“String” contains function in C

This function checks whether or not a substring needle exists in another string haystack and returns the position if it does or 0 if it doesn't, unless the position is 0 in which case, it won't be located.

Looking for ways to improve this code, specifically better error handling.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

size_t contains(const char * needle, const char *haystack);

int main(void)
{
char *needle = "test";
char *haystack = "This is a dinosaurtest.";
printf("Position: %lu", contains(needle, haystack));

return EXIT_SUCCESS;
}

size_t contains(const char * needle, const char *haystack)
{
if(needle == NULL || haystack == NULL)
{
return 0;
}
long int first_char_pos = -1;
size_t len_h = strlen(haystack);
size_t len_n = strlen(needle);
size_t i, j;
size_t exist_count = 0;

// Find the first character. If it doesn't exist, we're done.
for(i = 0; i < len_h; i++)
{
if((haystack[i] == needle[0]) && (first_char_pos == -1))
{
first_char_pos = i;
exist_count++;
}
}

if(first_char_pos == -1)
{
return 0;
}
printf("First char match index: %li\n", first_char_pos);
printf("Char: %c\n", haystack[first_char_pos]);
size_t current_index = (size_t) first_char_pos;
for(i = first_char_pos; i < len_h; i++)
{
if(haystack[i] == needle[exist_count] && (i == (current_index + 1)))
{
current_index = i;
exist_count++;
}
printf("Exist count: %lu\n", exist_count); //<--Debugging
if(exist_count == len_n)
{
return first_char_pos;
}
}
return 0;
}

• Please add some examples to show exactly what you mean by saying "contains". I doubt that your code works as intended. – Roland Illig Aug 10 '19 at 20:47
• This already exists: strstr(). There's a safer version called strnstr(). You can find an implementation here: github.com/lattera/freebsd/blob/master/lib/libc/string/… – alx Aug 10 '19 at 21:26
• @RolandIllig: why, can you explain? – Arkadiusz Drabczyk Aug 11 '19 at 16:14
• Should contains("tt", "test") return true? – Roland Illig Aug 11 '19 at 16:21
• @CacahueteFrito So, you pay a not inconsiderable cost in usability and performance to mask data-corruption in some rare circumstances, instead of relying on your programs invariants, patching the data or reporting the error? That seems a bad idea all around. – Deduplicator Sep 24 '19 at 11:56

Just a couple of remarks:

• You should add a newline after the last line:

  $./nh First char match index: 18 Char: t Exist count: 1 Exist count: 2 Exist count: 3 Exist count: 4 Position: 18$

• I don't know what compiler you use but with when compiled with gcc and -Wall -Wextra -pedantic you get:

gcc -O2 nh.c -lm -o nh -Wall -Wextra -pedantic
nh.c: In function ‘contains’:
nh.c:25:15: warning: unused variable ‘j’ [-Wunused-variable]
size_t i, j;
^

• Code formatting should be more consistent. For example, in this line you put a whitespace before needle but don't put a whitespace before haystack:

size_t contains(const char * needle, const char *haystack);

• %lu is not a portable specifier for size_t type, you should use %zu introduced in C99.

• You said:

returns the position if it does or 0 if it doesn't, unless the position is 0 in which case, it won't be located.

This is really not good. For example, with this it returns 0:

char *needle = "This";
char *haystack = "This is a dinosaurtest.";


With this, it also returns zero:

char *needle = "non-existent";
char *haystack = "This is a dinosaurtest.";


You can't tell the difference between success and failure in this two examples. Actually, atoi() has the same problem. I don't know what operating system you use but maybe you could use ssize_t as the return type if it's available and return -1 in case of failure.

A simple, trivial implementation of contains could be done like this:

ssize_t contains(const char * needle, const char *haystack)
{
char *needle_in_haystack;
if(!needle || !haystack) return -1;
needle_in_haystack = strstr(haystack, needle);
return needle_in_haystack ? needle_in_haystack - haystack : -1;
}


Then, this program (with a few changes as mentioned above) should work:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

ssize_t contains(const char * needle, const char *haystack)
{
char *needle_in_haystack;
if(!needle || !haystack) return -1;
needle_in_haystack = strstr(haystack, needle);
return needle_in_haystack ? needle_in_haystack - haystack : -1;
}

int main(void)
{
char *needle = "test";
char *haystack = "This is a dinosaurtest.";
char *haystack2 = "This does not contain the string.";
printf("Position: %zd\n", contains(needle, haystack));
printf("Position: %zd\n", contains(needle, haystack2));

return EXIT_SUCCESS;
}


Output:

Position: 18
Position: -1

• I would remove the check that the input is not NULL, and just use a language extension for that (__attribute__((nonnull)) in GCC). NULL is something that you would never expect as input for this function, and it adds one or two unnecessary lines of code. I would prefer to write in the documentation of the function something like: "If the input to this function is a NULL pointer, the behaviour is undefined.". – alx Aug 11 '19 at 23:50
• @CacahueteFrito The original code did it, and I want to strive for compatibility (who knows how the OP was using it?). – S.S. Anne Aug 11 '19 at 23:51
• Missing include for ssize_t: #include <sys/types.h>. Another option would be to use ptrdiff_t instead, from #include <stddef.h>; you are actually returning a pointer difference: ? needle_in_haystack - haystack : – alx Aug 12 '19 at 11:26

Your code doesn't work. It returns 0 for haystack "abbc" and needle "bc", even though haystack contains needle.

You don't need the first loop and all the length calculations. Btw., the function doesn't succeed, if the first char is found, but only the second occourrence of the first char fits with needle.

The task can be reduced to a few lines:

int contains(char *buf, char *needle) {
char *src, *srch, *srcp;
for(src=buf; *src; src++) {
for(srch = needle, srcp = src; *srch && *srcp && *srch == *srcp; srch++, srcp++);
if(!*srch) return src - buf;
}
return -1;
}

• What is a good way to get better at writing more compact, efficient C code like this? This sorta reminds me of K&R C. – the_endian Aug 14 '19 at 17:58