Undeclared Variable

syntax

error: 'x' undeclared (first use in this function)

Description

Using a variable that hasn't been declared. In C, all variables must be declared before use.

Common Causes

• Using a variable without declaring it first
• Typo in variable name (using 'counter' when you declared 'Counter')
• Using a variable outside its scope

Example of Error

Error Code

int main() {
    x = 10;
    printf("Value: %d\n", x);
    return 0;
}

Solution

Declare the variable before using it.

Corrected Code

int main() {
    int x;
    x = 10;
    printf("Value: %d\n", x);
    return 0;
}

Additional Tips

• C is case-sensitive, so 'count' and 'Count' are different variables
• Variables declared inside a block are only accessible within that block
• Consider using a linter to catch undeclared variables