Where is the itoa function in Linux

Question

itoa   is a really handy function to convert a number to a string  Linux does not seem to have itoa    is there an equivalent function or do I have to use sprintf str    d   num

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If you are calling it a lot  the advice of  just use snprintf  can be annoying  So here s what you probably want   const char  my itoa buf char  buf  size t len  int num      static char loc buf sizeof int    CHAR BITS      not thread safe       if   buf          buf   loc buf      len   sizeof loc buf          if  snprintf buf  len    d   num     -1      return        or whatever       return buf     const char  my itoa int num    return my itoa buf NULL  0  num

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I have used  itoa      on RedHat 6 and GCC compiler  It works

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You can use this program instead of sprintf   void itochar int x  char  buffer  int radix    int main         char buffer 10       itochar 725  buffer  10       printf    n  s  n   buffer       return 0     void itochar int x  char  buffer  int radix        int i   0   n s      n   s      while  n  gt  0                s   n radix          n   n radix          buffer i       0    s            buffer i      0       strrev buffer

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Reading the code of guys who do it for a living will get you a LONG WAY   Check out how guys from MySQL did it  The source is VERY WELL COMMENTED and will teach you much more than hacked up solutions found all over the place   MySQL s implementation of int2str  I provide the mentioned implementation here  the link is here for reference and should be used to read the full implementation   char   int2str long int val  char  dst  int radix           int upcase      char buffer 65     char  p    long int new val    char  dig vec  upcase    dig vec upper    dig vec lower    ulong uval   ulong  val     if  radix  lt  0          if  radix  lt  -36    radix  gt  -2        return NullS      if  val  lt  0               dst      -            Avoid integer overflow in  -val  for LLONG MIN  BUG 31799            uval    ulong 0 - uval            radix   -radix        else if  radix  gt  36    radix  lt  2      return NullS            The slightly contorted code which follows is due to the fact that     few machines directly support unsigned long   and     Certainly     the VAX C compiler generates a subroutine call   In the interests     of efficiency  hollow laugh  I let this happen for the first digit     only  after that  val  will be in range so that signed integer     division will do   Sorry  bout that   CHECK THE CODE PRODUCED BY     YOUR C COMPILER   The first   and   should be unsigned  the second       and   signed  but C compilers tend to be extraordinarily     sensitive to minor details of style   This works on a VAX  that s     all I claim for it         p    amp buffer sizeof buffer -1      p     0     new val  uval    ulong  radix     --p   dig vec  uchar   uval-  ulong  new val  ulong  radix      val   new val    while  val    0          ldiv t res      res ldiv val radix        --p   dig vec res rem       val  res quot        while    dst      p       0      return dst-1

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direct copy to buffer   64 bit integer itoa hex        char  itoah long num  char  s  int len                    long n  m   16              int i   16 2              int shift    a -   9  1                 if  s    len  lt  1                      return 0               n   num  lt  0   -1   1              n   n   num               len   len  gt  i   i   len              i   len  lt  i   len   i               s i-1    0              i--               if  num                                    if len  lt  2                              return  amp s i                        s i-1   0                       return  amp s i-1                              while i  amp  amp  n                                    s i-1    n   m    0                        if  s i-1   gt   9                               s i-1     shift                        n   n m                      i--                             if num  lt  0                                    if i                                                    s i-1     -                               i--                                                   return  amp s i           note  change long to long long for 32 bit machine  long to int in case for 32 bit integer  m is the radix  When decreasing radix  increase number of characters  variable i   When increasing radix  decrease number of characters  better   In case of unsigned data type  i just becomes 16   1

