Undefined Reference to Standard Library Function

linker

undefined reference to 'sqrt'

Description

The linker cannot find a standard library function that is used in the code.

Common Causes

• Forgetting to link the math library (-lm)
• Using a function from a library that isn't linked
• Typo in function name

Example of Error

Error Code

#include <math.h>
#include <stdio.h>

int main() {
    double result = sqrt(2.0);
    printf("Square root of 2 is %f\n", result);
    return 0;
}

// Compiled with: gcc -o program main.c
// Missing -lm flag

Solution

Link the appropriate library when compiling.

Corrected Code

// Compile with:
// gcc -o program main.c -lm

Additional Tips

• The math library requires explicit linking with -lm
• Some functions may be in libraries other than the standard C library
• Check the documentation to see which library a function belongs to