Concatenating a string to make up an SQL query in C

Question

I need to do this in two separate steps but so far I am not finding a way of doing this.

First, I need to convert a double variable, into a char variable (and to be saved in that variable). I have noticed type casting doesnt work the same in C as Java / other languages. How do I cast a variable to be a string / char?

Second, I need to concatenate the strings, there will be a total of 6 string variables that will need concatenating, I have only found the strcat function which only takes 2 arguments.

These are the strings I am trying to build:

char *queryOne = "INSERT INTO location (id, carid, ownerid, lat, long, speed) VALUES (,2, 1, ";
char *queryTwo = lat; // lat is a double
char *queryThree = ",";
char *queryFour = longatude; // longatude is a double
char *queryFive = ",";
char *querySix = speed; // speed is a double

And then I need the concatenated string to work in: (mysql_query(conn, query)) as one long string

Edit: So possibly, this should convert the datatype I think?

char buffer [50];

char *queryOne = "INSERT INTO location (id, carid, ownerid, lat, long, speed) VALUES (,2, 1, ";
char *queryTwo = sprintf (buffer, "%d", lat);
char *queryThree = ",";
char *queryFour = sprintf (buffer, "%d", longatude);
char *queryFive = ",";
char *querySix = sprintf (buffer, "%d", speed);

fprintf(stderr, "Dta: %s\n", queryOne);
fprintf(stderr, "Dta: %s\n", *queryTwo);
fprintf(stderr, "Dta: %s\n", queryThree);
fprintf(stderr, "Dta: %s\n", *queryFour);
fprintf(stderr, "Dta: %s\n", queryFive);
fprintf(stderr, "Dta: %s\n", *querySix);

@WeatherVane because I need to know how to convert to char from double, as I need one long string of chars to use with the SQL query — I Garratt
– I Garratt, Commented Aug 5, 2015 at 19:31

lost_in_the_source · Accepted Answer · 2015-08-05 19:41:33Z

1

In your case, you could use:

#define MAXSQL 256
char sql[MAXSQL];
snprintf(sql, MAXSQL,  "%s %f , %f , %f", queryOne, lat, longatude, speed);

The snprintf function writes onto the buffer, that is its first argument. http://www.cplusplus.com/reference/cstdio/snprintf/?kw=snprintf

Now you can use the sql string as you please.

Note that I used snprintf rather than sprintf. This is to avoid potential buffer overflows.

Also, don't use strcat so repeatedly, because that causes a Shlemiel the Painter algorithm, and every next call to strcat gets slower, because strcat has to start from the beginning and find the null terminator. See http://www.joelonsoftware.com/articles/fog0000000319.html for more info.

edited Aug 5, 2015 at 19:41

answered Aug 5, 2015 at 19:35

lost_in_the_source

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5 Comments

Weather Vane Over a year ago

Don't use %lf for doubles in printf family. Use it for doubles in scanf family. Here %f is sufficient for double.

lost_in_the_source Over a year ago

Changed as you said @WeatherVane

I Garratt Over a year ago

@cpp_prog thanks for the suggestion, is this what you mean? mysql_query(conn, snprintf(sql, MAXSQL, "%s %f , %f , %f", queryOne, lat, longatude, speed)) as it is producing a segfault

Weather Vane Over a year ago

No, you are passing the return value from snprintf which is its length. Make the string sql as advised, then mysql_query(conn, sql). I respectfully advise to read the manual.

I Garratt Over a year ago

@WeatherVane ah sorry I didn't realise sql was to be used as a variable, I thought that was just a buffer size

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