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EDIT  Sorry  I should have remembered that this machine is decidedly non-standard  having plugged in various non-standard libc implementations for academic purposes  -   As itoa   is indeed non-standard  as mentioned by several helpful commenters  it is best to use sprintf target string   d  source int  or  better yet  because it s safe from buffer overflows  snprintf target string  size of target string in bytes    d   source int    I know it s not quite as concise or cool as itoa    but at least you can Write Once  Run Everywhere  tm   -   Here s the old  edited  answer  You are correct in stating that the default gcc libc does not include itoa    like several other platforms  due to it not technically being a part of the standard   See here for a little more info   Note that you have to    include  lt stdlib h gt    Of course you already know this  because you wanted to use itoa   on Linux after presumably using it on another platform  but    the code  stolen from the link above  would look like   Example     itoa example     include  lt stdio h gt   include  lt stdlib h gt   int main        int i    char buffer  33     printf   Enter a number        scanf    d   amp i     itoa  i buffer 10     printf   decimal   s n  buffer     itoa  i buffer 16     printf   hexadecimal   s n  buffer     itoa  i buffer 2     printf   binary   s n  buffer     return 0      Output    Enter a number  1750 decimal  1750 hexadecimal  6d6 binary  11011010110    Hope this helps

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If you are calling it a lot  the advice of  just use snprintf  can be annoying  So here s what you probably want   const char  my itoa buf char  buf  size t len  int num      static char loc buf sizeof int    CHAR BITS      not thread safe       if   buf          buf   loc buf      len   sizeof loc buf          if  snprintf buf  len    d   num     -1      return        or whatever       return buf     const char  my itoa int num    return my itoa buf NULL  0  num

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Following function allocates just enough memory to keep string representation of the given number and then writes the string representation into this area using standard sprintf method   char  itoa long n        int len   n  0   1   floor log10l labs n    1      if  n lt 0  len       room for negative sign  -       char     buf   calloc sizeof char   len 1       1 for null     snprintf buf  len 1    ld   n       return   buf      Don t forget to free up allocated memory when out of need   char  num str   itoa 123456789L           free num str     N B  As snprintf copies n-1 bytes  we have to call snprintf buf  len 1    ld   n   not just snprintf buf  len    ld   n

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Edit  I just found out about std  to string which is identical in operation to my own function below  It was introduced in C  11 and is available in recent versions of gcc  at least as early as 4 5 if you enable the c  0x extensions   Not only is itoa missing from gcc  it s not the handiest function to use since you need to feed it a buffer  I needed something that could be used in an expression so I came up with this   std  string itos int n       const int max size   std  numeric limits lt int gt   digits10   1   sign     1   0-terminator       char buffer max size     0      sprintf buffer    d   n      return std  string buffer       Ordinarily it would be safer to use snprintf instead of sprintf but the buffer is carefully sized to be immune to overrun   See an example  http   ideone com mKmZVE

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If you just want to print them   void binary unsigned int n        for int shift sizeof int  8-1 shift gt  0 shift--               if  n  gt  gt  shift  amp  1           printf  1           else          printf  0               printf   n

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EDIT  Sorry  I should have remembered that this machine is decidedly non-standard  having plugged in various non-standard libc implementations for academic purposes  -   As itoa   is indeed non-standard  as mentioned by several helpful commenters  it is best to use sprintf target string   d  source int  or  better yet  because it s safe from buffer overflows  snprintf target string  size of target string in bytes    d   source int    I know it s not quite as concise or cool as itoa    but at least you can Write Once  Run Everywhere  tm   -   Here s the old  edited  answer  You are correct in stating that the default gcc libc does not include itoa    like several other platforms  due to it not technically being a part of the standard   See here for a little more info   Note that you have to    include  lt stdlib h gt    Of course you already know this  because you wanted to use itoa   on Linux after presumably using it on another platform  but    the code  stolen from the link above  would look like   Example     itoa example     include  lt stdio h gt   include  lt stdlib h gt   int main        int i    char buffer  33     printf   Enter a number        scanf    d   amp i     itoa  i buffer 10     printf   decimal   s n  buffer     itoa  i buffer 16     printf   hexadecimal   s n  buffer     itoa  i buffer 2     printf   binary   s n  buffer     return 0      Output    Enter a number  1750 decimal  1750 hexadecimal  6d6 binary  11011010110    Hope this helps

