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

Solution

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;
}

Comments

[Jonathan Leffler]

Just in case char is signed, you should use str[i] = tolower((unsigned char)str[i]);.

[mrtgnccn]

@BlueMoon Thank you sir, I did it like you said and it became alive. However I saw strcmp() as a standart C function and that is why I used it. Thank you again.

[P.P]

strcmp() is a standard function, while strlwr() isn't. The documentation of strlwr() would have mentioned about it (I hope!).

[Jonathan Leffler]

@BlueMoon: FYI: on Linux (Ubuntu 14.04 specifically, with GCC 4.8.2 specifically, but I think it is valid for x86 and x86_64 versions of Linux generically), plain char is a signed type; the range is CHAR_MIN == -128 && CHAR_MAX == +127. Ditto for Mac OS X 10.9.2 Mavericks and GCC 4.9.0.