1. libpq-fe.h介绍
libpq-fe.h头文件包含了PostgreSQL用于C/C++语言连接数据库的API接口函数声明及注释
2. 连接数据库
（1）数据库连接参数配置函数
extern PGconn *PQsetdbLogin(const char *pghost, const char *pgport, const char *pgoptions, const char *pgtty, const char *dbName, const char *login, const char *pwd);
返回值PGconn *指针，即连接指针。如果你要对PQsetdbLogin函数封装的话，记得将形参连接指针设成PGconn *&引用类型，因为连接函数需要对连接指针修改，而不是修改对象！
pghost：主机地址，本机为127.0.0.1或localhost；pgport：端口值，一般为5432；pgoptions：额外选项，NULL即可；pgtty：NULL即可；dbName：数据库名；user：用户名；pwd：密码；
（2）错误显示函数extern char *PQerrorMessage(const PGconn *conn)
当连接有误时，可以使用PQerrorMessage函数显示出错信息。
3. 执行SQL语句
执行SQL语句主要是增删改查，只有查询会返回有效记录集。
（1）SQL执行函数
extern PGresult *PQexec(PGconn *conn, const char *query);
参数：
conn：连接指针；query：SQL语句；
返回值:
PGresult *：查询集指针；
（2）元组数函数
extern int PQntuples(const PGresult *res)；
返回值：查询集中记录数；res：查询集指针；
（3）字段数函数
extern int PQnfields(const PGresult *res)；
返回值：每条记录中列数（字段数）；res：查询集指针；
（4）取值函数
extern char *PQgetvalue(const PGresult *res, int tup_num, int field_num);
返回值：查询集中每个位置的值；res：查询集指针；tup_num：行号，从0开始；field_num：列号，从0开始；
4. 关闭连接
（1）查询集清理函数
extern void PQclear(PGresult *res);
res：查询集指针；
（2）关闭连接函数
extern void PQfinish(PGconn *conn);
conn：连接指针；
5. 错误查询
许多时候执行SQL语句后，数据表没有变化，程序也不报错，这种情况很难发现错误。我们需要使用PostgreSQL提供的errorMessage和status函数追踪程序变量的状态。
比如：
（1）PQerrorMessage函数提供了PGconn连接指针的出错信息；
（2）PQresultErrorMessage函数提供了PGresult查询集指针的出错信息；
（3）PQresultStatus函数返回查询集指针的状态信息ExecStatusType，这是个枚举enum类型：
typedef enum { PGRES_EMPTY_QUERY = 0, /* empty query string was executed */ PGRES_COMMAND_OK, /* a query command that doesn't return * anything was executed properly by the * backend */ PGRES_TUPLES_OK, /* a query command that returns tuples was * executed properly by the backend, PGresult * contains the result tuples */ PGRES_COPY_OUT, /* Copy Out data transfer in progress */ PGRES_COPY_IN, /* Copy In data transfer in progress */ PGRES_BAD_RESPONSE, /* an unexpected response was recv'd from the * backend */ PGRES_NONFATAL_ERROR, /* notice or warning message */ PGRES_FATAL_ERROR, /* query failed */ PGRES_COPY_BOTH, /* Copy In/Out data transfer in progress */ PGRES_SINGLE_TUPLE /* single tuple from larger resultset */ } ExecStatusType;
6. libpq-fe.h
/*------------------------------------------------------------------------- * * libpq-fe.h * This file contains definitions for structures and * externs for functions used by frontend postgres applications. * * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * * src/interfaces/libpq/libpq-fe.h * *------------------------------------------------------------------------- */ #ifndef LIBPQ_FE_H #define LIBPQ_FE_H #ifdef __cplusplus extern "C" { #endif #include <stdio.h> /* * postgres_ext.h defines the backend's externally visible types, * such as Oid. */ #include "postgres_ext.h" /* * Option flags for PQcopyResult */ #define PG_COPYRES_ATTRS 0x01 #define PG_COPYRES_TUPLES 0x02 /* Implies PG_COPYRES_ATTRS */ #define PG_COPYRES_EVENTS 0x04 #define PG_COPYRES_NOTICEHOOKS 0x08 /* Application-visible enum types */ /* * Although it is okay to add to these lists, values which become unused * should never be removed, nor should constants be redefined - that would * break compatibility with existing code. */ typedef enum { CONNECTION_OK, CONNECTION_BAD, /* Non-blocking mode only below here */ /* * The existence of these should never be relied upon - they should only * be used for user feedback or similar purposes. */ CONNECTION_STARTED, /* Waiting for connection to be made. */ CONNECTION_MADE, /* Connection OK; waiting to send. */ CONNECTION_AWAITING_RESPONSE, /* Waiting for a response from the * postmaster. */ CONNECTION_AUTH_OK, /* Received authentication; waiting for * backend startup. */ CONNECTION_SETENV, /* Negotiating