6. Input and Output Primitives Manager — RTEMS POSIX API Guide 5.4-rc2 (21st August 2026) documentation

# 6. Input and Output Primitives Manager

## 6.1. Introduction

The input and output primitives manager is …

The directives provided by the input and output primitives manager are:

- pipe - Create an Inter-Process Channel

- dup - Duplicates an open file descriptor

- dup2 - Duplicates an open file descriptor

- close - Closes a file

- read - Reads from a file

- write - Writes to a file

- fcntl - Manipulates an open file descriptor

- lseek - Reposition read/write file offset

- fsync - Synchronize file complete in-core state with that on disk

- fdatasync - Synchronize file in-core data with that on disk

- sync - Schedule file system updates

- mount - Mount a file system

- unmount - Unmount file systems

- readv - Vectored read from a file

- writev - Vectored write to a file

- aio_read - Asynchronous Read

- aio_write - Asynchronous Write

- lio_listio - List Directed I/O

- aio_error - Retrieve Error Status of Asynchronous I/O Operation

- aio_return - Retrieve Return Status Asynchronous I/O Operation

- aio_cancel - Cancel Asynchronous I/O Request

- aio_suspend - Wait for Asynchronous I/O Request

- aio_fsync - Asynchronous File Synchronization

## 6.2. Background

There is currently no text in this section.

## 6.3. Operations

There is currently no text in this section.

## 6.4. Directives

This section details the input and output primitives manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 6.4.1. pipe - Create an Inter-Process Channel

CALLING SEQUENCE:

    #include <unistd.h>
    int pipe(
        int fildes[2]
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.2. dup - Duplicates an open file descriptor

CALLING SEQUENCE:

    #include <unistd.h>
    int dup(
        int fildes
    );

STATUS CODES:

`EBADF` | Invalid file descriptor.
`EINTR` | Function was interrupted by a signal.
`EMFILE` | The process already has the maximum number of file descriptors open and tried to open a new one.

DESCRIPTION:

The `dup` function returns the lowest numbered available file descriptor. This new desciptor refers to the same open file as the original descriptor and shares any locks.

NOTES:

NONE

### 6.4.3. dup2 - Duplicates an open file descriptor

CALLING SEQUENCE:

    #include <unistd.h>
    int dup2(
        int fildes,
        int fildes2
    );

STATUS CODES:

`EBADF` | Invalid file descriptor.
`EINTR` | Function was interrupted by a signal.
`EMFILE` | The process already has the maximum number of file descriptors open and tried to open a new one.

DESCRIPTION:

`dup2` creates a copy of the file descriptor `oldfd`.

The old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using `lseek` on one of the descriptors, the position is also changed for the other.

NOTES:

NONE

### 6.4.4. close - Closes a file

CALLING SEQUENCE:

    #include <unistd.h>
    int close(
        int fildes
    );

STATUS CODES:

`EBADF` | Invalid file descriptor
`EINTR` | Function was interrupted by a signal.

DESCRIPTION:

The `close()` function deallocates the file descriptor named by `fildes` and makes it available for reuse. All outstanding record locks owned by this process for the file are unlocked.

NOTES:

A signal can interrupt the `close()` function. In that case, `close()` returns -1 with `errno` set to EINTR. The file may or may not be closed.

### 6.4.5. read - Reads from a file

CALLING SEQUENCE:

    #include <unistd.h>
    ssize_t read(
        int fildes,
        void *buf,
        size_t nbyte
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EAGAIN` | The O_NONBLOCK flag is set for a file descriptor and the process would be delayed in the I/O operation.
`EBADF` | Invalid file descriptor
`EINTR` | Function was interrupted by a signal.
`EIO` | Input or output error
`EINVAL` | Bad buffer pointer

DESCRIPTION:

The `read()` function reads `nbyte` bytes from the file associated with `fildes` into the buffer pointed to by `buf`.

The `read()` function returns the number of bytes actually read and placed in the buffer. This will be less than `nbyte` if:

- The number of bytes left in the file is less than `nbyte`.

- The `read()` request was interrupted by a signal.

- The file is a pipe or FIFO or special file with less than `nbytes` immediately available for reading.

When attempting to read from any empty pipe or FIFO:

- If no process has the pipe open for writing, zero is returned to indicate end-of-file.

- If some process has the pipe open for writing and O_NONBLOCK is set, -1 is returned and `errno` is set to EAGAIN.