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If you are calling it a lot  the advice of  just use snprintf  can be annoying  So here s what you probably want   const char  my itoa buf char  buf  size t len  int num      static char loc buf sizeof int    CHAR BITS      not thread safe       if   buf          buf   loc buf      len   sizeof loc buf          if  snprintf buf  len    d   num     -1      return        or whatever       return buf     const char  my itoa int num    return my itoa buf NULL  0  num

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As Matt J wrote  there is itoa  but it s not standard  Your code will be more portable if you use snprintf

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i tried my own implementation of itoa    it seem s work in binary  octal  decimal and hex   define INT LEN  10   define HEX LEN  8   define BIN LEN  32   define OCT LEN  11   static char    my itoa   int value  char   str  int base         int i n  2 tmp      char buf BIN LEN 1         switch base                case 16              for i   0 i lt HEX LEN   i                                if value base gt 0                                        n                                                snprintf str  n    x   value               break          case 10              for i   0 i lt INT LEN   i                                if value base gt 0                                        n                                                snprintf str  n    d   value               break          case 8              for i   0 i lt OCT LEN   i                                if value base gt 0                                        n                                                snprintf str  n    o   value               break          case 2              for i   0 tmp   value i lt BIN LEN   i                                if tmp base gt 0                                        n                                      tmp  base                            for i   1  tmp   value  i lt n   i                                if tmp 2    0                                        buf n-i-1    1                                     else                                       buf n-i-1    0                                     tmp  base                            buf n-1      0               strcpy str buf               break          default              return NULL            return str

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EDIT  Sorry  I should have remembered that this machine is decidedly non-standard  having plugged in various non-standard libc implementations for academic purposes  -   As itoa   is indeed non-standard  as mentioned by several helpful commenters  it is best to use sprintf target string   d  source int  or  better yet  because it s safe from buffer overflows  snprintf target string  size of target string in bytes    d   source int    I know it s not quite as concise or cool as itoa    but at least you can Write Once  Run Everywhere  tm   -   Here s the old  edited  answer  You are correct in stating that the default gcc libc does not include itoa    like several other platforms  due to it not technically being a part of the standard   See here for a little more info   Note that you have to    include  lt stdlib h gt    Of course you already know this  because you wanted to use itoa   on Linux after presumably using it on another platform  but    the code  stolen from the link above  would look like   Example     itoa example     include  lt stdio h gt   include  lt stdlib h gt   int main        int i    char buffer  33     printf   Enter a number        scanf    d   amp i     itoa  i buffer 10     printf   decimal   s n  buffer     itoa  i buffer 16     printf   hexadecimal   s n  buffer     itoa  i buffer 2     printf   binary   s n  buffer     return 0      Output    Enter a number  1750 decimal  1750 hexadecimal  6d6 binary  11011010110    Hope this helps

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glibc internal implementation  glibc 2 28 has an internal implementation    stdio-common  itoa c sysdeps generic  itoa h   which is used in several places internally  but I could not find if it can be exposed or how   At least that should be a robust implementation if you are willing to extract it   This question asks how to roll your own  How to convert an int to string in C

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Following function allocates just enough memory to keep string representation of the given number and then writes the string representation into this area using standard sprintf method   char  itoa long n        int len   n  0   1   floor log10l labs n    1      if  n lt 0  len       room for negative sign  -       char     buf   calloc sizeof char   len 1       1 for null     snprintf buf  len 1    ld   n       return   buf      Don t forget to free up allocated memory when out of need   char  num str   itoa 123456789L           free num str     N B  As snprintf copies n-1 bytes  we have to call snprintf buf  len 1    ld   n   not just snprintf buf  len    ld   n

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glibc internal implementation  glibc 2 28 has an internal implementation    stdio-common  itoa c sysdeps generic  itoa h   which is used in several places internally  but I could not find if it can be exposed or how   At least that should be a robust implementation if you are willing to extract it   This question asks how to roll your own  How to convert an int to string in C

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As Matt J wrote  there is itoa  but it s not standard  Your code will be more portable if you use snprintf

