Symbol referencing errors

Undefined first referenced
symbol in file
logf /var/tmp//ccwztFsO.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status

float exponC(float mean)
{
index1++;
return -1* mean * logf(u1[index1]);
}

float exponS(float mean)
{
index2++;
return -1* mean * logf(u2[index2]);
}

I use unix system.

You need to add

-lm

to the command line. That's a hyphen, a lower-case 'L' followed by an 'm'