environment. */ CONNECTION_SSL_STARTUP, /* Negotiating SSL. */ CONNECTION_NEEDED /* Internal state: connect() needed */ } ConnStatusType; typedef enum { PGRES_POLLING_FAILED = 0, PGRES_POLLING_READING, /* These two indicate that one may */ PGRES_POLLING_WRITING, /* use select before polling again. */ PGRES_POLLING_OK, PGRES_POLLING_ACTIVE /* unused; keep for awhile for backwards * compatibility */ } PostgresPollingStatusType; typedef enum { PGRES_EMPTY_QUERY = 0, /* empty query string was executed */ PGRES_COMMAND_OK, /* a query command that doesn't return * anything was executed properly by the * backend */ PGRES_TUPLES_OK, /* a query command that returns tuples was * executed properly by the backend, PGresult * contains the result tuples */ PGRES_COPY_OUT, /* Copy Out data transfer in progress */ PGRES_COPY_IN, /* Copy In data transfer in progress */ PGRES_BAD_RESPONSE, /* an unexpected response was recv'd from the * backend */ PGRES_NONFATAL_ERROR, /* notice or warning message */ PGRES_FATAL_ERROR, /* query failed */ PGRES_COPY_BOTH, /* Copy In/Out data transfer in progress */ PGRES_SINGLE_TUPLE /* single tuple from larger resultset */ } ExecStatusType; typedef enum { PQTRANS_IDLE, /* connection idle */ PQTRANS_ACTIVE, /* command in progress */ PQTRANS_INTRANS, /* idle, within transaction block */ PQTRANS_INERROR, /* idle, within failed transaction */ PQTRANS_UNKNOWN /* cannot determine status */ } PGTransactionStatusType; typedef enum { PQERRORS_TERSE, /* single-line error messages */ PQERRORS_DEFAULT, /* recommended style */ PQERRORS_VERBOSE /* all the facts, ma'am */ } PGVerbosity; /* * PGPing - The ordering of this enum should not be altered because the * values are exposed externally via pg_isready. */ typedef enum { PQPING_OK, /* server is accepting connections */ PQPING_REJECT, /* server is alive but rejecting connections */ PQPING_NO_RESPONSE, /* could not establish connection */ PQPING_NO_ATTEMPT /* connection not attempted (bad params) */ } PGPing; /* PGconn encapsulates a connection to the backend. * The contents of this struct are not supposed to be known to applications. */ typedef struct pg_conn PGconn; /* PGresult encapsulates the result of a query (or more precisely, of a single * SQL command --- a query string given to PQsendQuery can contain multiple * commands and thus return multiple PGresult objects). * The contents of this struct are not supposed to be known to applications. */ typedef struct pg_result PGresult; /* PGcancel encapsulates the information needed to cancel a running * query on an existing connection. * The contents of this struct are not supposed to be known to applications. */ typedef struct pg_cancel PGcancel; /* PGnotify represents the occurrence of a NOTIFY message. * Ideally this would be an opaque typedef, but it's so simple that it's * unlikely to change. * NOTE: in Postgres 6.4 and later, the be_pid is the notifying backend's, * whereas in earlier versions it was always your own backend's PID. */ typedef struct pgNotify { char *relname; /* notification condition name */ int be_pid; /* process ID of notifying server process */ char *extra; /* notification parameter */ /* Fields below here are private to libpq; apps should not use 'em */ struct pgNotify *next; /* list link */ } PGnotify; /* Function types for notice-handling callbacks */ typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res); typedef void (*PQnoticeProcessor) (void *arg, const char *message); /* Print options for PQprint() */ typedef char pqbool; typedef struct _PQprintOpt { pqbool header; /* print output field headings and row count */ pqbool align; /* fill align the fields */ pqbool standard; /* old brain dead format */ pqbool html3; /* output html tables */ pqbool expanded; /* expand tables */ pqbool pager; /* use pager for output if needed */ char *fieldSep; /* field separator */ char *tableOpt; /* insert to HTML <table ...