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itoa is not a standard C function  You can implement your own  It appeared in the first edition of Kernighan and Ritchie s The C Programming Language  on page 60  The second edition of The C Programming Language   K amp R2   contains the following implementation of itoa  on page 64  The book notes several issues with this implementation  including the fact that it does not correctly handle the most negative number        itoa   convert n to characters in s     void itoa int n  char s            int i  sign        if   sign   n   lt  0      record sign             n   -n              make n positive         i   0       do            generate digits in reverse order             s i      n   10    0        get next digit           while   n    10   gt  0          delete it         if  sign  lt  0           s i       -        s i      0        reverse s         The function reverse used above is implemented two pages earlier     include  lt string h gt       reverse   reverse string s in place     void reverse char s            int i  j       char c        for  i   0  j   strlen s -1  i lt j  i    j--             c   s i            s i    s j            s j    c

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EDIT  Sorry  I should have remembered that this machine is decidedly non-standard  having plugged in various non-standard libc implementations for academic purposes  -   As itoa   is indeed non-standard  as mentioned by several helpful commenters  it is best to use sprintf target string   d  source int  or  better yet  because it s safe from buffer overflows  snprintf target string  size of target string in bytes    d   source int    I know it s not quite as concise or cool as itoa    but at least you can Write Once  Run Everywhere  tm   -   Here s the old  edited  answer  You are correct in stating that the default gcc libc does not include itoa    like several other platforms  due to it not technically being a part of the standard   See here for a little more info   Note that you have to    include  lt stdlib h gt    Of course you already know this  because you wanted to use itoa   on Linux after presumably using it on another platform  but    the code  stolen from the link above  would look like   Example     itoa example     include  lt stdio h gt   include  lt stdlib h gt   int main        int i    char buffer  33     printf   Enter a number        scanf    d   amp i     itoa  i buffer 10     printf   decimal   s n  buffer     itoa  i buffer 16     printf   hexadecimal   s n  buffer     itoa  i buffer 2     printf   binary   s n  buffer     return 0      Output    Enter a number  1750 decimal  1750 hexadecimal  6d6 binary  11011010110    Hope this helps

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Reading the code of guys who do it for a living will get you a LONG WAY   Check out how guys from MySQL did it  The source is VERY WELL COMMENTED and will teach you much more than hacked up solutions found all over the place   MySQL s implementation of int2str  I provide the mentioned implementation here  the link is here for reference and should be used to read the full implementation   char   int2str long int val  char  dst  int radix           int upcase      char buffer 65     char  p    long int new val    char  dig vec  upcase    dig vec upper    dig vec lower    ulong uval   ulong  val     if  radix  lt  0          if  radix  lt  -36    radix  gt  -2        return NullS      if  val  lt  0               dst      -            Avoid integer overflow in  -val  for LLONG MIN  BUG 31799            uval    ulong 0 - uval            radix   -radix        else if  radix  gt  36    radix  lt  2      return NullS            The slightly contorted code which follows is due to the fact that     few machines directly support unsigned long   and     Certainly     the VAX C compiler generates a subroutine call   In the interests     of efficiency  hollow laugh  I let this happen for the first digit     only  after that  val  will be in range so that signed integer     division will do   Sorry  bout that   CHECK THE CODE PRODUCED BY     YOUR C COMPILER   The first   and   should be unsigned  the second       and   signed  but C compilers tend to be extraordinarily     sensitive to minor details of style   This works on a VAX  that s     all I claim for it         p    amp buffer sizeof buffer -1      p     0     new val  uval    ulong  radix     --p   dig vec  uchar   uval-  ulong  new val  ulong  radix      val   new val    while  val    0          ldiv t res      res ldiv val radix        --p   dig vec res rem       val  res quot        while    dst      p       0      return dst-1

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I would prefer this  https   github com wsq003 itoa for linux  It should be the fastest itoa   ever  We use itoa   instead of sprintf   for performance reason  so a fastest itoa   with limited feature is reasonable and worthwhile

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The replacement with snprintf is NOT complete   It covers only bases  2  8  10  16  whereas itoa works for bases between 2 and 36   Since I was searching a replacement for base 32  I guess I ll have to code my own

