Client 4—An Interactive Query Processor
To wrap up this chapter, you'll convert the interactive query processor from the previous chapter into a libpgeasy client.
Most of the code remains the same, so I'll point out only the differences. The most important change is that you no longer have to pass the PGconn * (connection handle) to every function-libpgeasy is managing the connection handle for you.
1 /*
2 ** File: client4.c
3 */
4
5 #include <stdlib.h>
6 #include <string.h>
7 #include <libpq-fe.h>
8 #include <libpgeasy.h>
9 #include <readline/readline.h>
10 #include <readline/history.h>
11
12 typedef enum { FALSE, TRUE } bool;
13
14 #define MAX_PRINT_LEN 40
15
16 static char separator[MAX_PRINT_LEN+1];
17
18 void print_result_set( PGresult * result )
19 {
20 int col;
21 int row;
22 int * sizes;
23
24 /*
25 ** Compute the size for each column
26 */
27 sizes = (int *)calloc( PQnfields( result ), sizeof( int ));
28
29 for( col = 0; col < PQnfields( result ); col++ )
30 {
31 int len = 0;
32
33 for( row = 0; row < PQntuples( result ); row++ )
34 {
35 if( PQgetisnull( result, row, col ))
36 len = 0;
37 else
38 len = PQgetlength( result, row, col );
39
40 if( len > sizes[col] )
41 sizes[col] = len;
42 }
43
44 if(( len = strlen( PQfname( result, col ))) > sizes[col] )
45 sizes[col] = len;
46
47 if( sizes[col] > MAX_PRINT_LEN )
48 sizes[col] = MAX_PRINT_LEN;
49 }
50
51 /*
52 ** Print the field names.
53 */
54 for( col = 0; col < PQnfields( result ); col++ )
55 {
56 printf( "%-*s ", sizes[col], PQfname( result, col ));
57 }
58
59 printf( "
" );
60
61 /*
62 ** Print the separator line
63 */
64 memset( separator, '-', MAX_PRINT_LEN );
65
66 for( col = 0; col < PQnfields( result ); col++ )
67 {
68 printf( "%*.*s ", sizes[col], sizes[col], separator );
69 }
70
71 printf( "
" );
72
73 /*
74 ** Now loop through each of the tuples returned by
75 ** our query and print the results.
76 */
77 for( row = 0; row < PQntuples( result ); row++ )
78 {
79 for( col = 0; col < PQnfields( result ); col++ )
80 {
81 if( PQgetisnull( result, row, col ))
82 printf( "%*s", sizes[col], "" );
83 else
84 printf( "%*s ", sizes[col], PQgetvalue(result, row, col));
85 }
86
87 printf( "
" );
88
89 }
90 printf( "(%d rows)
", PQntuples( result ));
91
92 free( sizes );
93 }
You can't use the fetch() or fetchwithnulls() in the print_result_set() function. There is no way to construct a call to these functions because you can't know (at the time the program is compiled) how many columns will be returned by a query.
The process_query() function is very simple. The call to doquery() sends the command to the server and returns a pointer to the result set.
95 void process_query( char * buf )
96 {
97 PGresult * result;
98
99 result = doquery( buf );
100
101 if( PQresultStatus( result ) == PGRES_TUPLES_OK )
102 {
103 print_result_set( result );
104 }
105 else if( PQresultStatus( result ) == PGRES_COMMAND_OK )
106 {
107 printf( "%s", PQcmdStatus( result ));
108
109 if( strlen( PQcmdTuples( result )))
110 printf( " - %s rows
", PQcmdTuples( result ));
111 else
112 printf( "
" );
113 }
114 else
115 {
116 printf( "%s
", PQresultErrorMessage( result ));
117 }
118 }
The main() function is largely unchanged. I don't bother to save the connection handle returned by connectdb() because libpgeasy remembers it for me. The only other change in main() is that you set the error-handling mode calling on_error_continue(). If you don't set the error-handling mode, libpgeasy assumes that it should terminate your application if an error is encountered.
120 int main( int argc, char * argv[] )
121 {
122 char * buf;
123
124 connectdb( argc > 1 ? argv[1] : "" );
125
126 on_error_continue();
127
128 using_history();
129 read_history( ".pg_history" );
130
131 while(( buf = readline( "->" )) != NULL )
132 {
133 if( strncmp( buf, "quit", sizeof( "quit" ) - 1 ) == 0 )
134 {
135 break;
136 }
137 else
138 {
139 if( strlen( buf ) != 0 )
140 {
141 add_history( buf );
142 process_query( buf );
143 }
144 free( buf );
145 }
146 }
147
148 write_history( ".pg_history" );
149
150 disconnectdb();
151
152 exit( EXIT_SUCCESS );
153 }