Warning while using strlen in C

Question

I am new to C programming.I am trying to take input from a file line by line and print length of each line. This is my code-

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

int main()
 {

    char name[100];
    printf("Enter file name\n");
    scanf("%s",name);



    FILE * input;
    input=fopen(name,"r");

    char line[100];
    fscanf(input,"%s",line);

    while(!feof(input))
    {

        printf("%d\n",strlen(line));
        fscanf(input,"%s",line);
    }


    return 0;
 }

This is my input file-

172.24.2.1
172.24.2.225
172.17.4.41
172.24.4.42
202.16.112.150
172.24.152.10

This is correctly printing the length of each line.But during compilation I get this warning-

In function ‘main’:
main.c:24:4: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t’ [-Wformat=]
    printf("%d\n",strlen(line));

So my question is why am I getting this warning although my output is correct and how can I fix this? Thanks in advance.

Answer 1

The function strlen returns an object of type size_t (cf. ISO 9899:2011§7.24.6.3). You need to specify that the argument you pass to printf has type size_t, as opposed to int. You can do that by adding a z length modifier (cf. ISO 9899:2011§7.21.6.1/7) specifying an object of type size_t. You should also use the u formatting specifier as size_t is an unsigned type.

printf("%zu\n",strlen(line));