How to generate a random int in C

Question

Is there a function to generate a random int number in C   Or will I have to use a third party library

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You can use the concept of a dangling pointer  A pointer pointing to a memory location that has been deleted  or freed  is called dangling pointer  It will show random values while printing

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The rand   function in  lt stdlib h gt  returns a  pseudo-random integer between 0 and RAND MAX  You can use srand unsigned int seed  to set a seed   It s common practice to use the   operator in conjunction with rand   to get a different range  though bear in mind that this throws off the uniformity somewhat   For example      random int between 0 and 19    int r   rand     20    If you really care about uniformity you can do something like this      Returns an integer in the range  0  n         Uses rand    and so is affected-by affects the same seed      int randint int n      if   n - 1     RAND MAX        return rand        else          Supporting larger values for n would requires an even more        elaborate implementation that combines multiple calls to rand       assert  n  lt   RAND MAX          Chop off all of the values that would cause skew        int end   RAND MAX   n     truncate skew     assert  end  gt  0       end    n              and ignore results from rand   that fall above that limit          Worst case the loop condition should succeed 50  of the time         so we can expect to bail out of this loop pretty quickly       int r      while   r   rand     gt   end        return r   n

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STL doesn t exist for C  You have to call rand  or better yet  random  These are declared in the standard library header stdlib h  rand is POSIX  random is a BSD spec function   The difference between rand and random is that random returns a much more usable 32-bit random number  and rand typically returns a 16-bit number  The BSD manpages show that the lower bits of rand are cyclic and predictable  so rand is potentially useless for small numbers

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This is a good way to get a random number between two numbers of your choice    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt        define randnum min  max              rand      int    max    1  -  min       min    int main         srand time NULL         printf   d n   randnum 1  70        Output the first time  39  Output the second time  61  Output the third time  65  You can change the values after randnum to whatever numbers you choose  and it will generate a random number for you between those two numbers

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include  lt stdio h gt   include  lt dos h gt   int random int range    int main void        printf   d   random 10        return 0     int random int range        struct time t      int r       gettime  amp t       r   t ti sec   range      return r

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Well  STL is C    not C  so I don t know what you want  If you want C  however  there is the rand   and srand   functions   int rand void    void srand unsigned seed     These are both part of ANSI C  There is also the random   function   long random void     But as far as I can tell  random   is not standard ANSI C  A third-party library may not be a bad idea  but it all depends on how random of a number you really need to generate

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The standard C function is rand     It s good enough to deal cards for solitaire  but it s awful   Many implementations of rand   cycle through a short list of numbers  and the low bits have shorter cycles   The way that some programs call rand   is awful  and calculating a good seed to pass to srand   is hard   The best way to generate random numbers in C is to use a third-party library like OpenSSL   For example    include  lt stdint h gt   include  lt stdio h gt   include  lt stdlib h gt   include  lt openssl rand h gt      Random integer in  0  limit     unsigned int random uint unsigned int limit        union           unsigned int i          unsigned char c sizeof unsigned int          u       do           if   RAND bytes u c  sizeof u c                  fprintf stderr   Can t get random bytes  n                exit 1                   while  u i  lt   -limit   limit       u i  lt   2  size   limit         return u i   limit        Random double in  0 0  1 0     double random double         union           uint64 t i          unsigned char c sizeof uint64 t          u       if   RAND bytes u c  sizeof u c              fprintf stderr   Can t get random bytes  n            exit 1                53 bits   2  53        return  u i  gt  gt  11     1 0 9007199254740992 0      int main         printf  Dice   d n    int  random uint 6    1        printf  Double   f n   random double         return 0      Why so much code   Other languages like Java and Ruby have functions for random integers or floats   OpenSSL only gives random bytes  so I try to mimic how Java or Ruby would transform them into integers or floats   For integers  we want to avoid modulo bias   Suppose that we got some random 4 digit integers from rand     10000  but rand   can only return 0 to 32767  as it does in Microsoft Windows    Each number from 0 to 2767 would appear more often than each number from 2768 to 9999   To remove the bias  we can retry rand   while the value is below 2768  because the 30000 values from 2768 to 32767 map uniformly onto the 10000 values from 0 to 9999   For floats  we want 53 random bits  because a double holds 53 bits of precision  assuming it s an IEEE double    If we use more than 53 bits  we get rounding bias   Some programmers write code like rand      double RAND MAX  but rand   might return only 31 bits  or only 15 bits in Windows   OpenSSL s RAND bytes   seeds itself  perhaps by reading  dev urandom in Linux   If we need many random numbers  it would be too slow to read them all from  dev urandom  because they must be copied from the kernel   It is faster to allow OpenSSL to generate more random numbers from a seed   More about random numbers    Perl s Perl seed   is an example of how to calculate a seed in C for srand     It mixes bits from the current time  the process ID  and some pointers  if it can t read  dev urandom  OpenBSD s arc4random uniform   explains modulo bias  Java API for java util Random describes algorithms for removing bias from random integers  and packing 53 bits into random floats

