readv(2) [redhat man page]
READV(2) Linux Programmer's Manual READV(2) NAME
readv, writev - read or write a vector SYNOPSIS
#include <sys/uio.h> ssize_t readv(int fd, const struct iovec *vector, int count); ssize_t writev(int fd, const struct iovec *vector, int count); struct iovec { void *iov_base; /* Starting address */ size_t iov_len; /* Length in bytes */ }; DESCRIPTION
readv reads data from file descriptor fd, and puts the result in the buffers described by vector. The number of buffers is specified by count. The buffers are filled in the order specified. Operates just like read except that data is put in vector instead of a contiguous buffer. writev writes data to file descriptor fd, and from the buffers described by vector. The number of buffers is specified by count. The buf- fers are used in the order specified. Operates just like write except that data is taken from vector instead of a contiguous buffer. RETURN VALUE
On success readv returns the number of bytes read. On success writev returns the number of bytes written. On error, -1 is returned, and errno is set appropriately. ERRORS
EINVAL An invalid argument was given. For instance count might be greater than MAX_IOVEC, or zero. fd could also be attached to an object which is unsuitable for reading (for readv) or writing (for writev). EFAULT "Segmentation fault." Most likely vector or some of the iov_base pointers points to memory that is not properly allocated. EBADF The file descriptor fd is not valid. EINTR The call was interrupted by a signal before any data was read/written. EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading. (Or the file descriptor fd is for an object that is locked.) EISDIR fd refers to a directory. EOPNOTSUPP fd refers to a socket or device that does not support reading/writing. ENOMEM Insufficient kernel memory was available. Other errors may occur, depending on the object connected to fd. CONFORMING TO
4.4BSD (the readv and writev functions first appeared in BSD 4.2), Unix98. Linux libc5 used size_t as the type of the count parameter. SEE ALSO
read(2), write(2), fprintf(3), fscanf(3) Linux 2.2.0-pre8 1999-01-20 READV(2)
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READV(2) Linux Programmer's Manual READV(2) NAME
readv, writev - read or write data into multiple buffers SYNOPSIS
#include <sys/uio.h> ssize_t readv(int fd, const struct iovec *iov, int iovcnt); ssize_t writev(int fd, const struct iovec *iov, int iovcnt); DESCRIPTION
The readv() function reads iovcnt buffers from the file associated with the file descriptor fd into the buffers described by iov ("scatter input"). The writev() function writes iovcnt buffers of data described by iov to the file associated with the file descriptor fd ("gather output"). The pointer iov points to an array of iovec structures, defined in <sys/uio.h> as: struct iovec { void *iov_base; /* Starting address */ size_t iov_len; /* Number of bytes to transfer */ }; The readv() function works just like read(2) except that multiple buffers are filled. The writev() function works just like write(2) except that multiple buffers are written out. Buffers are processed in array order. This means that readv() completely fills iov[0] before proceeding to iov[1], and so on. (If there is insufficient data, then not all buffers pointed to by iov may be filled.) Similarly, writev() writes out the entire contents of iov[0] before proceeding to iov[1], and so on. The data transfers performed by readv() and writev() are atomic: the data written by writev() is written as a single block that is not intermingled with output from writes in other processes (but see pipe(7) for an exception); analogously, readv() is guaranteed to read a contiguous block of data from the file, regardless of read operations performed in other threads or processes that have file descriptors referring to the same open file description (see open(2)). RETURN VALUE
On success, the readv() function returns the number of bytes read; the writev() function returns the number of bytes written. On error, -1 is returned, and errno is set appropriately. ERRORS
The errors are as given for read(2) and write(2). Additionally the following error is defined: EINVAL The sum of the iov_len values overflows an ssize_t value. Or, the vector count iovcnt is less than zero or greater than the permit- ted maximum. CONFORMING TO
4.4BSD (the readv() and writev() functions first appeared in 4.2BSD), POSIX.1-2001. Linux libc5 used size_t as the type of the iovcnt argument, and int as return type for these functions. NOTES
Linux Notes POSIX.1-2001 allows an implementation to place a limit on the number of items that can be passed in iov. An implementation can advertise its limit by defining IOV_MAX in <limits.h> or at run time via the return value from sysconf(_SC_IOV_MAX). On Linux, the limit advertised by these mechanisms is 1024, which is the true kernel limit. However, the glibc wrapper functions do some extra work if they detect that the underlying kernel system call failed because this limit was exceeded. In the case of readv() the wrapper function allocates a tempo- rary buffer large enough for all of the items specified by iov, passes that buffer in a call to read(2), copies data from the buffer to the locations specified by the iov_base fields of the elements of iov, and then frees the buffer. The wrapper function for writev() performs the analogous task using a temporary buffer and a call to write(2). BUGS
It is not advisable to mix calls to functions like readv() or writev(), which operate on file descriptors, with the functions from the stdio library; the results will be undefined and probably not what you want. EXAMPLE
The following code sample demonstrates the use of writev(): char *str0 = "hello "; char *str1 = "world "; struct iovec iov[2]; ssize_t nwritten; iov[0].iov_base = str0; iov[0].iov_len = strlen(str0); iov[1].iov_base = str1; iov[1].iov_len = strlen(str1); nwritten = writev(STDOUT_FILENO, iov, 2); SEE ALSO
read(2), write(2) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2002-10-17 READV(2)