> */ char *caption; /* HTML <caption> */ char **fieldName; /* null terminated array of replacement field * names */ } PQprintOpt; /* ---------------- * Structure for the conninfo parameter definitions returned by PQconndefaults * or PQconninfoParse. * * All fields except "val" point at static strings which must not be altered. * "val" is either NULL or a malloc'd current-value string. PQconninfoFree() * will release both the val strings and the PQconninfoOption array itself. * ---------------- */ typedef struct _PQconninfoOption { char *keyword; /* The keyword of the option */ char *envvar; /* Fallback environment variable name */ char *compiled; /* Fallback compiled in default value */ char *val; /* Option's current value, or NULL */ char *label; /* Label for field in connect dialog */ char *dispchar; /* Indicates how to display this field in a * connect dialog. Values are: "" Display * entered value as is "*" Password field - * hide value "D" Debug option - don't show * by default */ int dispsize; /* Field size in characters for dialog */ } PQconninfoOption; /* ---------------- * PQArgBlock -- structure for PQfn() arguments * ---------------- */ typedef struct { int len; int isint; union { int *ptr; /* can't use void (dec compiler barfs) */ int integer; } u; } PQArgBlock; /* ---------------- * PGresAttDesc -- Data about a single attribute (column) of a query result * ---------------- */ typedef struct pgresAttDesc { char *name; /* column name */ Oid tableid; /* source table, if known */ int columnid; /* source column, if known */ int format; /* format code for value (text/binary) */ Oid typid; /* type id */ int typlen; /* type size */ int atttypmod; /* type-specific modifier info */ } PGresAttDesc; /* ---------------- * Exported functions of libpq * ---------------- */ /* === in fe-connect.c === */ /* make a new client connection to the backend */ /* Asynchronous (non-blocking) */ extern PGconn *PQconnectStart(const char *conninfo); extern PGconn *PQconnectStartParams(const char *const * keywords, const char *const * values, int expand_dbname); extern PostgresPollingStatusType PQconnectPoll(PGconn *conn); /* Synchronous (blocking) */ extern PGconn *PQconnectdb(const char *conninfo); extern PGconn *PQconnectdbParams(const char *const * keywords, const char *const * values, int expand_dbname); extern PGconn *PQsetdbLogin(const char *pghost, const char *pgport, const char *pgoptions, const char *pgtty, const char *dbName, const char *login, const char *pwd); #define PQsetdb(M_PGHOST,M_PGPORT,M_PGOPT,M_PGTTY,M_DBNAME) \ PQsetdbLogin(M_PGHOST, M_PGPORT, M_PGOPT, M_PGTTY, M_DBNAME, NULL, NULL) /* close the current connection and free the PGconn data structure */ extern void PQfinish(PGconn *conn); /* get info about connection options known to PQconnectdb */ extern PQconninfoOption *PQconndefaults(void); /* parse connection options in same way as PQconnectdb */ extern PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg); /* return the connection options used by a live connection */ extern PQconninfoOption *PQconninfo(PGconn *conn); /* free the data structure returned by PQconndefaults() or PQconninfoParse() */ extern void PQconninfoFree(PQconninfoOption *connOptions); /* * close the current connection and restablish a new one with the same * parameters */ /* Asynchronous (non-blocking) */ extern int PQresetStart(PGconn *conn); extern PostgresPollingStatusType PQresetPoll(PGconn *conn); /* Synchronous (blocking) */ extern void PQreset(PGconn *conn); /* request a cancel structure */ extern PGcancel *PQgetCancel(PGconn *conn); /* free a cancel structure */ extern void PQfreeCancel(PGcancel *cancel); /* issue a cancel request */ extern int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize); /* backwards compatible version of PQcancel; not thread-safe */ extern int PQrequestCancel(PGconn *conn); /* Accessor functions for PGconn objects */ extern char *PQdb(const PGconn *conn); extern char *PQuser(const PGconn *conn); extern char *PQpass(const PGconn *conn); extern char *PQhost(const PGconn *conn); extern char *PQport(const PGconn *conn); extern char *PQtty(const PGconn *conn); extern char *PQoptions(const PGconn *conn); extern ConnStatusType PQstatus(const PGconn *conn); extern PGTransactionStatusType PQtransactionStatus(const PGconn *conn); extern const char *PQparameterStatus(const PGconn *conn, const char *paramName); extern int PQprotocolVersion(const PGconn *conn); extern int PQserverVersion(const PGconn *conn); extern char *PQerrorMessage(const PGconn *conn); extern int PQsocket(const PGconn *conn); extern int PQbackendPID(const PGconn *conn); extern int PQconnectionNeedsPassword(const PGconn *conn); extern int PQconnectionUsedPassword(const PGconn *conn); extern int PQclientEncoding(const PGconn *conn); extern int PQsetClientEncoding(PGconn *conn, const char *encoding); /* Get the OpenSSL structure associated with a connection. Returns NULL for * unencrypted connections or if any other TLS library is in use. */ extern void *PQgetssl(PGconn *conn); /* Tell libpq whether it needs to initialize OpenSSL */ extern void PQinitSSL(int do_init); /* More detailed way to tell libpq whether it needs to initialize OpenSSL */ extern void PQinitOpenSSL(int do_ssl, int do_crypto); /* Set verbosity for PQerrorMessage and PQresultErrorMessage */ extern PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity); /* Enable/disable tracing */ extern void PQtrace(PGconn *conn, FILE *debug_port); extern void PQuntrace(PGconn *conn); /* Override default notice handling routines */ extern PQnoticeReceiver PQsetNoticeReceiver(PGconn *conn, PQnoticeReceiver proc, void *arg); extern PQnoticeProcessor PQsetNoticeProcessor(PGconn *conn, PQnoticeProcessor proc, void *arg); /* * Used to set callback that prevents concurrent access to * non-thread safe functions that libpq needs. * The default implementation uses a libpq internal mutex. * Only required for multithreaded apps that use kerberos * both within their app and for postgresql connections. */ typedef void (*pgthreadlock_t) (int acquire); extern pgthreadlock_t PQregisterThreadLock(pgthreadlock_t newhandler); /* === in fe-exec.c === */ /* Simple synchronous query */ extern PGresult *PQexec(PGconn *conn, const char *query); extern PGresult *PQexecParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char *const * paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); extern PGresult *PQprepare(PGconn *conn, const char *stmtName, const char *query, int nParams, const Oid *paramTypes); extern PGresult *PQexecPrepared(PGconn *conn, const char *stmtName, int nParams, const char *const * paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); /* Interface for multiple-result or asynchronous queries */ extern int PQsendQuery(PGconn *conn, const char *query); extern int PQsendQueryParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char *const * paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); extern int PQsendPrepare(PGconn *conn, const char *stmtName, const char *query, int nParams, const Oid *paramTypes); extern int PQsendQueryPrepared(PGconn *conn, const