PostgreSQL
35.3. Client Interfaces
This section describes the facilities that PostgreSQL’s libpq client interface library provides for accessing large objects. The PostgreSQL large object interface is modeled after the Unix file-system interface, with analogues of open, read, write, lseek, etc.
All large object manipulation using these functions must take place within an SQL transaction block, since large object file descriptors are only valid for the duration of a transaction.
If an error occurs while executing any one of these functions, the function will return an otherwise-impossible value, typically 0 or -1. A message describing the error is stored in the connection object and can be retrieved with PQerrorMessage.
Client applications that use these functions should include the header file libpq/libpq-fs.h and link with the libpq library.
Client applications cannot use these functions while a libpq connection is in pipeline mode.
35.3.1. Creating a Large Object
The function
Oid lo_create(PGconn *conn, Oid lobjId);creates a new large object. The OID to be assigned can be specified by `lobjId; if so, failure occurs if that OID is already in use for some large object. If lobjId is `InvalidOid (zero) then lo_create assigns an unused OID. The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure.
An example:
inv_oid = lo_create(conn, desired_oid);The older function
Oid lo_creat(PGconn *conn, int mode);also creates a new large object, always assigning an unused OID. The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure.
In PostgreSQL releases 8.1 and later, the `mode is ignored, so that `lo_creat is exactly equivalent to lo_create with a zero second argument. However, there is little reason to use lo_creat unless you need to work with servers older than 8.1. To work with such an old server, you must use lo_creat not lo_create, and you must set `mode to one of `INV_READ, INV_WRITE, or INV_READ | INV_WRITE. (These symbolic constants are defined in the header file libpq/libpq-fs.h.)
An example:
inv_oid = lo_creat(conn, INV_READ|INV_WRITE);35.3.2. 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 file system and be readable by the client application.
The function
Oid lo_import_with_oid(PGconn *conn, const char *filename, Oid lobjId);also imports a new large object. The OID to be assigned can be specified by `lobjId; if so, failure occurs if that OID is already in use for some large object. If lobjId is `InvalidOid (zero) then lo_import_with_oid assigns an unused OID (this is the same behavior as lo_import). The return value is the OID that was assigned to the new large object, or InvalidOid (zero) on failure.
lo_import_with_oid is new as of PostgreSQL 8.4 and uses lo_create internally which is new in 8.1; if this function is run against 8.0 or before, it will fail and return InvalidOid.
35.3.3. 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.
35.3.4. 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. (These symbolic constants are defined in the header file `libpq/libpq-fs.h.) lo_open returns a (non-negative) large object descriptor for later use in lo_read, lo_write, lo_lseek, lo_lseek64, lo_tell, lo_tell64, lo_truncate, lo_truncate64, and lo_close. The descriptor is only valid for the duration of the current transaction. On failure, -1 is returned.
The server currently does not distinguish between modes INV_WRITE and INV_READ | INV_WRITE: you are allowed to read from the descriptor in either case. However there is a significant difference between these modes and INV_READ alone: with INV_READ you cannot write on the descriptor, and the data read from it will reflect the contents of the large object at the time of the transaction snapshot that was active when lo_open was executed, regardless of later writes by this or other transactions. Reading from a descriptor opened with INV_WRITE returns data that reflects all writes of other committed transactions as well as writes of the current transaction. This is similar to the behavior of REPEATABLE READ versus READ COMMITTED transaction modes for ordinary SQL SELECT commands.
lo_open will fail if SELECT privilege is not available for the large object, or if INV_WRITE is specified and UPDATE privilege is not available. (Prior to PostgreSQL 11, these privilege checks were instead performed at the first actual read or write call using the descriptor.) These privilege checks can be disabled with the lo_compat_privileges run-time parameter.
An example:
inv_fd = lo_open(conn, inv_oid, INV_READ|INV_WRITE);35.3.5. 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 (which must be of size len) 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 current implementation, this will always equal `len` unless there is an error). In the event of an error, the return value is -1.
Although the `len parameter is declared as `size_t, this function will reject length values larger than INT_MAX. In practice, it’s best to transfer data in chunks of at most a few megabytes anyway.
35.3.6. Reading Data from a Large Object
The function
int lo_read(PGconn *conn, int fd, char *buf, size_t len);reads up to `len bytes from large object descriptor fd into buf (which must be of size len). The fd argument must have been returned by a previous `lo_open. The number of bytes actually read is returned; this will be less than `len` if the end of the large object is reached first. In the event of an error, the return value is -1.
Although the `len parameter is declared as `size_t, this function will reject length values larger than INT_MAX. In practice, it’s best to transfer data in chunks of at most a few megabytes anyway.
35.3.7. 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.
When dealing with large objects that might exceed 2GB in size, instead use
pg_int64 lo_lseek64(PGconn *conn, int fd, pg_int64 offset, int whence);This function has the same behavior as lo_lseek, but it can accept an `offset larger than 2GB and/or deliver a result larger than 2GB. Note that `lo_lseek will fail if the new location pointer would be greater than 2GB.
lo_lseek64 is new as of PostgreSQL 9.3. If this function is run against an older server version, it will fail and return -1.
35.3.8. 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 -1.
When dealing with large objects that might exceed 2GB in size, instead use
pg_int64 lo_tell64(PGconn *conn, int fd);This function has the same behavior as lo_tell, but it can deliver a result larger than 2GB. Note that lo_tell will fail if the current read/write location is greater than 2GB.
lo_tell64 is new as of PostgreSQL 9.3. If this function is run against an older server version, it will fail and return -1.
35.3.9. Truncating a Large Object
To truncate a large object to a given length, call
int lo_truncate(PGconn *conn, int fd, size_t len);This function truncates the large object descriptor `fd to length len. The fd argument must have been returned by a previous `lo_open. If `len is greater than the large object’s current length, the large object is extended to the specified length with null bytes ('\0'). On success, `lo_truncate returns zero. On error, the return value is -1.
The read/write location associated with the descriptor `fd` is not changed.
Although the `len parameter is declared as `size_t, lo_truncate will reject length values larger than INT_MAX.
When dealing with large objects that might exceed 2GB in size, instead use
int lo_truncate64(PGconn *conn, int fd, pg_int64 len);This function has the same behavior as lo_truncate, but it can accept a `len` value exceeding 2GB.
lo_truncate is new as of PostgreSQL 8.3; if this function is run against an older server version, it will fail and return -1.
lo_truncate64 is new as of PostgreSQL 9.3; if this function is run against an older server version, it will fail and return -1.
35.3.10. Closing a Large Object Descriptor
A large object descriptor can 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 -1.
Any large object descriptors that remain open at the end of a transaction will be closed automatically.
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