--- name: "preadv" section: "2" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/readv" --- # preadv(2) read or write data into multiple buffers Part of [readv(2)](/readv/2.md). ## NAME readv, writev, preadv, pwritev, preadv2, pwritev2 - read or write data into multiple buffers ## LIBRARY Standard C library (*libc*, *-lc*) ## SYNOPSIS ``` #include ``` **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)](/feature_test_macros/7.md)): **preadv**(), **pwritev**(): ``` Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE ``` ## DESCRIPTION The **readv**() system call reads *iovcnt* buffers from the file associated with the file descriptor *fd* into the buffers described by *iov* ("scatter input"). 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, described in [**iovec**(3type)](/iovec/3type.md). The **readv**() system call works just like [**read**(2)](/read/2.md) except that multiple buffers are filled. The **writev**() system call works just like [**write**(2)](/write/2.md) 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; 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)](/open/2.md)). ### preadv() and pwritev() The **preadv**() system call combines the functionality of **readv**() and [**pread**(2)](/pread/2.md). It performs the same task as **readv**(), but adds a fourth argument, *offset*, which specifies the file offset at which the input operation is to be performed. The **pwritev**() system call combines the functionality of **writev**() and [**pwrite**(2)](/pwrite/2.md). 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. ### preadv2() and pwritev2() 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: - **RWF\_DSYNC** (since Linux 4.7) Provide a per-write equivalent of the **O\_DSYNC** [**open**(2)](/open/2.md) flag. This flag is meaningful only for **pwritev2**(), and its effect applies only to the data range written by the system call. - **RWF\_HIPRI** (since Linux 4.6) High priority read/write. Allows block-based filesystems to use polling of the device, which provides lower latency, but may use additional resources. (Currently, this feature is usable only on a file descriptor opened using the **O\_DIRECT** flag.) - **RWF\_SYNC** (since Linux 4.7) Provide a per-write equivalent of the **O\_SYNC** [**open**(2)](/open/2.md) flag. This flag is meaningful only for **pwritev2**(), and its effect applies only to the data range written by the system call. - **RWF\_NOWAIT** (since Linux 4.14) Do not wait for data which is not immediately available. If this flag is specified, the **preadv2**() system call will return instantly if it would have to read data from the backing storage or wait for a lock. If some data was successfully read, it will return the number of bytes read. If no bytes were read, it will return -1 and set *errno* to **EAGAIN** (but see **BUGS**). Currently, this flag is meaningful only for **preadv2**(). - **RWF\_APPEND** (since Linux 4.16) Provide a per-write equivalent of the **O\_APPEND** [**open**(2)](/open/2.md) flag. This flag is meaningful only for **pwritev2**(), and its effect applies only to the data range written by the system call. The *offset* argument does not affect the write operation; the data is always appended to the end of the file. However, if the *offset* argument is -1, the current file offset is updated. - **RWF\_NOAPPEND** (since Linux 6.9) Do not honor the **O\_APPEND** [**open**(2)](/open/2.md) flag. This flag is meaningful only for **pwritev2**(). Historically, Linux honored **O\_APPEND** flag if set and ignored the offset argument, which is a bug. For **pwritev2**(), the *offset* argument is honored as expected if **RWF\_NOAPPEND** flag is set, the same as if **O\_APPEND** flag were not set. - **RWF\_ATOMIC** (since Linux 6.11) Requires that writes to regular files in block-based filesystems be issued with torn-write protection. Torn-write protection means that for a power or any other hardware failure, all or none of the data from the write will be stored, but never a mix of old and new data. This flag is meaningful only for **pwritev2**(), and its effect applies only to the data range written by the system call. The total write length must be power-of-2 and must be sized in the range \[*stx\_atomic\_write\_unit\_min*, *stx\_atomic\_write\_unit\_max*\]. The write must be at a naturally-aligned offset within the file with respect to the total write length. For example, a write of length 32KiB at a file offset of 32KiB is permitted, however a write of length 32KiB at a file offset of 48KiB is not permitted. The upper limit of *iovcnt* for **pwritev2**() is given by the value in *stx\_atomic\_write\_segments\_max.