- If some process has the pipe open for writing and O_NONBLOCK is clear, `read()` waits for some data to be written or the pipe to be closed.

When attempting to read from a file other than a pipe or FIFO and no data is available.

- If O_NONBLOCK is set, -1 is returned and `errno` is set to EAGAIN.

- If O_NONBLOCK is clear, `read()` waits for some data to become available.

- The O_NONBLOCK flag is ignored if data is available.

NOTES:

NONE

### 6.4.6. write - Writes to a file

CALLING SEQUENCE:

    #include <unistd.h>
    ssize_t write(
        int fildes,
        const void *buf,
        size_t nbyte
    );

STATUS CODES:

`EAGAIN` | The O_NONBLOCK flag is set for a file descriptor and the process would be delayed in the I/O operation.
`EBADF` | Invalid file descriptor
`EFBIG` | An attempt was made to write to a file that exceeds the maximum file size
`EINTR` | The function was interrupted by a signal.
`EIO` | Input or output error.
`ENOSPC` | No space left on disk.
`EPIPE` | Attempt to write to a pope or FIFO with no reader.
`EINVAL` | Bad buffer pointer

DESCRIPTION:

The `write()` function writes `nbyte` from the array pointed to by `buf` into the file associated with `fildes`.

If `nybte` is zero and the file is a regular file, the `write()` function returns zero and has no other effect. If `nbyte` is zero and the file is a special file, te results are not portable.

The `write()` function returns the number of bytes written. This number will be less than `nbytes` if there is an error. It will never be greater than `nbytes`.

NOTES:

NONE

### 6.4.7. fcntl - Manipulates an open file descriptor

CALLING SEQUENCE:

    #include <fcntl.h>
    int fcntl(
        int fildes,
        int cmd,
        ...
    );

STATUS CODES:

`EACCESS` | Search permission is denied for a direcotry in a file’s path prefix.
`EAGAIN` | The O_NONBLOCK flag is set for a file descriptor and the process would be delayed in the I/O operation.
`EBADF` | Invalid file descriptor
`EDEADLK` | An `fcntl` with function `F_SETLKW` would cause a deadlock.
`EINTR` | The functioin was interrupted by a signal.
`EINVAL` | Invalid argument
`EMFILE` | Too many file descriptor or in use by the process.
`ENOLCK` | No locks available

DESCRIPTION:

`fcntl()` performs one of various miscellaneous operations on``fd``. The operation in question is determined by `cmd`:

`F_DUPFD` | Makes `arg` be a copy of `fd`, closing `fd` first if necessary. The same functionality can be more easily achieved by using `dup2()`. The old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using `lseek()` on one of the descriptors, the position is also changed for the other. The two descriptors do not share the close-on-exec flag, however. The close-on-exec flag of the copy is off, meaning that it will be closed on exec. On success, the new descriptor is returned.
`F_GETFD` | Read the close-on-exec flag. If the low-order bit is 0, the file will remain open across exec, otherwise it will be closed.
`F_SETFD` | Set the close-on-exec flag to the value specified by `arg` (only the least significant bit is used).
`F_GETFL` | Read the descriptor’s flags (all flags (as set by open()) are returned).
`F_SETFL` | Set the descriptor’s flags to the value specified by `arg`. Only``O_APPEND`` and `O_NONBLOCK` may be set. The flags are shared between copies (made with `dup()` etc.) of the same file descriptor. The flags and their semantics are described in `open()`.
`F_GETLK`, `F_SETLK` and `F_SETLKW` | Manage discretionary file locks. The third argument `arg` is a pointer to a struct flock (that may be overwritten by this call).
`F_GETLK` | Return the flock structure that prevents us from obtaining the lock, or set the``l_type`` field of the lock to `F_UNLCK` if there is no obstruction.
`F_SETLK` | The lock is set (when `l_type` is `F_RDLCK` or `F_WRLCK`) or cleared (when it is `F_UNLCK`. If lock is held by someone else, this call returns -1 and sets `errno` to EACCES or EAGAIN.
`F_SETLKW` | Like `F_SETLK`, but instead of returning an error we wait for the lock to be released.
`F_GETOWN` | Get the process ID (or process group) of the owner of a socket. Process groups are returned as negative values.
`F_SETOWN` | Set the process or process group that owns a socket. For these commands, ownership means receiving `SIGIO` or `SIGURG` signals. Process groups are specified using negative values.

NOTES:

The errors returned by `dup2` are different from those returned by `F_DUPFD`.

