time_t只是8字节的类型定义(long long/__int64)，所有编译器和操作系统都理解。在过去，它只用于长int(4字节)，但现在不是了。如果你查看crtdef .h中的time_t，你会发现这两种实现，但操作系统将使用long long。

您可以使用typeid来找出time_t在您的系统中是如何定义的。

#include <iostream> // cout
#include <ctime>    // time_t
#include <typeinfo> // typeid, name

using namespace std;

int main()
{
    cout << "Test 1: The type of time_t is: \t\t" 
         << typeid(time_t).name() << endl;
    cout << "Test 2: time_t is a signed long?: \t"
         << (typeid(time_t) == typeid(signed long) ? "true" : "false") << endl;
    cout << "Test 3: time_t is an unsigned long?: \t" 
         << (typeid(time_t) == typeid(unsigned long) ? "true" : "false") << endl;
    return 0;
}

在我的系统中，输出是:

Test 1: The type of time_t is:          l
Test 2: time_t is a signed long?:       true
Test 3: time_t is an unsigned long?:    false

在64位机器上，Time_t的类型为long int，否则为long long int。

你可以在这些头文件中验证这一点:

time.h: /usr/include Types.h和typeszes .h: /usr/include/x86_64-linux-gnu/bits

(下面的陈述不是一个接一个的。他们可以在答复中找到。头文件使用Ctrl+f搜索。)

1)在time.h

typedef __time_t time_t;

2)在types.h

# define __STD_TYPE     typedef  
__STD_TYPE __TIME_T_TYPE __time_t;  

3)在typesizes.h

#define __TIME_T_TYPE       __SYSCALL_SLONG_TYPE  
#if defined __x86_64__ && defined __ILP32__  
# define __SYSCALL_SLONG_TYPE   __SQUAD_TYPE  
#else
# define __SYSCALL_SLONG_TYPE   __SLONGWORD_TYPE
#endif  

4)再次在types.h

#define __SLONGWORD_TYPE    long int
#if __WORDSIZE == 32
# define __SQUAD_TYPE       __quad_t
#elif __WORDSIZE == 64
# define __SQUAD_TYPE       long int  

#if __WORDSIZE == 64
typedef long int __quad_t;  
#else
__extension__ typedef long long int __quad_t;

在Visual Studio 2008中，它默认为__int64，除非你定义了_USE_32BIT_TIME_T。最好是假装不知道它的定义，因为它可以(也将)在不同平台之间改变。

time_t维基百科的文章给出了一些解释。归根结底，在C规范中，time_t的类型是不保证的。

The time_t datatype is a data type in the ISO C library defined for storing system time values. Such values are returned from the standard time() library function. This type is a typedef defined in the standard header. ISO C defines time_t as an arithmetic type, but does not specify any particular type, range, resolution, or encoding for it. Also unspecified are the meanings of arithmetic operations applied to time values. Unix and POSIX-compliant systems implement the time_t type as a signed integer (typically 32 or 64 bits wide) which represents the number of seconds since the start of the Unix epoch: midnight UTC of January 1, 1970 (not counting leap seconds). Some systems correctly handle negative time values, while others do not. Systems using a 32-bit time_t type are susceptible to the Year 2038 problem.

[根]#猫时间

#include <time.h>

int main(int argc, char** argv)
{
        time_t test;
        return 0;
}

[root]# gcc -E time.c | grep __time_t .c

类型定义long int __time_t;

它在$INCDIR/bits/types.h中定义:

# 131 "/usr/include/bits/types.h" 3 4
# 1 "/usr/include/bits/typesizes.h" 1 3 4
# 132 "/usr/include/bits/types.h" 2 3 4