Help Center/ GaussDB/ Developer Guide(Centralized_3.x)/ Application Development Guide/ ECPG-based Development/ Host Variables/ Handling Character Strings
Updated on 2024-05-07 GMT+08:00
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Handling Character Strings

To handle SQL character string data types, such as varchar and text, there are two possible methods to declare the host variables.
  1. Method 1: Use char[] (a char string), which is the most common method for processing character data in C programs. 
    EXEC SQL BEGIN DECLARE SECTION;
    char str[50];
EXEC SQL END DECLARE SECTION;

    Note that you have to take care of the length yourself. If you use this host variable as the target variable of a query which returns a string with more than 49 characters, a buffer overflow occurs.

  2. Method 2: Use the VARCHAR type, which is a special type provided by ecpg. The definition on an array of type VARCHAR is converted into a named struct for every variable. The statement is as follows: 
    VARCHAR var[180];

    It will be converted into:

    struct varchar_var 
{ 
    int len; 
    char arr[180]; 
} var;

    To store a string in a VARCHAR host variable, the host variable has to be declared as a string including the zero-byte terminator. arr stores the string including a terminating zero byte. len stores the length of the string stored in arr without the terminating zero byte. The terminator is not included when the length is calculated. When a host variable is used as input for a query, if the values of strlen(arr) and len are different, the shorter one is used.

    • VARCHAR can be written in upper or lower case, but not in mixed case.
    • char and VARCHAR host variables can also hold values of other SQL types, which will be stored in their string forms.
Parent topic: Host Variables