char *stmtName, int nParams, const char *const * paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); extern int PQsetSingleRowMode(PGconn *conn); extern PGresult *PQgetResult(PGconn *conn); /* Routines for managing an asynchronous query */ extern int PQisBusy(PGconn *conn); extern int PQconsumeInput(PGconn *conn); /* LISTEN/NOTIFY support */ extern PGnotify *PQnotifies(PGconn *conn); /* Routines for copy in/out */ extern int PQputCopyData(PGconn *conn, const char *buffer, int nbytes); extern int PQputCopyEnd(PGconn *conn, const char *errormsg); extern int PQgetCopyData(PGconn *conn, char **buffer, int async); /* Deprecated routines for copy in/out */ extern int PQgetline(PGconn *conn, char *string, int length); extern int PQputline(PGconn *conn, const char *string); extern int PQgetlineAsync(PGconn *conn, char *buffer, int bufsize); extern int PQputnbytes(PGconn *conn, const char *buffer, int nbytes); extern int PQendcopy(PGconn *conn); /* Set blocking/nonblocking connection to the backend */ extern int PQsetnonblocking(PGconn *conn, int arg); extern int PQisnonblocking(const PGconn *conn); extern int PQisthreadsafe(void); extern PGPing PQping(const char *conninfo); extern PGPing PQpingParams(const char *const * keywords, const char *const * values, int expand_dbname); /* Force the write buffer to be written (or at least try) */ extern int PQflush(PGconn *conn); /* * "Fast path" interface --- not really recommended for application * use */ extern PGresult *PQfn(PGconn *conn, int fnid, int *result_buf, int *result_len, int result_is_int, const PQArgBlock *args, int nargs); /* Accessor functions for PGresult objects */ extern ExecStatusType PQresultStatus(const PGresult *res); extern char *PQresStatus(ExecStatusType status); extern char *PQresultErrorMessage(const PGresult *res); extern char *PQresultErrorField(const PGresult *res, int fieldcode); extern int PQntuples(const PGresult *res); extern int PQnfields(const PGresult *res); extern int PQbinaryTuples(const PGresult *res); extern char *PQfname(const PGresult *res, int field_num); extern int PQfnumber(const PGresult *res, const char *field_name); extern Oid PQftable(const PGresult *res, int field_num); extern int PQftablecol(const PGresult *res, int field_num); extern int PQfformat(const PGresult *res, int field_num); extern Oid PQftype(const PGresult *res, int field_num); extern int PQfsize(const PGresult *res, int field_num); extern int PQfmod(const PGresult *res, int field_num); extern char *PQcmdStatus(PGresult *res); extern char *PQoidStatus(const PGresult *res); /* old and ugly */ extern Oid PQoidValue(const PGresult *res); /* new and improved */ extern char *PQcmdTuples(PGresult *res); extern char *PQgetvalue(const PGresult *res, int tup_num, int field_num); extern int PQgetlength(const PGresult *res, int tup_num, int field_num); extern int PQgetisnull(const PGresult *res, int tup_num, int field_num); extern int PQnparams(const PGresult *res); extern Oid PQparamtype(const PGresult *res, int param_num); /* Describe prepared statements and portals */ extern PGresult *PQdescribePrepared(PGconn *conn, const char *stmt); extern PGresult *PQdescribePortal(PGconn *conn, const char *portal); extern int PQsendDescribePrepared(PGconn *conn, const char *stmt); extern int PQsendDescribePortal(PGconn *conn, const char *portal); /* Delete a PGresult */ extern void PQclear(PGresult *res); /* For freeing other alloc'd results, such as PGnotify structs */ extern void PQfreemem(void *ptr); /* Exists for backward compatibility. bjm 2003-03-24 */ #define PQfreeNotify(ptr) PQfreemem(ptr) /* Error when no password was given. */ /* Note: depending on this is deprecated; use