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writev() - Unix, Linux System Call

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readv, writev - read or write data into multiple buffers


#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);


Thereadv() function reads count blocks from the file associated with the file descriptor fd into the multiple buffers described by vector.

The writev() function writes at most count blocks described by vector to the file associated with the file descriptor fd.

The pointer vector points to a struct iovec defined in <sys/uio.h> as :

struct iovec {
    void *iov_base;   /* Starting address */
    size_t iov_len;   /* Number of bytes */

Buffers are processed in the order specified. 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.


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.


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 count is less than zero or greater than the permitted maximum.


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 count parameter, and int as return type for these functions.


POSIX.1-2001 allows an implementation to place a limit on the number of items that can be passed in vector. 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 temporary buffer large enough for all of the items specified by vector, passes that buffer in a call to read(), copies data from the buffer to the locations specified by the iov_base fields of the elements of vector, and then frees the buffer.

The wrapper function for writev() performs the analogous task using a temporary buffer and a call to write().


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.


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