strlen returns size_t which is an unsigned integer. So strlen(str)-1 would produce SIZE_MAX (the maximum value size_t can hold) if strlen(str) is 0.

You should be using %zu to print size_t values.

This statement

printf(" %d,  %ul,  %u, %d,  %ul, %u", strlen(str),strlen(str),strlen(str),strlen(str)-1,strlen(str)-1,strlen(str)-1);

shows that when strlen( str ) - 1 is outputed like unsigned integer for example using format specifier %ul its value is 4294967295l

In the condition of the loop

for (int i=0;i<strlen(str)-1;i++)

the compiler has to determine the common type of the left and right operands that to determine the type of the result of the condition

i<strlen(str)-1

The right operand strlen(str)-1 has type size_t (the return type of function strlen is size_t). It is unsigned integer type usually that corresponds to unsigned long. It can not have negative values. Any value that stored in an object of this type is interpretated as a non-negative value and as the output shows the value of strlen(str)-1 is equal to 4294967295l. (The actual value you could get if you used type specifier %zu because it may not be excluded that size_t can correspond even to unsigned long long)

The right operand has type int. Its rank is at least not greater than the rank of size_type. So the both operands are converted to type size_t and have non-negative values.

This procedure of determining of the common type is called the usual arithmetic conversions. It is obvious that 4294967295l is greater than 0. Thus the loop will iterate 4294967295l times if it does not have a break statement.

You could get the expected result if you rewrite the condition in the loop the following way

for ( int i = 0; i < ( int )strlen( str ) - 1; i++ )