### 6.4.8. lseek - Reposition read/write file offset

CALLING SEQUENCE:

    #include <unistd.h>
    off_t lseek(
        int fildes,
        off_t offset,
        int whence
    );

STATUS CODES:

`EBADF` | `fildes` is not an open file descriptor.
`ESPIPE` | `fildes` is associated with a pipe, socket or FIFO.
`EINVAL` | `whence` is not a proper value.

DESCRIPTION:

The `lseek` function repositions the offset of the file descriptor `fildes` to the argument offset according to the directive whence. The argument `fildes` must be an open file descriptor. `Lseek` repositions the file pointer fildes as follows:

- If `whence` is SEEK_SET, the offset is set to `offset` bytes.

- If `whence` is SEEK_CUR, the offset is set to its current location plus offset bytes.

- If `whence` is SEEK_END, the offset is set to the size of the file plus `offset` bytes.

The `lseek` function allows the file offset to be set beyond the end of the existing end-of-file of the file. If data is later written at this point, subsequent reads of the data in the gap return bytes of zeros (until data is actually written into the gap).

Some devices are incapable of seeking. The value of the pointer associated with such a device is undefined.

NOTES:

NONE

### 6.4.9. fsync - Synchronize file complete in-core state with that on disk

CALLING SEQUENCE:

    #include <unistd.h>
    int fsync(
        int fildes
    );

STATUS CODES:

On success, zero is returned. On error, -1 is returned, and `errno` is set appropriately.

`EBADF` | `fd` is not a valid descriptor open for writing
`EINVAL` | `fd` is bound to a special file which does not support support synchronization
`EROFS` | `fd` is bound to a special file which does not support support synchronization
`EIO` | An error occurred during synchronization

DESCRIPTION:

`fsync` copies all in-core parts of a file to disk.

NOTES:

NONE

### 6.4.10. fdatasync - Synchronize file in-core data with that on disk

CALLING SEQUENCE:

    #include <unistd.h>
    int fdatasync(
        int fildes
    );

STATUS CODES:

On success, zero is returned. On error, -1 is returned, and `errno` is set appropriately.

`EBADF` | `fd` is not a valid file descriptor open for writing.
`EINVAL` | `fd` is bound to a special file which does not support synchronization.
`EIO` | An error occurred during synchronization.
`EROFS` | `fd` is bound to a special file which dows not support synchronization.

DESCRIPTION:

`fdatasync` flushes all data buffers of a file to disk (before the system call returns). It resembles `fsync` but is not required to update the metadata such as access time.

Applications that access databases or log files often write a tiny data fragment (e.g., one line in a log file) and then call `fsync` immediately in order to ensure that the written data is physically stored on the harddisk. Unfortunately, fsync will always initiate two write operations: one for the newly written data and another one in order to update the modification time stored in the inode. If the modification time is not a part of the transaction concept `fdatasync` can be used to avoid unnecessary inode disk write operations.

NOTES:

NONE

### 6.4.11. sync - Schedule file system updates

CALLING SEQUENCE:

    #include <unistd.h>
    void sync(
        void
    );

STATUS CODES:

NONE

DESCRIPTION:

The `sync` service causes all information in memory that updates file systems to be scheduled for writing out to all file systems.

NOTES:

The writing of data to the file systems is only guaranteed to be scheduled upon return. It is not necessarily complete upon return from `sync`.

### 6.4.12. mount - Mount a file system

CALLING SEQUENCE:

    #include <libio.h>
    int mount(
        rtems_filesystem_mount_table_entry_t **mt_entry,
        rtems_filesystem_operations_table *fs_ops,
        rtems_filesystem_options_t fsoptions,
        char *device,
        char *mount_point
    );

STATUS CODES:

> - - `ENOMEM`

>   - Unable to allocate memory needed.

> - - `EINVAL`

>   - The filesystem does not support being mounted.

> - - `EINVAL`

>   - Attempt to mount a read-only filesystem as writeable.

DESCRIPTION:

The `mount` routines mounts the filesystem class which uses the filesystem operations specified by `fs_ops` and `fsoptions`. The filesystem is mounted at the directory `mount_point` and the mode of the mounted filesystem is specified by `fsoptions`. If this filesystem class requires a device, then the name of the device must be specified by `device`.

If this operation succeeds, the mount table entry for the mounted filesystem is returned in `mt_entry`.

