undefined reference to `strlwr'

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;
}