PQconnectionNeedsPassword(). */ #define PQnoPasswordSupplied "fe_sendauth: no password supplied\n" /* Create and manipulate PGresults */ extern PGresult *PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status); extern PGresult *PQcopyResult(const PGresult *src, int flags); extern int PQsetResultAttrs(PGresult *res, int numAttributes, PGresAttDesc *attDescs); extern void *PQresultAlloc(PGresult *res, size_t nBytes); extern int PQsetvalue(PGresult *res, int tup_num, int field_num, char *value, int len); /* Quoting strings before inclusion in queries. */ extern size_t PQescapeStringConn(PGconn *conn, char *to, const char *from, size_t length, int *error); extern char *PQescapeLiteral(PGconn *conn, const char *str, size_t len); extern char *PQescapeIdentifier(PGconn *conn, const char *str, size_t len); extern unsigned char *PQescapeByteaConn(PGconn *conn, const unsigned char *from, size_t from_length, size_t *to_length); extern unsigned char *PQunescapeBytea(const unsigned char *strtext, size_t *retbuflen); /* These forms are deprecated! */ extern size_t PQescapeString(char *to, const char *from, size_t length); extern unsigned char *PQescapeBytea(const unsigned char *from, size_t from_length, size_t *to_length); /* === in fe-print.c === */ extern void PQprint(FILE *fout, /* output stream */ const PGresult *res, const PQprintOpt *ps); /* option structure */ /* * really old printing routines */ extern void PQdisplayTuples(const PGresult *res, FILE *fp, /* where to send the output */ int fillAlign, /* pad the fields with spaces */ const char *fieldSep, /* field separator */ int printHeader, /* display headers? */ int quiet); extern void PQprintTuples(const PGresult *res, FILE *fout, /* output stream */ int printAttName, /* print attribute names */ int terseOutput, /* delimiter bars */ int width); /* width of column, if 0, use variable width */ /* === in fe-lobj.c === */ /* Large-object access routines */ extern int lo_open(PGconn *conn, Oid lobjId, int mode); extern int lo_close(PGconn *conn, int fd); extern int lo_read(PGconn *conn, int fd, char *buf, size_t len); extern int lo_write(PGconn *conn, int fd, const char *buf, size_t len); extern int lo_lseek(PGconn *conn, int fd, int offset, int whence); extern pg_int64 lo_lseek64(PGconn *conn, int fd, pg_int64 offset, int whence); extern Oid lo_creat(PGconn *conn, int mode); extern Oid lo_create(PGconn *conn, Oid lobjId); extern int lo_tell(PGconn *conn, int fd); extern pg_int64 lo_tell64(PGconn *conn, int fd); extern int lo_truncate(PGconn *conn, int fd, size_t len); extern int lo_truncate64(PGconn *conn, int fd, pg_int64 len); extern int lo_unlink(PGconn *conn, Oid lobjId); extern Oid lo_import(PGconn *conn, const char *filename); extern Oid lo_import_with_oid(PGconn *conn, const char *filename, Oid lobjId); extern int lo_export(PGconn *conn, Oid lobjId, const char *filename); /* === in fe-misc.c === */ /* Get the version of the libpq library in use */ extern int PQlibVersion(void); /* Determine length of multibyte encoded char at *s */ extern int PQmblen(const char *s, int encoding); /* Determine display length of multibyte encoded char at *s */ extern int PQdsplen(const char *s, int encoding); /* Get encoding id from environment variable PGCLIENTENCODING */ extern int PQenv2encoding(void); /* === in fe-auth.c === */ extern char *PQencryptPassword(const char *passwd, const char *user); /* === in encnames.c === */ extern int pg_char_to_encoding(const char *name); extern const char *pg_encoding_to_char(int encoding); extern int pg_valid_server_encoding_id(int encoding); #ifdef __cplusplus } #endif #endif /* LIBPQ_FE_H */
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