NOTES:

This method is not defined in the POSIX standard.

### 6.4.13. unmount - Unmount file systems

CALLING SEQUENCE:

    #include <libio.h>
    int unmount(
        const char *mount_path
    );

STATUS CODES:

> - - `EBUSY`

>   - Filesystem is in use or the root filesystem.

> - - `EACCESS`

>   - Unable to allocate memory needed.

DESCRIPTION:

The `unmount` routine removes the attachment of the filesystem specified by `mount_path`.

NOTES:

This method is not defined in the POSIX standard.

### 6.4.14. readv - Vectored read from a file

CALLING SEQUENCE:

    #include <sys/uio.h>
    ssize_t readv(
        int fildes,
        const struct iovec *iov,
        int iovcnt
    );

STATUS CODES:

In addition to the errors detected by Input and Output Primitives Manager read - Reads from a file, read(), this routine may return -1 and sets `errno` based upon the following errors:

`EINVAL` | The sum of the `iov_len` values in the iov array overflowed an `ssize_t`.
`EINVAL` | The `iovcnt` argument was less than or equal to 0, or greater than `IOV_MAX`.

DESCRIPTION:

The `readv()` function is equivalent to `read()` except as described here. The `readv()` function shall place the input data into the `iovcnt` buffers specified by the members of the `iov` array: `iov[0], iov[1], ..., iov[iovcnt-1]`.

Each `iovec` entry specifies the base address and length of an area in memory where data should be placed. The `readv()` function always fills an area completely before proceeding to the next.

NOTES:

NONE

### 6.4.15. writev - Vectored write to a file

CALLING SEQUENCE:

    #include <sys/uio.h>
    ssize_t writev(
        int fildes,
        const struct iovec *iov,
        int iovcnt
    );

STATUS CODES:

In addition to the errors detected by Input and Output Primitives Manager write - Write to a file, write(), this routine may return -1 and sets `errno` based upon the following errors:

`EINVAL` | The sum of the `iov_len` values in the iov array overflowed an `ssize_t`.
`EINVAL` | The `iovcnt` argument was less than or equal to 0, or greater than `IOV_MAX`.

DESCRIPTION:

The `writev()` function is equivalent to `write()`, except as noted here. The `writev()` function gathers output data from the `iovcnt` buffers specified by the members of the `iov array`: `iov[0], iov[1], ..., iov[iovcnt-1]`. The `iovcnt` argument is valid if greater than 0 and less than or equal to `IOV_MAX`.

Each `iovec` entry specifies the base address and length of an area in memory from which data should be written. The `writev()` function always writes a complete area before proceeding to the next.

If `fd` refers to a regular file and all of the `iov_len` members in the array pointed to by `iov` are 0, `writev()` returns 0 and has no other effect. For other file types, the behavior is unspecified by POSIX.

NOTES:

NONE

### 6.4.16. aio_read - Asynchronous Read

CALLING SEQUENCE:

    #include <aio.h>
    int aio_read(
        struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.17. aio_write - Asynchronous Write

CALLING SEQUENCE:

    #include <aio.h>
    int aio_write(
        struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.18. lio_listio - List Directed I/O

CALLING SEQUENCE:

    #include <aio.h>
    int lio_listio(
        int mode,
        struct aiocb *restrict const list[restrict],
        int nent,
        struct sigevent *restrict sig
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.19. aio_error - Retrieve Error Status of Asynchronous I/O Operation

CALLING SEQUENCE:

    #include <aio.h>
    int aio_error(
        const struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.20. aio_return - Retrieve Return Status Asynchronous I/O Operation

CALLING SEQUENCE:

    #include <aio.h>
    ssize_t aio_return(
        struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.21. aio_cancel - Cancel Asynchronous I/O Request

CALLING SEQUENCE:

    #include <aio.h>
    int aio_cancel(
        int fildes,
        struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.22. aio_suspend - Wait for Asynchronous I/O Request

CALLING SEQUENCE:

    #include <aio.h>
    int aio_suspend(
        const struct aiocb *const list[],
        int nent,
        const struct timespec *timeout
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.

### 6.4.23. aio_fsync - Asynchronous File Synchronization

CALLING SEQUENCE:

    #include <aio.h>
    int aio_fsync(
        int op,
        struct aiocb *aiocbp
    );

STATUS CODES:

`E` | The

DESCRIPTION:

NOTES:

This routine is not currently supported by RTEMS but could be in a future version.