What causes an "incompatible implicit declaration" warning?

Question

I am trying find out the number of characters in a string using the strlen() function. The user inputs a sentence and the code must display the number of characters in the string.

int l;
char string[64];
printf("Enter a statement: \n");
scanf("%s",&string);
l = strlen(string);
printf("%d",l);

The code runs incorrectly with the following warning : [Warning] incompatible implicit declaration of built-in function 'strlen'

I did find the correct code online, but what exactly is wrong with my logic ? A little help required.

The title is the first problem with this question. What do you mean by "pass a string to an integer"? How does it relate to the rest of the question? — Antti Haapala -- Слава Україні, Aug 13 '19 at 07:35
Your code is incomplete; in particular, it seems to be missing a `main()` function and at least one `#include`. Please [edit] your code so it's a [mcve] of your problem (including any necessary inputs, but preferably not needing any), then we can try to reproduce and solve it. You should also read [ask]. — Toby Speight, Aug 13 '19 at 08:31

score 7 · Answer 1 · edited Aug 13 '19 at 08:33

Observation, firstly

The code runs incorrectly with the following warning : [Warning] incompatible implicit declaration of built-in function 'strlen'

This is because you didn't include the necessary header for strlen.

#include <string.h>

Secondly, here:

scanf("%s",&string);

string is character array and array name itself address, hence while scanning & is not required.

scanf("%s", string); /* remove & */

Also, always check the return value of functions like scanf. Check the manual page of scanf() to know what it returns. For example:

int ret = scanf("%s", string);
if (ret == 1) {
    /* success, proceed further */
}
else {
   /* proper error handling */
}

`&` is not required, but it does work (the address of an array is the same as its value, just a different type). — S.S. Anne, Aug 20 '19 at 16:01

score 2 · Answer 2 · answered Aug 13 '19 at 09:33

The reason for the warning is forgetting to include string.h

What the warning means in detail, is that in older C from the 90s, it was valid to use a function without declaring it. So if you didn't provide the relevant header like string.h where the declaration is found, the compiler would then "guess" what function format you want and try to silently declare the function "between the lines".

The compiler would have implicitly given you this:

int strlen (); // accepts any parameters

But the real and correct strlen declaration is this:

size_t strlen (const char *s);

These aren't compatible types, hence the warning about incompatible declaration. The actual definition of the strlen function matches the latter, but not the former, so this is a bug.

From the year 1999 and beyond, implicit function declarations were removed from the C language, since they were plain dangerous. The gcc compiler still allows obsolete style C though, if you configure it a certain way.

If you are a beginner, you should always compile with a strict and standard compliant setup:

gcc -std=c11 -pedantic-errors -Wall -Wextra

This will swap the mentioned warning for an error, which is nice since this was a program-breaking bug.

What causes an "incompatible implicit declaration" warning?

2 Answers2