undefined reference to `strlwr' - c

Question

My code is like a text compressor, reading normal text and turns into numbers, every word has a number. It compiles in DevC++ but does not end, however, it does not compile in Ubuntu 13.10. I'm getting an error like in the title in Ubuntu "undefined reference to `strlwr'", my code is a little long so I am not able to post it here, but one of the error is from here:
//operatinal funcitons here
int main()
{
int i = 0, select;
char filename[50], textword[40], find[20], insert[20], delete[20];
FILE *fp, *fp2, *fp3;
printf("Enter the file name: ");
fflush(stdout);
scanf("%s", filename);
fp = fopen(filename, "r");
fp2 = fopen("text.txt", "w+");
while (fp == NULL)
{
printf("Wrong file name, please enter file name again: ");
fflush(stdout);
scanf("%s", filename);
fp = fopen(filename, "r");
}
while (!feof(fp))
{
while(fscanf(fp, "%s", textword) == 1)
{
strlwr(textword);
//some other logic
}
}
.... //main continues

Answer 1

strlwr() is not standard C function. Probably it's provided by one implementation while the other compiler you use don't.
You can easily implement it yourself:
#include <string.h>
#include<ctype.h>
char *strlwr(char *str)
{
unsigned char *p = (unsigned char *)str;
while (*p) {
*p = tolower((unsigned char)*p);
p++;
}
return str;
}

Answer 2

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
char* strlwr(char* );
int main()
{
printf("Please Enter Size Of The String: \n");
int a,b;
scanf("%d",&a);
char* str;
str=(char*)malloc(sizeof(char)*a);
scanf("\n%[^\n]",str);
char* x;
x=strlwr(str);
for(b=0;x[b]!='\0';b++)
{
printf("%c",x[b]);
}
free(str);
return 0;
}
char* strlwr(char* x)
{
int b;
for(b=0;x[b]!='\0';b++)
{
if(x[b]>='A'&&x[b]<='Z')
{
x[b]=x[b]-'A'+'a';
}
}
return x;
}