Does UINT_MAX have all bits set to 1?

In C, the maximum value for an unsigned integer must be in the form¹: 2^N – 1.

Thus all value bits of the value UINT_MAX will be set to 1. There may be padding bits, whose values are unspecified.

¹ (Quoted from: ISO/IEC 9899:201x 6.2.6.2 Integer types 1)
For unsigned integer types other than unsigned char, the bits of the object representation shall be divided into two groups: value bits and padding bits (there need not be any of the latter). If there are N value bits, each bit shall represent a different power of 2 between 1 and 2^N−1 , so that objects of that type shall be capable of representing values from 0 to 2^N−1 using a pure binary representation; this shall be known as the value representation. The values of any padding bits are unspecified.