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Here is a much improved version of Archana s solution  It works for any radix 1-16  and numbers  lt   0  and it shouldn t clobber memory    static char  numberSystem      0123456789ABCDEF   static char  twosComp      FEDCBA9876543210    static void safestrrev char  buffer  const int bufferSize  const int strlen        int len   strlen      if  len  gt  bufferSize                len   bufferSize            for  int index   0  index  lt   len   2   index                  char ch   buffer index           buffer index    buffer len - index - 1           buffer len - index - 1    ch           static int negateBuffer char  buffer  const int bufferSize  const int strlen  const int radix        int len   strlen      if  len  gt  bufferSize                len   bufferSize            if  radix    10                if  len  lt   bufferSize - 1                         buffer len       -               buffer len      0                       else               int twosCompIndex   0          for  int index   0  index  lt  len  index                          if   buffer index   gt    0    amp  amp   buffer index   lt    9                                  twosCompIndex   buffer index  -  0                             else if   buffer index   gt    A    amp  amp   buffer index   lt    F                                  twosCompIndex   buffer index  -  A    10                            else if   buffer index   gt    a    amp  amp   buffer index   lt    f                                  twosCompIndex   buffer index  -  a    10                            twosCompIndex     16 - radix               buffer index     twosComp twosCompIndex                     if  len  lt   bufferSize - 1                         buffer len       numberSystem radix - 1               buffer len    0                      return len     static int twosNegation const int x  const int radix        int n   x      if  x  lt  0                if  radix    10                        n   -x                    else                       n    x                      return n     static char  safeitoa const int x  char  buffer  const int bufferSize  const int radix        int strlen   0      int n   twosNegation x  radix       int nuberSystemIndex   0       if  radix  lt   16                do                       if  strlen  lt   bufferSize - 1                                 nuberSystemIndex    n   radix                   buffer strlen       numberSystem nuberSystemIndex                   buffer strlen      0                   n   n   radix                            else                               break                          while  n    0           if  x  lt  0                        strlen   negateBuffer buffer  bufferSize  strlen  radix                     safestrrev buffer  bufferSize  strlen           return buffer            return NULL

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Edit  I just found out about std  to string which is identical in operation to my own function below  It was introduced in C  11 and is available in recent versions of gcc  at least as early as 4 5 if you enable the c  0x extensions   Not only is itoa missing from gcc  it s not the handiest function to use since you need to feed it a buffer  I needed something that could be used in an expression so I came up with this   std  string itos int n       const int max size   std  numeric limits lt int gt   digits10   1   sign     1   0-terminator       char buffer max size     0      sprintf buffer    d   n      return std  string buffer       Ordinarily it would be safer to use snprintf instead of sprintf but the buffer is carefully sized to be immune to overrun   See an example  http   ideone com mKmZVE

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I would prefer this  https   github com wsq003 itoa for linux  It should be the fastest itoa   ever  We use itoa   instead of sprintf   for performance reason  so a fastest itoa   with limited feature is reasonable and worthwhile

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I have used  itoa      on RedHat 6 and GCC compiler  It works

