What is time t ultimately a typedef to

Question

I searched my Linux box and saw this typedef   typedef   time t time t    But I could not find the   time t  definition

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It s a 32-bit signed integer type on most legacy platforms   However  that causes your code to suffer from the year 2038 bug   So modern C libraries should be defining it to be a signed 64-bit int instead  which is safe for a few billion years

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The time t Wikipedia article article sheds some light on this  The bottom line is that the type of time t is not guaranteed in the C specification      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

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root   cat time c   include  lt time h gt   int main int argc  char   argv            time t test          return 0       root   gcc -E time c   grep   time t  typedef long int   time t   It s defined in  INCDIR bits types h through     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

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Under Visual Studio 2008  it defaults to an   int64 unless you define  USE 32BIT TIME T   You re better off just pretending that you don t know what it s defined as  since it can  and will  change from platform to platform

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root   cat time c   include  lt time h gt   int main int argc  char   argv            time t test          return 0       root   gcc -E time c   grep   time t  typedef long int   time t   It s defined in  INCDIR bits types h through     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

User · Answer

It s a 32-bit signed integer type on most legacy platforms   However  that causes your code to suffer from the year 2038 bug   So modern C libraries should be defining it to be a signed 64-bit int instead  which is safe for a few billion years

User · Answer

What is ultimately a time t typedef to    Robust code does not care what the type is   C species time t to be a real type like double  long long  int64 t  int  etc      It even could be unsigned as the return values from many time function indicating error is not -1  but  time t  -1  - This implementation choice is uncommon   The point is that the  need-to-know  the type is rare   Code should be written to avoid the need     Yet a common  need-to-know  occurs when code wants to print the raw time t   Casting to the widest integer type will accommodate most modern cases   time t now   0  time  amp now   printf   jd    intmax t  now      or  printf   lld    long long  now     Casting to a double or long double will work too  yet could provide inexact decimal output  printf    16e    double  now

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Typically you will find these underlying implementation-specific typedefs for gcc in the bits or asm header directory   For me  it s  usr include x86 64-linux-gnu bits types h   You can just grep  or use a preprocessor invocation like that suggested by Quassnoi to see which specific header

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The answer is definitely implementation-specific  To find out definitively for your platform compiler  just add this output somewhere in your code   printf   sizeof time t is   d n   sizeof time t      If the answer is 4  32 bits  and your data is meant to go beyond 2038  then you have 25 years to migrate your code   Your data will be fine if you store your data as a string  even if it s something simple like   FILE  stream    stream file pointer that you ve opened correctly   fprintf  stream    d n    int time t     Then just read it back the same way  fread  fscanf  etc  into an int   and you have your epoch offset time  A similar workaround exists in  Net  I pass 64-bit epoch numbers between Win and Linux systems with no problem  over a communications channel   That brings up byte-ordering issues  but that s another subject   To answer paxdiablo s query  I d say that it printed  19100  because the program was written this way  and I admit I did this myself in the  80 s    time t now  struct tm local date time  now   time NULL      convert  then copy internal object to our object memcpy   amp local date time  localtime  amp now   sizeof local date time    printf   Year is  19 02d n   local date time tm year     The printf statement prints the fixed string  Year is  19  followed by a zero-padded string with the  years since 1900   definition of tm- gt tm year   In 2000  that value is 100  obviously    02d  pads with two zeros but does not truncate if longer than two digits   The correct way is  change to last line only    printf   Year is   d n   local date time tm year   1900     New question  What s the rationale for that thinking

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time t is of type long int on 64 bit machines  else it is long long int   You could verify this in these header files     time h   usr include types h and typesizes h   usr include x86 64-linux-gnu bits   The statements below are not one after another  They could be found in the resp  header file using Ctrl f search     1 In time h    typedef   time t time t    2 In types h      define   STD TYPE     typedef     STD TYPE   TIME T TYPE   time t      3 In typesizes h     define   TIME T TYPE         SYSCALL SLONG TYPE    if defined   x86 64    amp  amp  defined   ILP32       define   SYSCALL SLONG TYPE     SQUAD TYPE    else   define   SYSCALL SLONG TYPE     SLONGWORD TYPE  endif     4  Again in 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

User · Answer

It s a 32-bit signed integer type on most legacy platforms   However  that causes your code to suffer from the year 2038 bug   So modern C libraries should be defining it to be a signed 64-bit int instead  which is safe for a few billion years

User · Answer

The time t Wikipedia article article sheds some light on this  The bottom line is that the type of time t is not guaranteed in the C specification      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

User · Answer

What is ultimately a time t typedef to    Robust code does not care what the type is   C species time t to be a real type like double  long long  int64 t  int  etc      It even could be unsigned as the return values from many time function indicating error is not -1  but  time t  -1  - This implementation choice is uncommon   The point is that the  need-to-know  the type is rare   Code should be written to avoid the need     Yet a common  need-to-know  occurs when code wants to print the raw time t   Casting to the widest integer type will accommodate most modern cases   time t now   0  time  amp now   printf   jd    intmax t  now      or  printf   lld    long long  now     Casting to a double or long double will work too  yet could provide inexact decimal output  printf    16e    double  now

