C : warning: assignment makes pointer from integer without a cast [enabled by default]

Question

This is my code

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

void main() {
    FILE *fp;
    char * word;
    char line[255];
    fp=fopen("input.txt","r");
    while(fgets(line,255,fp)){
        word=strtok(line," ");
        while(word){
            printf("%s",word);
            word=strtok(NULL," ");
        }
    }
}

This the warning I get.

token.c:10:7: warning: assignment makes pointer from integer without a cast [enabled by default]
   word=strtok(line," ");
       ^

token.c:13:8: warning: assignment makes pointer from integer without a cast [enabled by default]
    word=strtok(NULL," ");
        ^

The word is declared as char*. Then why this warning arises?

Answer 1

Include #include <string.h> to get the prototype for strtok().

Your compiler (like in pre-C99 C) assumed strtok() returns an int because of that. But not providing function declaration/prototype is not valid in modern C (since C99).

There used an old rule in C which allowed implicit function declarations. But implicit int rule has been removed from C language since C99.

See: C function calls: Understanding the "implicit int" rule

Answer 2

strtok() is prototyped in <string.h>, you need to include it.

Otherwise, with the lack of forward declaration, your compiler assumes that any function used for which the signature is not known to it, returns an int and accepts any number of parameters. This was called implicit declarration .

FWIW, implicit function declaration is now invalid as per the latest standard. Quoting C11, Foreward, "Major changes in the second edition included:"

• remove implicit function declaration

Answer 3

You need to include string.h

#include <string.h>