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#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); ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset); ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset); ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags); ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
preadv(),
pwritev():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
The writev() system call 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() system call works just like read(2) except that multiple buffers are filled.
The writev() system call 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)).
The pwritev() system call combines the functionality of writev() and pwrite(2). It performs the same task as writev(), but adds a fourth argument, offset, which specifies the file offset at which the output operation is to be performed.
The file offset is not changed by these system calls. The file referred to by fd must be capable of seeking.
These system calls are similar to preadv() and pwritev() calls, but add a fifth argument, flags, which modifies the behavior on a per-call basis.
Unlike preadv() and pwritev(), if the offset argument is -1, then the current file offset is used and updated.
The flags argument contains a bitwise OR of zero or more of the following flags:
Note that is not an error for a successful call to transfer fewer bytes than requested (see read(2) and write(2)).
On error, -1 is returned, and errno is set appropriately.
preadv2() and pwritev2() first appeared in Linux 4.6
preadv(), pwritev(): nonstandard, but present also on the modern BSDs.
preadv2(), pwritev2(): nonstandard Linux extension.
unsigned long pos_l, unsigned long pos
These arguments contain, respectively, the low order and high order 32 bits of offset.
The need for this extra effort in the glibc wrapper functions went away with Linux 2.2 and later. However, glibc continued to provide this behavior until version 2.10. Starting with glibc version 2.9, the wrapper functions provide this behavior only if the library detects that the system is running a Linux kernel older than version 2.6.18 (an arbitrarily selected kernel version). And since glibc 2.20 (which requires a minimum Linux kernel version of 2.6.32), the glibc wrapper functions always just directly invoke the system calls.
It is not advisable to mix calls to 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.
char *str0 = "hello "; char *str1 = "world\n"; 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);