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rand   is the most convenient way to generate random numbers   You may also catch random number from any online service like random org

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On modern x86 64 CPUs you can use the hardware random number generator via  rdrand64 step    Example code    include  lt immintrin h gt   uint64 t randVal  if   rdrand64 step  amp randVal          Report an error here  random number generation has failed       If no error occured  randVal contains a random 64-bit number

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Lets go through this  First we use the srand   function to seed the randomizer  Basically  the computer can generate random numbers based on the number that is fed to srand    If you gave the same seed value  then the same random numbers would be generated every time  Therefore  we have to seed the randomizer with a value that is always changing  We do this by feeding it the value of the current time with the time   function  Now  when we call rand    a new random number will be produced every time       include  lt stdio h gt       int random number int min num  int max num        int main void                printf  quot Min   1 Max   40  d n quot   random number 1 40            printf  quot Min   100 Max   1000  d n quot  random number 100 1000            return 0             int random number int min num  int max num                int result   0  low num   0  hi num   0           if  min num  lt  max num                        low num   min num              hi num   max num   1     include max num in output           else               low num   max num   1     include max num in output             hi num   min num                     srand time NULL            result    rand      hi num - low num     low num          return result

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For Linux C applications   This is my reworked code from an answer above that follows my C code practices and returns a random buffer of any size  with proper return codes  etc     Make sure to call urandom open   once at the beginning of your program   int gUrandomFd   -1   int urandom open void        if  gUrandomFd    -1            gUrandomFd   open   dev urandom   O RDONLY              if  gUrandomFd    -1            fprintf stderr   Error opening  dev urandom  errno   d   strerrer   s  n                     errno  strerror errno            return -1        else           return 0            void urandom close void        close gUrandomFd       gUrandomFd   -1            This link essentially validates the merits of  dev urandom     http   sockpuppet org blog 2014 02 25 safely-generate-random-numbers     int getRandomBuffer uint8 t  buf  int size        int ret   0     Return value      if  gUrandomFd    -1            fprintf stderr   Urandom   dev urandom  file not open n            return -1             ret   read gUrandomFd  buf  size        if  ret    size            fprintf stderr   Only read   d  bytes  expected   d  n                    ret  size           return -1        else           return 0

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You want to use rand     Note  VERY IMPORTANT   make sure to set the seed for the rand function   If you do not  your random numbers are not truly random   This is very  very  very important   Thankfully  you can usually use some combination of the system ticks timer and the date to get a good seed

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Hearing a good explanation of why using rand   to produce uniformly distributed random numbers in a given range is a bad idea  I decided to take a look at how skewed the output actually is  My test case was fair dice throwing  Here s the C code    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   int main int argc  char  argv          int i      int dice 6        for  i   0  i  lt  6  i           dice i    0      srand time NULL         const int TOTAL   10000000      for  i   0  i  lt  TOTAL  i          dice  rand     6      1       double pers   0 0  tpers   0 0      for  i   0  i  lt  6  i            pers    dice i    100 0    TOTAL        printf   t 1d   5 2f   n   dice i   pers         tpers    pers            printf   ttotal    6 2f   n   tpers       and here s its output      gcc -o t3 t3 c      t3          1666598  16 67               1668630  16 69          1667682  16 68          1666049  16 66          1665948  16 66          1665093  16 65          total   100 00       t3              1667634  16 68          1665914  16 66          1665542  16 66          1667828  16 68          1663649  16 64          1669433  16 69          total   100 00    I don t know how uniform you need your random numbers to be  but the above appears uniform enough for most needs   Edit  it would be a good idea to initialize the PRNG with something better than time NULL