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Where is the itoa function in Linux    As itoa   is not standard in C  various versions with various function signatures exists  char  itoa int value  char  str  int base   is common in  nix   Should it be missing from Linux or if code does not want to limit portability  code could make it own   Below is a version that does not have trouble with INT MIN and handles problem buffers  NULL or an insufficient buffer returns NULL    include  lt stdlib h gt   include  lt limits h gt   include  lt string h gt      Buffer sized for a decimal string of a  signed int   28 93  gt  log10 2   define SIGNED PRINT SIZE object     sizeof object    CHAR BIT - 1   28   93   3   char  itoa x int number  char  dest  size t dest size      if  dest    NULL        return NULL         char buf SIGNED PRINT SIZE number      char  p    amp buf sizeof buf - 1         Work with negative absolute value   int neg num   number  lt  0   number   -number        Form string    p     0     do        --p    char    0  - neg num   10       neg num    10      while  neg num     if  number  lt  0         --p    -             Copy string   size t src size    size t    amp buf sizeof buf  - p     if  src size  gt  dest size           Not enough room     return NULL        return memcpy dest  p  src size         Below is a C99 or later version that handles any base  2   36   char  itoa x int number  char  dest  size t dest size  int base      if  dest    NULL    base  lt  2    base  gt  36        return NULL         char buf sizeof number   CHAR BIT   2      worst case  itoa INT MIN   2    char  p    amp buf sizeof buf - 1         Work with negative absolute value to avoid UB of  abs INT MIN     int neg num   number  lt  0   number   -number        Form string    p     0     do        --p    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ  - neg num   base        neg num    base      while  neg num     if  number  lt  0         --p    -             Copy string   size t src size    size t    amp buf sizeof buf  - p     if  src size  gt  dest size           Not enough room     return NULL        return memcpy dest  p  src size       For a C89 and onward compliant code  replace inner loop with    div t qr    do       qr   div neg num  base        --p    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ  -qr rem       neg num   qr quot      while  neg num

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Where is the itoa function in Linux    There is no such function in Linux  I use this code instead                    itoa  Convert integer to string  PARAMS  - value     A 64-bit number to convert - str       Destination buffer  should be 66 characters long for radix2  24 - radix8  22 - radix10  18 - radix16  - radix     Radix must be in range -36    36  Negative values used for signed numbers                    char  itoa  unsigned long long  value   char str     int radix        char        buf  66       char        dest   buf   sizeof buf       boolean     sign   false       if  value    0            memcpy  str   0   2           return str             if  radix  lt  0            radix   -radix          if    long long  value  lt  0                value   -value              sign   true                        --dest     0        switch  radix            case 16          while  value                  --dest    0     value  amp  0xF               if   dest  gt   9    dest     A  -  9  - 1              value  gt  gt   4                    break      case 10          while  value                 --dest    0     value   10               value    10                    break       case 8          while  value                 --dest    0     value  amp  7               value  gt  gt   3                    break       case 2          while  value                 --dest    0     value  amp  1               value  gt  gt   1                    break       default                The slow version  but universal         while  value                 --dest    0     value   radix               if   dest  gt   9    dest     A  -  9  - 1              value    radix                    break             if  sign   --dest    -        memcpy  str  dest  buf  sizeof buf  - dest       return str

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i tried my own implementation of itoa    it seem s work in binary  octal  decimal and hex   define INT LEN  10   define HEX LEN  8   define BIN LEN  32   define OCT LEN  11   static char    my itoa   int value  char   str  int base         int i n  2 tmp      char buf BIN LEN 1         switch base                case 16              for i   0 i lt HEX LEN   i                                if value base gt 0                                        n                                                snprintf str  n    x   value               break          case 10              for i   0 i lt INT LEN   i                                if value base gt 0                                        n                                                snprintf str  n    d   value               break          case 8              for i   0 i lt OCT LEN   i                                if value base gt 0                                        n                                                snprintf str  n    o   value               break          case 2              for i   0 tmp   value i lt BIN LEN   i                                if tmp base gt 0                                        n                                      tmp  base                            for i   1  tmp   value  i lt n   i                                if tmp 2    0                                        buf n-i-1    1                                     else                                       buf n-i-1    0                                     tmp  base                            buf n-1      0               strcpy str buf               break          default              return NULL            return str

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Where is the itoa function in Linux    There is no such function in Linux  I use this code instead                    itoa  Convert integer to string  PARAMS  - value     A 64-bit number to convert - str       Destination buffer  should be 66 characters long for radix2  24 - radix8  22 - radix10  18 - radix16  - radix     Radix must be in range -36    36  Negative values used for signed numbers                    char  itoa  unsigned long long  value   char str     int radix        char        buf  66       char        dest   buf   sizeof buf       boolean     sign   false       if  value    0            memcpy  str   0   2           return str             if  radix  lt  0            radix   -radix          if    long long  value  lt  0                value   -value              sign   true                        --dest     0        switch  radix            case 16          while  value                  --dest    0     value  amp  0xF               if   dest  gt   9    dest     A  -  9  - 1              value  gt  gt   4                    break      case 10          while  value                 --dest    0     value   10               value    10                    break       case 8          while  value                 --dest    0     value  amp  7               value  gt  gt   3                    break       case 2          while  value                 --dest    0     value  amp  1               value  gt  gt   1                    break       default                The slow version  but universal         while  value                 --dest    0     value   radix               if   dest  gt   9    dest     A  -  9  - 1              value    radix                    break             if  sign   --dest    -        memcpy  str  dest  buf  sizeof buf  - dest       return str

