警告unsigned long to int in C语言

Question

I'd have a quick question over my codes:

int main(void)
{
    int i,j,key[5][5],ikey[5][5],row,col,plen,suc;
    int devide,count,h,k,no,p1[100],e1[100],d1[100];
    char p[100],e[100],d[100],clen;

    printf("Enter your plaintext::::::::");
    gets(p);
    plen = strlen(p); // this line gets error

warning line is:

plen = strlen(p);

implicit conversion loses integer precision unsigned long to int.

Answer 1

As already mentioned the function strlen(p) returns a value of type size_t . Here is an abstract from the C99 standard:

size_t

which is the unsigned integer type

Now the point here is that what exact type it is( unsigned , unsigned long etc.) is actually implementation-defined, which means it varies depending on the platform. Obviously you have it defined as unsigned long which means that by

plen = strlen(p);

you convert unsigned long to int which the compiler can't ignore because in some cases this conversion may lose the value's precision.

Answer 2

The correct declaration:

#include <stddef.h>

size_t plen;
// ...
plen = strlen(p);

As others have already pointed out, strlen() returns a size_t .