C语言警告提示

[Warning] incompatible implicit declaration of built-in function 'strlen' [enabled by default]

提示：strlen()  不兼容的函数声明（字符串长度函数）

位于头文件<string.h>

需要添加：#include <string.h>

#include <stdio.h>
#include <string.h>
//strnchr(const char *str,char ch){

size_t strnchr(const char *str, char ch){

    int i, n = 0, len = strlen(str);
    for(i=0; i<len; i++){
        if(str[i] == ch){
            n++;
        }
    }

    return n;
}
int main(){
    char *str = "abcddtsfsdfdsf.net";
    char ch = 't';
    int n = strnchr(str, ch);
    printf("%d\n", n);
    return 0;
}