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As Matt J wrote  there is itoa  but it s not standard  Your code will be more portable if you use snprintf

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You can use this program instead of sprintf   void itochar int x  char  buffer  int radix    int main         char buffer 10       itochar 725  buffer  10       printf    n  s  n   buffer       return 0     void itochar int x  char  buffer  int radix        int i   0   n s      n   s      while  n  gt  0                s   n radix          n   n radix          buffer i       0    s            buffer i      0       strrev buffer

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I have used  itoa      on RedHat 6 and GCC compiler  It works

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direct copy to buffer   64 bit integer itoa hex        char  itoah long num  char  s  int len                    long n  m   16              int i   16 2              int shift    a -   9  1                 if  s    len  lt  1                      return 0               n   num  lt  0   -1   1              n   n   num               len   len  gt  i   i   len              i   len  lt  i   len   i               s i-1    0              i--               if  num                                    if len  lt  2                              return  amp s i                        s i-1   0                       return  amp s i-1                              while i  amp  amp  n                                    s i-1    n   m    0                        if  s i-1   gt   9                               s i-1     shift                        n   n m                      i--                             if num  lt  0                                    if i                                                    s i-1     -                               i--                                                   return  amp s i           note  change long to long long for 32 bit machine  long to int in case for 32 bit integer  m is the radix  When decreasing radix  increase number of characters  variable i   When increasing radix  decrease number of characters  better   In case of unsigned data type  i just becomes 16   1

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Where is the itoa function in Linux    As itoa   is not standard in C  various versions with various function signatures exists  char  itoa int value  char  str  int base   is common in  nix   Should it be missing from Linux or if code does not want to limit portability  code could make it own   Below is a version that does not have trouble with INT MIN and handles problem buffers  NULL or an insufficient buffer returns NULL    include  lt stdlib h gt   include  lt limits h gt   include  lt string h gt      Buffer sized for a decimal string of a  signed int   28 93  gt  log10 2   define SIGNED PRINT SIZE object     sizeof object    CHAR BIT - 1   28   93   3   char  itoa x int number  char  dest  size t dest size      if  dest    NULL        return NULL         char buf SIGNED PRINT SIZE number      char  p    amp buf sizeof buf - 1         Work with negative absolute value   int neg num   number  lt  0   number   -number        Form string    p     0     do        --p    char    0  - neg num   10       neg num    10      while  neg num     if  number  lt  0         --p    -             Copy string   size t src size    size t    amp buf sizeof buf  - p     if  src size  gt  dest size           Not enough room     return NULL        return memcpy dest  p  src size         Below is a C99 or later version that handles any base  2   36   char  itoa x int number  char  dest  size t dest size  int base      if  dest    NULL    base  lt  2    base  gt  36        return NULL         char buf sizeof number   CHAR BIT   2      worst case  itoa INT MIN   2    char  p    amp buf sizeof buf - 1         Work with negative absolute value to avoid UB of  abs INT MIN     int neg num   number  lt  0   number   -number        Form string    p     0     do        --p    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ  - neg num   base        neg num    base      while  neg num     if  number  lt  0         --p    -             Copy string   size t src size    size t    amp buf sizeof buf  - p     if  src size  gt  dest size           Not enough room     return NULL        return memcpy dest  p  src size       For a C89 and onward compliant code  replace inner loop with    div t qr    do       qr   div neg num  base        --p    0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ  -qr rem       neg num   qr quot      while  neg num

