offset is invalid. [ESPIPE] The file descriptor is associated with a pipe, socket, or FIFO. SEE ALSO dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2), socket(2), socketpair(2), fread(3), readdir(3) STANDARDS The read() system call is expected to conform to ISO/IEC 9945-1:1990 ("POSIX.1"). The readv() and pread() system calls are expected to con- form to X/Open Portability Guide Issue 4, Version 2 ("XPG4.2"). HISTORY The pread() function appeared in AT&T System V Release 4 UNIX. The readv() system call appeared in 4.2BSD. The read() function appeared in Version 6 AT&T UNIX. BSD February 26, 1994 BSD
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY
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