Converting a MySQL result into a JSON string in C - mysql

Question

How do I convert a result from a MySQL query to a JSON string in C?
Of course I know how to do it, I just wondered if there's already a solution to copy-paste, realizing I don't want to write boiler-plate code.
And how do I Google for this question? Google just disregard the c and shows results for PHP etc.

Answer 1

The C file I made to do this is four times faster than the corresponding PHP file, using ab to measure performance:
ab -k -c 300 -n 10000 localhost/tiny.php
Time per request: 393.072 [ms] (mean)
With C:
ab -k -c 300 -n 10000 localhost/cgi/tiny.fcgi
Time per request: 98.237 [ms] (mean)
This is assuming Apache spawns 10 processes of tiny.fcgi, and PHP does not use FastCGI.
FastCgiServer /var/www/cgi/tiny.fcgi -processes 10
This is the PHP code, which connects to MySQL, fetch a query result and echo the JSON representation:
<?php
$mysqli = mysqli_connect("localhost", "user", "password", "db");
mysqli_set_charset($mysqli, "utf8");
if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}
$result = mysqli_query($mysqli, "SELECT * FROM table");
$rows = array();
while ($row = mysqli_fetch_assoc($result)) {
$rows[] = $row;
}
echo json_encode($rows);
Let's step through the C code. I will use the PHP internal struct smart_str to handle strings (this is what PHP use in the json_encode function). Using the naive char* for strings will be catastrophical, because the number of malloc and free will be huge due to string append usage.
We want to use FastCGI:
#include "fcgi_stdio.h"
int main(void)
{
while(FCGI_Accept() >= 0)
{
}
FCGI_Finish();
return 0;
}
Analysing this with valgrind gives 768 bytes still reachable at end, which we will ignore (bug in FastCGI probably).
Next comes MySQL connection and query:
MYSQL* connection = NULL;
MYSQL_RES* result = NULL;
connection = mysql_init(NULL);
if (connection == NULL)
{
(void) FCGI_fprintf(FCGI_stderr, "Could not connect to MySQL: %s\n", mysql_error(connection));
continue;
}
// Connect to database
if (mysql_real_connect(connection, "localhost", "user", "password", "db", 0, NULL, 0) == NULL)
{
close_mysql_with_error(connection);
continue;
}
// Select from pages
if (mysql_query(connection, "SELECT * FROM table") != 0)
{
close_mysql_with_error(connection);
continue;
}
// Get result
result = mysql_store_result(connection);
// Abort if no result
if (result == NULL)
{
close_mysql_with_error(connection);
continue;
}
(I use continue instead of exit or return, because this code is within the while loop seen above.)
Nothing strange here, right?
The next part will create our smart_str JSON variable, pass it to function result_to_json and then echo the result.
smart_str json = {0, 0, 0};
result_to_json(result, &json);
if (json.c != NULL)
(void) FCGI_printf("json = %s\n", json.c);
smart_str_free(&json);
The result_to_json is just a loop over the rows in the MySQL result:
static void result_to_json(MYSQL_RES *result, smart_str* json)
{
MYSQL_ROW row;
int i;
int num_fields = (int) mysql_num_fields(result);
smart_str** fields = get_field_names(result, num_fields);
if (fields == NULL)
{
return;
}
smart_str_appendc(json, '[');
while ((row = mysql_fetch_row(result, num_fields)))
{
smart_str_appendl(json, "{", 1);
for (i = 0; i < num_fields; i++)
{
// key
smart_str_appendl(json, "\"", 1);
smart_str_appendl(json, fields[i]->c, fields[i]->len);
smart_str_appendl(json, "\": ", 3);
if (row[i] == NULL)
{
smart_str_appendl(json, "null", 4);
smart_str_appendl(json, ", ", 2);
}
else
{
smart_str_appendl(json, "\"", 1);
smart_str_appendl(json, row[i], strlen(row[i]));
smart_str_appendl(json, "\", ", 3);
}
}
if (json == NULL) {
free_field_names(fields, num_fields);
return;
}
// Strip last ','
json->len--;
json->len--;
smart_str_appendl(json, "}, ", 3);
}
if (json == NULL)
{
free_field_names(fields, num_fields);
return;
}
// Strip last ','
json->len--;
json->len--;
smart_str_appendl(json, "]", 1);
smart_str_0(json);
free_field_names(fields, num_fields);
return;
}
Finally, get_field_names and free_field_names:
static smart_str** get_field_names(MYSQL_RES *my_result, int num_fields)
{
smart_str** fields; // Array of pointers
MYSQL_FIELD *field = NULL;
int i;
// Allocate size of array
fields = malloc(num_fields * sizeof(smart_str*));
if (fields == NULL)
{
return NULL;
}
for (i = 0; i < num_fields; i++)
{
field = mysql_fetch_field(my_result);
if (field == NULL) {
// TODO: Free fields[]
free(fields);
return NULL;
}
fields[i] = malloc(sizeof(smart_str));
if (fields[i] == NULL) {
// TODO: Free fields[]
free(fields);
return NULL;
}
else
{
fields[i]->c = NULL;
smart_str_appendl(fields[i], field->name, strlen(field->name));
}
return fields;
}
static void free_field_names(smart_str** strings, int size)
{
int i;
for (i = 0; i < size; i++)
{
smart_str_free(strings[i]);
free(strings[i]);
}
free(strings);
}
There you go! What's left to do is to measure against FastCGI enabled PHP, to see how much the PHP performance improves.