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My minimalistic solution should work for random numbers in range  min  max   Use srand time NULL   before invoking the function   int range rand int min num  int max num        if  min num  gt   max num            fprintf stderr   min num is greater or equal than max num  n               return min num    rand      max num - min num

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FWIW  the answer is that yes  there is a stdlib h function called rand  this function is tuned primarily for speed and distribution  not for unpredictability  Almost all built-in random functions for various languages and frameworks use this function by default  There are also  cryptographic  random number generators that are much less predictable  but run much slower  These should be used in any sort of security-related application

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If you need secure random characters or integers   As addressed in how to safely generate random numbers in various programming languages  you ll want to do one of the following    Use libsodium s randombytes API Re-implement what you need from libsodium s sysrandom implementation yourself  very carefully More broadly  use  dev urandom  not  dev random  Not OpenSSL  or other userspace PRNGs     For example    include  sodium h   int foo         char myString 32       uint32 t myInt       if  sodium init    lt  0               panic  the library couldn t be initialized  it is not safe to use            return 1                  myString will be an array of 32 random bytes  not null-terminated                randombytes buf myString  32           myInt will be a random number between 0 and 9        myInt   randombytes uniform 10       randombytes uniform   is cryptographically secure and unbiased

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include lt stdio h gt   include lt stdlib h gt   include lt time h gt     generate number in range  min max  int random int min  int max       int number   min   rand      max - min       return number        Driver code int main        srand time NULL        for int i   1  i  lt   10  i             printf   d t   random 10  100              return 0

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Despite all the people suggestion rand   here  you don t want to use rand   unless you have to  The random numbers that rand   produces are often very bad  To quote from the Linux man page      The versions of rand   and srand   in the Linux C Library use the same random number generator as random 3  and srandom 3   so the lower-order bits should be as random as the higher-order bits  However  on older rand   implementations  and on current implementations on different systems  the lower-order bits are much less random than the higher-order bits  Do not use this function in applications intended to be portable when good randomness is needed   Use random 3  instead      Regarding portability  random   is also defined by the POSIX standard for quite some time now  rand   is older  it appeared already in the first POSIX 1 spec  IEEE Std 1003 1-1988   whereas random   first appeared in POSIX 1-2001  IEEE Std 1003 1-2001   yet the current POSIX standard is already POSIX 1-2008  IEEE Std 1003 1-2008   which received an update just a year ago  IEEE Std 1003 1-2008  2016 Edition   So I would consider random   to be very portable   POSIX 1-2001 also introduced the lrand48   and mrand48   functions  see here       This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic    And pretty good pseudo random source is the arc4random   function that is available on many systems  Not part of any official standard  appeared in BSD around 1997 but you can find it on systems like Linux and macOS iOS

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Note  Don t use rand   for security  If you need a cryptographically secure number  see this answer instead    include  lt time h gt   include  lt stdlib h gt   srand time NULL         Initialization  should only be called once  int r   rand            Returns a pseudo-random integer between 0 and RAND MAX   On Linux  you might prefer to use random and srandom

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This is hopefully a bit more random than just using srand time NULL      include  lt time h gt   include  lt stdio h gt   include  lt stdlib h gt   int main int argc  char   argv        srand  unsigned int   main    unsigned int  amp argc    unsigned int time NULL        srand rand          for  int i   0  i  lt  10  i            printf   d n   rand

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If you need better quality pseudo random numbers than what stdlib provides  check out Mersenne Twister  It s faster  too  Sample implementations are plentiful  for example here

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I had a serious issue with pseudo random number generator in my recent application  I repeatidly called my C program via a pyhton script and I was using as seed the following code   srand time NULL     However  since    rand will generate the same pseudo random sequence give the same seed in srand  see man srand   As already stated  time function changes only second from second  if your application is run multiple times within the same second  time will return the same value each time    My program generated the same sequence of numbers  You can do 3 things to solve this problem    mix time output with some other information changing on runs  in my application  the output name    srand time NULL    getHashOfString outputName     I used djb2 as my hash function  Increase time resolution  On my platform  clock gettime was available  so I use it    include lt time h gt  struct timespec nanos  clock gettime CLOCK MONOTONIC   amp nanos  srand nanos tv nsec    Use both methods together    include lt time h gt  struct timespec nanos  clock gettime CLOCK MONOTONIC   amp nanos  srand nanos tv nsec   getHashOfString outputName       Option 3 ensures you  as far as i know  the best seed randomity  but it may create a difference only on very fast application  In my opinion option 2 is a safe bet

