libpq library

Estimated sirs:

I am a Computers Engineering student. Currently I am doing a program in C which uses the PostgreSQL's library "libpq" and I have a problem:

I can't introduce in the data base a simple quote. For example:

char *sent_sql;
char *dato;
strcpy(dato, "grocery's");
sprintf(sent_sql, "insert into table values ('%s')", dato);

However, I don't have problems with the SQL interpreter, eg.:
insert into table values ('grocery\'s');
In this case, the data will be stored as "grocery's" (OK!), but I don't find the way I could do that using the library.

I just can't introduce the '\' character previous to the simple quote because I will loose the final character of the string. I can't define the field as type text because I need that the field had a defined size (I got to store encrypted data).

If you know a solution, I would thank you.

Sorry for my english and thanks for paying attention to this e-mail,

DAVID.