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The replacement with snprintf is NOT complete   It covers only bases  2  8  10  16  whereas itoa works for bases between 2 and 36   Since I was searching a replacement for base 32  I guess I ll have to code my own

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If you just want to print them   void binary unsigned int n        for int shift sizeof int  8-1 shift gt  0 shift--               if  n  gt  gt  shift  amp  1           printf  1           else          printf  0               printf   n

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itoa is not a standard C function  You can implement your own  It appeared in the first edition of Kernighan and Ritchie s The C Programming Language  on page 60  The second edition of The C Programming Language   K amp R2   contains the following implementation of itoa  on page 64  The book notes several issues with this implementation  including the fact that it does not correctly handle the most negative number        itoa   convert n to characters in s     void itoa int n  char s            int i  sign        if   sign   n   lt  0      record sign             n   -n              make n positive         i   0       do            generate digits in reverse order             s i      n   10    0        get next digit           while   n    10   gt  0          delete it         if  sign  lt  0           s i       -        s i      0        reverse s         The function reverse used above is implemented two pages earlier     include  lt string h gt       reverse   reverse string s in place     void reverse char s            int i  j       char c        for  i   0  j   strlen s -1  i lt j  i    j--             c   s i            s i    s j            s j    c

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If you are calling it a lot  the advice of  just use snprintf  can be annoying  So here s what you probably want   const char  my itoa buf char  buf  size t len  int num      static char loc buf sizeof int    CHAR BITS      not thread safe       if   buf          buf   loc buf      len   sizeof loc buf          if  snprintf buf  len    d   num     -1      return        or whatever       return buf     const char  my itoa int num    return my itoa buf NULL  0  num

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I have used  itoa      on RedHat 6 and GCC compiler  It works

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Here is a much improved version of Archana s solution  It works for any radix 1-16  and numbers  lt   0  and it shouldn t clobber memory    static char  numberSystem      0123456789ABCDEF   static char  twosComp      FEDCBA9876543210    static void safestrrev char  buffer  const int bufferSize  const int strlen        int len   strlen      if  len  gt  bufferSize                len   bufferSize            for  int index   0  index  lt   len   2   index                  char ch   buffer index           buffer index    buffer len - index - 1           buffer len - index - 1    ch           static int negateBuffer char  buffer  const int bufferSize  const int strlen  const int radix        int len   strlen      if  len  gt  bufferSize                len   bufferSize            if  radix    10                if  len  lt   bufferSize - 1                         buffer len       -               buffer len      0                       else               int twosCompIndex   0          for  int index   0  index  lt  len  index                          if   buffer index   gt    0    amp  amp   buffer index   lt    9                                  twosCompIndex   buffer index  -  0                             else if   buffer index   gt    A    amp  amp   buffer index   lt    F                                  twosCompIndex   buffer index  -  A    10                            else if   buffer index   gt    a    amp  amp   buffer index   lt    f                                  twosCompIndex   buffer index  -  a    10                            twosCompIndex     16 - radix               buffer index     twosComp twosCompIndex                     if  len  lt   bufferSize - 1                         buffer len       numberSystem radix - 1               buffer len    0                      return len     static int twosNegation const int x  const int radix        int n   x      if  x  lt  0                if  radix    10                        n   -x                    else                       n    x                      return n     static char  safeitoa const int x  char  buffer  const int bufferSize  const int radix        int strlen   0      int n   twosNegation x  radix       int nuberSystemIndex   0       if  radix  lt   16                do                       if  strlen  lt   bufferSize - 1                                 nuberSystemIndex    n   radix                   buffer strlen       numberSystem nuberSystemIndex                   buffer strlen      0                   n   n   radix                            else                               break                          while  n    0           if  x  lt  0                        strlen   negateBuffer buffer  bufferSize  strlen  radix                     safestrrev buffer  bufferSize  strlen           return buffer            return NULL

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As Matt J wrote  there is itoa  but it s not standard  Your code will be more portable if you use snprintf

[c] Where is the itoa function in Linux?

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