warning: implicit declaration of function and undefined reference - c

Question

I need to write a function that search inside a string a sequence of letters from the abc ( not numbers) remove them and leave only the first and last two of the sequence. For example if the input string is: dabcemoqmnopqrrtaduvwxaz the output should be: da-cemoqm-rrtadu-xaz
Or is the input is : dabcefLMNOpQrstuv567zyx, the output is : da-cefL-OpQr-v567zyx.
I have a main file abc.c and abc_functions.c.
I am getting a few errors-
abc.c: In function ‘main’:
abc.c:10:5: warning: implicit declaration of function ‘abc_functions’ [-Wimplicit-function-declaration]
abc_functions(str);
^
/tmp/ccmsgqvg.o: In function `main':
abc.c:(.text+0x5f): undefined reference to `abc_functions'
collect2: error: ld returned 1 exit status
Any ideas?
`
#include "shortend_string.h"
#include <string.h>
void abc_functions (char str[])
{
char str[max_size];
char *dst = str;
int j = 0;
int i;
int curr;
for (i=0; i < strlen(str); i++)
{
if ((str[i] >= 'a' && str[i] <= 'z') || (str[i] >= 'A' && str[i] <= 'Z')) /*checking that the sequences involves the abc letters only*/
{
for (curr = i; curr < strlen(str); curr++)
{
if (str[curr+1] != str[curr]+1)/* sequences ending point*/
break;
}
}
if (curr >= i+2) /*if sequences is larger than or equal to 2 modify the string*/
{
dst[j++] = str[i];
dst[j++] = '-';
dst[j++] = str[curr];
i = curr; /*resetting the loop*/
}
else
dst[j++] = str[i];
}
dst[j] = '\0';
return(0);
}
`
`
#include <stdio.h>
#include "abc_functions.h"
int main()
{
char str[max_size];
printf("please enter a string:");
fgets(str, max_size, stdin);
printf("\nThe String is:%s", str);
abc_functions(str);
printf("\nThe output is :%s\n" , str);
return 0;
}
`

Answer 1

Note the first error:
implicit declaration of function ‘abc_functions’
The compiler said you lacked to supply an abc_functions() declaration.
Please check:
abc_functions.h contains the proper declartion.
abc_functions.h base directory is given to the compiler (use -Ipath_to_dir).
Also, whenever asking about compilation failures, it's a good idea to provide the compilation command line.