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include  lt stdio h gt   include  lt stdlib h gt   void main          int visited 100       int randValue  a  b  vindex   0       randValue    rand     100    1       while  vindex  lt  100            for  b   0  b  lt  vindex  b                  if  visited b     randValue                    randValue    rand     100    1                  b   0                                   visited vindex      randValue             for  a   0  a  lt  100  a            printf   d    visited a

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Try this  I put it together from some of the concepts already referenced above          Uses the srand   function to seed the random number generator based on time value  then returns an integer in the range 1 to max  Call this with random n  where n is an integer  and you get an integer as a return value       int random int max        srand  unsigned  time NULL        return  rand     max    1

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If your system supports the arc4random family of functions I would recommend using those instead the standard rand function  The arc4random family includes  uint32 t arc4random void  void arc4random buf void  buf  size t bytes  uint32 t arc4random uniform uint32 t limit  void arc4random stir void  void arc4random addrandom unsigned char  dat  int datlen   arc4random returns a random 32-bit unsigned integer  arc4random buf puts random content in it s parameter buf   void    The amount of content is determined by the bytes   size t parameter  arc4random uniform returns a random 32-bit unsigned integer which follows the rule  0  lt   arc4random uniform limit   lt  limit  where limit is also an unsigned 32-bit integer  arc4random stir reads data from  dev urandom and passes the data to arc4random addrandom to additionally randomize it s internal random number pool  arc4random addrandom is used by arc4random stir to populate it s internal random number pool according to the data passed to it  If you do not have these functions  but you are on Unix  then you can use this code     This is C  not C       include  lt sys types h gt   include  lt sys stat h gt   include  lt fcntl h gt   include  lt errno h gt   include  lt unistd h gt   include  lt stdlib h gt     exit     include  lt stdio h gt     printf     int urandom fd   -2   void urandom init       urandom fd   open  quot  dev urandom quot   O RDONLY      if  urandom fd    -1        int errsv   urandom fd      printf  quot Error opening   dev urandom    i n quot   errsv       exit 1          unsigned long urandom       unsigned long buf impl    unsigned long  buf    amp buf impl     if  urandom fd    -2        urandom init              Read sizeof long  bytes  usually 8  into  buf  which points to buf impl      read urandom fd  buf  sizeof long      return buf impl     The urandom init function opens the  dev urandom device  and puts the file descriptor in urandom fd  The urandom function is basically the same as a call to rand  except more secure  and it returns a long  easily changeable   However   dev urandom can be a little slow  so it is recommended that you use it as a seed for a different random number generator  If your system does not have a  dev urandom  but does have a  dev random or similar file  then you can simply change the path passed to open in urandom init  The calls and APIs used in urandom init and urandom are  I believe  POSIX-compliant  and as such  should work on most  if not all POSIX compliant systems  Notes  A read from  dev urandom will NOT block if there is insufficient entropy available  so values generated under such circumstances may be cryptographically insecure  If you are worried about that  then use  dev random  which will always block  if there is insufficient entropy  If you are on another system i e  Windows   then use rand or some internal Windows specific platform-dependent non-portable API  Wrapper function for urandom  rand  or arc4random calls   define RAND IMPL    urandom see large code block    rand   arc4random     int myRandom int bottom  int top       return  RAND IMPL      top - bottom     bottom

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C Program to generate random number between 9 and 50   include  lt time h gt   include  lt stdlib h gt   int main         srand time NULL        int lowerLimit   10  upperLimit   50      int r    lowerLimit   rand      upperLimit - lowerLimit       printf   d   r       In general we can generate a random number between lowerLimit  and upperLimit-1   i e lowerLimit is inclusive or say r     lowerLimit  upperLimit

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Have a look at ISAAC  Indirection  Shift  Accumulate  Add  and Count   Its uniformly distributed and has an average cycle length of 2 8295

[c] How to generate a random int in C?

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