User · Answer

The answer is definitely implementation-specific  To find out definitively for your platform compiler  just add this output somewhere in your code   printf   sizeof time t is   d n   sizeof time t      If the answer is 4  32 bits  and your data is meant to go beyond 2038  then you have 25 years to migrate your code   Your data will be fine if you store your data as a string  even if it s something simple like   FILE  stream    stream file pointer that you ve opened correctly   fprintf  stream    d n    int time t     Then just read it back the same way  fread  fscanf  etc  into an int   and you have your epoch offset time  A similar workaround exists in  Net  I pass 64-bit epoch numbers between Win and Linux systems with no problem  over a communications channel   That brings up byte-ordering issues  but that s another subject   To answer paxdiablo s query  I d say that it printed  19100  because the program was written this way  and I admit I did this myself in the  80 s    time t now  struct tm local date time  now   time NULL      convert  then copy internal object to our object memcpy   amp local date time  localtime  amp now   sizeof local date time    printf   Year is  19 02d n   local date time tm year     The printf statement prints the fixed string  Year is  19  followed by a zero-padded string with the  years since 1900   definition of tm- gt tm year   In 2000  that value is 100  obviously    02d  pads with two zeros but does not truncate if longer than two digits   The correct way is  change to last line only    printf   Year is   d n   local date time tm year   1900     New question  What s the rationale for that thinking

User · Answer

Under Visual Studio 2008  it defaults to an   int64 unless you define  USE 32BIT TIME T   You re better off just pretending that you don t know what it s defined as  since it can  and will  change from platform to platform

User · Answer

time t is just typedef for 8 bytes  long long   int64  which all compilers and OS s understand  Back in the days  it used to be just for long int  4 bytes  but not now  If you look at the time t in crtdefs h you will find both implementations but the OS will use long long

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root   cat time c   include  lt time h gt   int main int argc  char   argv            time t test          return 0       root   gcc -E time c   grep   time t  typedef long int   time t   It s defined in  INCDIR bits types h through     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

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Standards  William Brendel quoted Wikipedia  but I prefer it from the horse s mouth   C99 N1256 standard draft 7 23 1 3  Components of time  says      The types declared are size t  described in 7 17  clock t and time t which are arithmetic types capable of representing times   and 6 2 5 18  Types  says      Integer and floating types are collectively called arithmetic types    POSIX 7 sys types h says       CX  time t shall be an integer type    where  CX  is defined as       CX  Extension to the ISO C standard    It is an extension because it makes a stronger guarantee  floating points are out   gcc one-liner  No need to create a file as mentioned by Quassnoi   echo   gcc -E -xc -include  time h  -   grep time t   On Ubuntu 15 10 GCC 5 2 the top two lines are   typedef long int   time t  typedef   time t time t    Command breakdown with some quotes from man gcc    -E   Stop after the preprocessing stage  do not run the compiler proper   -xc  Specify C language  since input comes from stdin which has no file extension  -include file   Process file as if   include  file   appeared as the first line of the primary source file   -  input from stdin

User · Answer

Standards  William Brendel quoted Wikipedia  but I prefer it from the horse s mouth   C99 N1256 standard draft 7 23 1 3  Components of time  says      The types declared are size t  described in 7 17  clock t and time t which are arithmetic types capable of representing times   and 6 2 5 18  Types  says      Integer and floating types are collectively called arithmetic types    POSIX 7 sys types h says       CX  time t shall be an integer type    where  CX  is defined as       CX  Extension to the ISO C standard    It is an extension because it makes a stronger guarantee  floating points are out   gcc one-liner  No need to create a file as mentioned by Quassnoi   echo   gcc -E -xc -include  time h  -   grep time t   On Ubuntu 15 10 GCC 5 2 the top two lines are   typedef long int   time t  typedef   time t time t    Command breakdown with some quotes from man gcc    -E   Stop after the preprocessing stage  do not run the compiler proper   -xc  Specify C language  since input comes from stdin which has no file extension  -include file   Process file as if   include  file   appeared as the first line of the primary source file   -  input from stdin

User · Answer

It s a 32-bit signed integer type on most legacy platforms   However  that causes your code to suffer from the year 2038 bug   So modern C libraries should be defining it to be a signed 64-bit int instead  which is safe for a few billion years

User · Answer

time t is just typedef for 8 bytes  long long   int64  which all compilers and OS s understand  Back in the days  it used to be just for long int  4 bytes  but not now  If you look at the time t in crtdefs h you will find both implementations but the OS will use long long

User · Answer

root   cat time c   include  lt time h gt   int main int argc  char   argv            time t test          return 0       root   gcc -E time c   grep   time t  typedef long int   time t   It s defined in  INCDIR bits types h through     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

User · Answer

Under Visual Studio 2008  it defaults to an   int64 unless you define  USE 32BIT TIME T   You re better off just pretending that you don t know what it s defined as  since it can  and will  change from platform to platform

User · Answer

Under Visual Studio 2008  it defaults to an   int64 unless you define  USE 32BIT TIME T   You re better off just pretending that you don t know what it s defined as  since it can  and will  change from platform to platform

User · Answer

The time t Wikipedia article article sheds some light on this  The bottom line is that the type of time t is not guaranteed in the C specification      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

User · Answer

time t is of type long int on 64 bit machines  else it is long long int   You could verify this in these header files     time h   usr include types h and typesizes h   usr include x86 64-linux-gnu bits   The statements below are not one after another  They could be found in the resp  header file using Ctrl f search     1 In time h    typedef   time t time t    2 In types h      define   STD TYPE     typedef     STD TYPE   TIME T TYPE   time t      3 In typesizes h     define   TIME T TYPE         SYSCALL SLONG TYPE    if defined   x86 64    amp  amp  defined   ILP32       define   SYSCALL SLONG TYPE     SQUAD TYPE    else   define   SYSCALL SLONG TYPE     SLONGWORD TYPE  endif     4  Again in 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

User · Answer

The time t Wikipedia article article sheds some light on this  The bottom line is that the type of time t is not guaranteed in the C specification      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

User · Answer

Typically you will find these underlying implementation-specific typedefs for gcc in the bits or asm header directory   For me  it s  usr include x86 64-linux-gnu bits types h   You can just grep  or use a preprocessor invocation like that suggested by Quassnoi to see which specific header

[c] What is time_t ultimately a typedef to?

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