* Torn-write protection only works with **O\_DIRECT** flag, that is, buffered writes are not supported. To guarantee consistency from the write between a file's in-core state with the storage device, **O\_SYNC** or **O\_DSYNC** must be specified for [**open**(2)](/open/2.md). The same synchronized I/O guarantees as described in [**open**(2)](/open/2.md) are provided when these flags or their equivalent flags and system calls are used (e.g., if **RWF\_SYNC** is specified for **pwritev2**()). - **RWF\_DONTCACHE** (since Linux 6.14) Reads or writes to a regular file will prune instantiated page cache content when the operation completes. This is different than normal buffered I/O, where the data usually remains in cache until such time that it gets reclaimed due to memory pressure. If ranges of the read or written I/O were already in cache before this read or write, then those ranges will not be pruned at I/O completion time. - Additionally, any range dirtied by a write operation with **RWF\_DONTCACHE** set will get kicked off for writeback. This is similar to calling [**sync\_file\_range**(2)](/sync_file_range/2.md) with *SYNC\_FILE\_RANGE\_WRITE* to start writeback on the given range. **RWF\_DONTCACHE** is a hint, or best effort, where no hard guarantees are given on the state of the page cache once the operation completes. - If used on a file system or block device that doesn't support it, it will return -1, and *errno* will be set to **EOPNOTSUPP**. ## RETURN VALUE On success, **readv**(), **preadv**(), and **preadv2**() return the number of bytes read; **writev**(), **pwritev**(), and **pwritev2**() return the number of bytes written. Note that it is not an error for a successful call to transfer fewer bytes than requested (see [**read**(2)](/read/2.md) and [**write**(2)](/write/2.md)). On error, -1 is returned, and *errno* is set to indicate the error. ## ERRORS The errors are as given for [**read**(2)](/read/2.md) and [**write**(2)](/write/2.md). Furthermore, **preadv**(), **preadv2**(), **pwritev**(), and **pwritev2**() can also fail for the same reasons as [**lseek**(2)](/lseek/2.md). Additionally, the following errors are defined: - **EINVAL** The sum of the *iov\_len* values overflows an *ssize\_t* value. - **EINVAL** If **RWF\_ATOMIC** is specified, the combination of the sum of the *iov\_len* values and the *offset* value does not comply with the length and offset torn-write protection rules. - **EINVAL** The vector count, *iovcnt*, is less than zero or greater than the permitted maximum. If **RWF\_ATOMIC** is specified, this maximum is given by the *stx\_atomic\_write\_segments\_max* value from *statx.* - **EOPNOTSUPP** **RWF\_DONTCACHE** was set in *flags* and the file doesn't support it. - **EOPNOTSUPP** An unknown flag is specified in *flags*. ## VERSIONS ### C library/kernel differences The raw **preadv**() and **pwritev**() system calls have call signatures that differ slightly from that of the corresponding GNU C library wrapper functions shown in the SYNOPSIS. The final argument, *offset*, is unpacked by the wrapper functions into two arguments in the system calls: **unsigned long** *pos\_l***, unsigned long** *pos* These arguments contain, respectively, the low order and high order 32 bits of *offset*. ## STANDARDS - **readv**() **writev**() POSIX.1-2024. - **preadv**() **pwritev**() BSD. - **preadv2**() **pwritev2**() Linux. ## HISTORY - **readv**() **writev**() POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD). - **preadv**() **pwritev**() Linux 2.6.30, glibc 2.10. - **preadv2**() **pwritev2**() Linux 4.6, glibc 2.26. ### Historical C library/kernel differences To deal with the fact that **IOV\_MAX** was so low on early versions of Linux, the glibc wrapper functions for **readv**() and **writev**() did some extra work if they detected that the underlying kernel system call failed because this limit was exceeded. In the case of **readv**(), the wrapper function allocated a temporary buffer large enough for all of the items specified by *iov*, passed that buffer in a call to [**read**(2)](/read/2.md), copied data from the buffer to the locations specified by the *iov\_base* fields of the elements of *iov*, and then freed the buffer. The wrapper function for **writev**() performed the analogous task using a temporary buffer and a call to [**write**(2)](/write/2.md). 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 glibc 2.10. Starting with glibc 2.9, the wrapper functions provide this behavior only if the library detects that the system is running a Linux kernel older than Linux 2.6.18 (an arbitrarily selected kernel version). And since glibc 2.20 (which requires a minimum of Linux 2.6.32), the glibc wrapper functions always just directly invoke the system calls. ## NOTES POSIX.1 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 *\* or at run time via the return value from *sysconf(\_SC\_IOV\_MAX)*. On modern Linux systems, the limit is 1024. Back in Linux 2.0 days, this limit was 16. ## BUGS Linux 5.9 and Linux 5.10 have a bug where **preadv2**() with the **RWF\_NOWAIT** flag may return 0 even when not at end of file. ## EXAMPLES The following code sample demonstrates the use of **writev**(): ``` char *str0 = "hello "; char *str1 = "world\n"; ssize_t nwritten; struct iovec iov[2]; 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 [**pread**(2)](/pread/2.md), [**read**(2)](/read/2.md), [**write**(2)](/write/2.md)