readline in C doesn't work

I've never encountered such a problem before. I was writing simple C program on Mac and compiled as usual with gcc. ``` #include <stdio.h> #include <string.h> #include <stdlib.h> #include <readline/readline.h> #include <readline/history.h> #define MAXLINES 5 char *lineptr[MAXLINES]; void writel(char *lineptr[], int nlines); void quicksort(char *lineptr[], int left, int right); void swap(char *v[], int i, int j); /* sort input lines */ int main() { int nlines; /* number of lines to read */ int i = 0; /* saves lines in the array lineptr */ while (i < MAXLINES) { lineptr[i] = readline("Enter a line: \n"); i++; } quicksort(lineptr, 0, MAXLINES-1); writel(lineptr, MAXLINES); return 0; } ``` It seems that readline was causing the trouble. Once I commented out `lineptr[i] = readline("Enter a line: \n");` it compiled okay. But I don't understand what is wrong with readline here... The error is: Undefined symbols for architecture x86_64: "_readline", referenced from: _main in cckHOwOt.o ld: symbol(s) not found for architecture x86_64 Thanks for anyone who can give some advice. Thanks!