The log function calculates the natural logarithm for the floating-point number val. The logarithm is the exponent to which the base (e or 2.718282... in the case of natural logs) must be raised to equal val. Or,

log_e e^x = val

The log function returns the floating-point natural logarithm of val.

• If x < 0, log returns NaN.
• If x = 0, log returns -INF.

exp, log10

#include <math.h>
#include <stdio.h> /* for printf */

void tst_log (void) {
  float x;
  float y;

  x = 2.718282;
  x *= x;

  y = log (x); /* y = 2 */

  printf ("LOG(%f) = %f\n", x, y);
}