How to convert unsigned long to string

Question

In the C language  how do I convert unsigned long  value to a string  char    and keep my source code portable or just recompile it to work on other platform  without rewriting code   For example  if I have sprintf buffer  format  value   how do I determine the size of buffer with platform-independent manner

User · Answer

Try using sprintf   unsigned long x 1000000  char buffer 21   sprintf buffer   lu   x     Edit   Notice that you have to allocate a buffer in advance  and have no idea how long the numbers will actually be when you do so  I m assuming 32bit longs  which can produce numbers as big as 10 digits    See Carl Smotricz s answer for a better explanation of the issues involved

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The standard approach is to use sprintf buffer    lu   value   to write a string rep of value to buffer  However  overflow is a potential problem  as sprintf will happily  and unknowingly  write over the end of your buffer   This is actually a big weakness of sprintf  partially fixed in C   by using streams rather than buffers  The usual  answer  is to allocate a very generous buffer unlikely to overflow  let sprintf output to that  and then use strlen to determine the actual string length produced  calloc a buffer of  that size   1  and copy the string to that    This site discusses this and related problems at some length   Some libraries offer snprintf as an alternative which lets you specify a maximum buffer size

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you can write a function which converts from unsigned long to str  similar to ltostr library function   char  ultostr unsigned long value  char  ptr  int base      unsigned long t   0  res   0    unsigned long tmp   value    int count   0     if  NULL    ptr          return NULL         if  tmp    0          count           while tmp  gt  0          tmp   tmp base      count           ptr    count      ptr     0      do         res   value - base    t   value   base       if  res  lt  10                -- ptr    0    res            else if   res  gt   10   amp  amp   res  lt  16                   --ptr    A  - 10   res            while   value   t     0      return ptr       you can refer to my blog here which explains implementation and usage with example

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const int n   snprintf NULL  0    lu   ulong value   assert n  gt  0   char buf n 1   int c   snprintf buf  n 1    lu   ulong value   assert buf n       0    assert c    n

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char buffer  50    unsigned long a   5   int n sprintf  buffer    lu   a

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For a long value you need to add the length info  l  and  u  for unsigned decimal integer   as a reference of available options see sprintf   include  lt stdio h gt       int main                unsigned long lval   123        char buffer  50         sprintf  buffer    lu    lval

[c] How to convert unsigned long to string

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