warning: this decimal constant is unsigned only in ISO C90

关于c语言中的大整数常量--by流氓兔斯基之前编译程序的时候发现了这个问题：warning: this decimal constant is unsigned only in ISO C90
找了一下解决方式，记录一下：

1 在常数后面增加一个UL标识，或者ULL表示，如4294967295UL，这样就不会报警了
2 使用十六进制的数字，如0xFFFFFFFF
3 使用gcc -std=c99 用99标准来编译

看看c的标准里面是怎么说的：

The C90 rule that the default type of a decimal integer constant is either int, long, or
unsigned long, depending on which type is large enough to hold the value without overflow,
simplifies the use of constants. The choices in C99 are int, long and long long.
C89 added the suffixes U and u to specify unsigned numbers. C99 adds LL to specify long
long.
Unlike decimal constants, octal and hexadecimal constants too large to be ints are typed as
unsigned int if within range of that type, since it is more likely that they represent bit
patterns or masks, which are generally best treated as unsigned, rather than “real” numbers.
Little support was expressed for the old practice of permitting the digits 8 and 9 in an octal
constant, so it was dropped in C89.
A proposal to add binary constants was rejected due to lack of precedent and insufficient utility.
Despite a concern that a “lower-case-l” could be taken for the numeral one at the end of a
numeric literal, the C89 Committee rejected proposals to remove this usage, primarily on the
grounds of sanctioning existing practice.
搞明白了吧，出现这个告警，其实也是gcc提醒你该升级你的编译选项了。不过最安全的还是使用十六进制，或者加上UL或者ULL这样的说明。

链接自 http://forum.ubuntu.org.cn/viewtopic.php?t=288895