Client Interfaces

PostgreSQL 8.0.1 Documentation
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# 28.3. Client Interfaces

This section describes the facilities that PostgreSQL client interface libraries provide for accessing large objects. All large object manipulation using these functions must take place within an SQL transaction block. (This requirement is strictly enforced as of PostgreSQL 6.5, though it has been an implicit requirement in previous versions, resulting in misbehavior if ignored.) The PostgreSQL large object interface is modeled after the Unix file-system interface, with analogues of `open`, `read`, `write`, `lseek`, etc.

Client applications which use the large object interface in libpq should include the header file `libpq/libpq-fs.h` and link with the libpq library.

## 28.3.1. Creating a Large Object

The function

    Oid lo_creat(PGconn *conn, int mode);

creates a new large object. `mode` is a bit mask describing several different attributes of the new object. The symbolic constants used here are defined in the header file `libpq/libpq-fs.h`. The access type (read, write, or both) is controlled by or'ing together the bits `INV_READ` and `INV_WRITE`. The low-order sixteen bits of the mask have historically been used at Berkeley to designate the storage manager number on which the large object should reside. These bits should always be zero now. (The access type does not actually do anything anymore either, but one or both flag bits must be set to avoid an error.) The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure.

An example:

    inv_oid = lo_creat(conn, INV_READ|INV_WRITE);

## 28.3.2. Importing a Large Object

To import an operating system file as a large object, call

    Oid lo_import(PGconn *conn, const char *filename);

`filename` specifies the operating system name of the file to be imported as a large object. The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure. Note that the file is read by the client interface library, not by the server; so it must exist in the client filesystem and be readable by the client application.

## 28.3.3. Exporting a Large Object

To export a large object into an operating system file, call

    int lo_export(PGconn *conn, Oid lobjId, const char *filename);

The `lobjId` argument specifies the OID of the large object to export and the `filename` argument specifies the operating system name of the file. Note that the file is written by the client interface library, not by the server. Returns 1 on success, -1 on failure.

## 28.3.4. Opening an Existing Large Object

To open an existing large object for reading or writing, call

    int lo_open(PGconn *conn, Oid lobjId, int mode);

The `lobjId` argument specifies the OID of the large object to open. The `mode` bits control whether the object is opened for reading (`INV_READ`), writing (`INV_WRITE`), or both. A large object cannot be opened before it is created. `lo_open` returns a (non-negative) large object descriptor for later use in `lo_read`, `lo_write`, `lo_lseek`, `lo_tell`, and `lo_close`. The descriptor is only valid for the duration of the current transaction. On failure, -1 is returned.

## 28.3.5. Writing Data to a Large Object

The function

    int lo_write(PGconn *conn, int fd, const char *buf, size_t len);

writes `len` bytes from `buf` to large object descriptor `fd`. The `fd` argument must have been returned by a previous `lo_open`. The number of bytes actually written is returned. In the event of an error, the return value is negative.

## 28.3.6. Reading Data from a Large Object

The function

    int lo_read(PGconn *conn, int fd, char *buf, size_t len);

reads `len` bytes from large object descriptor `fd` into `buf`. The `fd` argument must have been returned by a previous `lo_open`. The number of bytes actually read is returned. In the event of an error, the return value is negative.

## 28.3.7. Seeking in a Large Object

To change the current read or write location associated with a large object descriptor, call

    int lo_lseek(PGconn *conn, int fd, int offset, int whence);

This function moves the current location pointer for the large object descriptor identified by `fd` to the new location specified by `offset`. The valid values for `whence` are `SEEK_SET` (seek from object start), `SEEK_CUR` (seek from current position), and `SEEK_END` (seek from object end). The return value is the new location pointer, or -1 on error.

## 28.3.8. Obtaining the Seek Position of a Large Object

To obtain the current read or write location of a large object descriptor, call

    int lo_tell(PGconn *conn, int fd);

If there is an error, the return value is negative.

## 28.3.9. Closing a Large Object Descriptor

A large object descriptor may be closed by calling

    int lo_close(PGconn *conn, int fd);

where `fd` is a large object descriptor returned by `lo_open`. On success, `lo_close` returns zero. On error, the return value is negative.

Any large object descriptors that remain open at the end of a transaction will be closed automatically.

## 28.3.10. Removing a Large Object

To remove a large object from the database, call

    int lo_unlink(PGconn *conn, Oid lobjId);

The `lobjId` argument specifies the OID of the large object to remove. Returns 1 if successful, -1 on failure.

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