How does strcmp work

Question

I ve been looking around a fair bit for an answer  I m going to make a series of my own string functions like my strcmp    my strcat    etc   Does strcmp   work through each index of two arrays of characters and if the ASCII value is smaller at an identical index of two strings  that string is there alphabetically greater and therefore a 0 or 1 or 2 is returned  I guess what Im asking is  does it use the ASCII values of characters to return these results   Any help would be greatly appreciated    REVISED   OK  so I have come up with this    it works for all cases except when the second string is greater than the first   Any tips    int my strcmp char s1    char s2             int i   0      while   s1 i       0                  if  s2 i       0      return 1            else if  s1 i   lt  s2 i      return -1            else if  s1 i   gt  s2 i      return 1            i                 return 0      int main  int argc  char  argv          int result   my strcmp argv 1   argv 2         printf  Value   d  n   result        return 0

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This code is equivalent  shorter  and more readable   int8 t strcmp  const uint8 t  s1  const uint8 t  s2        while    s1    0    amp  amp    s1   s2             s1             s2               return  int8 t  s1 -  int8 t  s2      We only need to test for end of s1  because if we reach the end of s2 before end of s1  the loop will terminate  since  s2     s1    The return expression calculates the correct value in every case  provided we are only using 7-bit  pure ASCII  characters   Careful thought is needed to produce correct code for 8-bit characters  because of the risk of integer overflow

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The pseudo-code  implementation  of strcmp would go something like   define strcmp  s1  s2       p1   address of first character of str1     p2   address of first character of str2      while contents of p1 not equal to null          if contents of p2 equal to null               return 1          if contents of p2 greater than contents of p1              return -1          if contents of p1 greater than contents of p2              return 1          advance p1         advance p2      if contents of p2 not equal to null          return -1      return 0   That s basically it  Each character is compared in turn an a decision is made as to whether the first or second string is greater  based on that character   Only if the characters are identical do you move to the next character and  if all the characters were identical  zero is returned   Note that you may not necessarily get 1 and -1  the specs say that any positive or negative value will suffice  so you should always check the return value with  lt  0   gt  0 or    0   Turning that into real C would be relatively simple   int myStrCmp  const char  s1  const char  s2        const unsigned char  p1    const unsigned char   s1      const unsigned char  p2    const unsigned char   s2       while   p1      0             if   p2      0   return  1          if   p2  gt   p1    return -1          if   p1  gt   p2    return  1           p1            p2               if   p2      0   return -1       return 0        Also keep in mind that  greater  in the context of characters is not necessarily based on simple ASCII ordering for all string functions   C has a concept called  locales  which specify  among other things  collation  or ordering of the underlying character set and you may find  for example  that the characters a         and    are all considered identical  This will happen for functions like strcoll

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This  from the masters themselves  K amp R  2nd ed   pg  106       strcmp  return  lt  0 if s  lt  t  0 if s    t   gt  0 if s  gt  t int strcmp char  s  char  t         int i       for  i   0  s i     t i   i            if  s i       0               return 0      return s i  - t i

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Here is the BSD implementation   int strcmp s1  s2      register const char  s1   s2        while   s1     s2            if   s1      0              return  0       return    const unsigned char   s1 -   const unsigned char    s2 - 1        Once there is a mismatch between two characters  it just returns the difference between those two characters

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Is just this   int strcmp char  str1  char  str2       while    str1     str2   amp  amp    str1    0                str1             str2            return   str1- str2       if you want more fast  you can add  register   before type  like this  register char  then  like this   int strcmp register char  str1  register char  str2       while    str1     str2   amp  amp    str1    0                str1             str2            return   str1- str2       this way  if possible  the register of the ALU are used

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This is how I implemented my strcmp  it works like this  it compares first letter of the two strings  if it is identical  it continues to the next letter  If not  it returns the corresponding value  It is very simple and easy to understand       include    function declaration  int strcmp char string1    char string2      int main         char string1     The San Antonio spurs       char string2     will be champins again          calling the function- strcmp     printf   n number returned by the strcmp function   d   strcmp string1  string2        getch        return 0         This function calculates the dictionary value of the string and compares it to another string  it returns a number bigger than 0 if the first string is bigger than the second it returns a number smaller than 0 if the second string is bigger than the first input  string1  string2 output  value- can be 1  0 or -1 according to the case   int strcmp char string1    char string2          int i 0      int value 2       this initialization value could be any number but the numbers that can be      returned by the function     while value  2                if  string1 i  gt string2 i                         value 1                    else if  string1 i  lt string2 i                         value -1                    else                       i                        return value

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It uses the byte values of the characters  returning a negative value if the first string appears before the second  ordered by byte values   zero if they are equal  and a positive value if the first appears after the second  Since it operates on bytes  it is not encoding-aware   For example   strcmp  abc    def    lt  0 strcmp  abc    abcd    lt  0    null character is less than  d  strcmp  abc    ABC    gt  0     a   gt   A  in ASCII strcmp  abc    abc      0   More precisely  as described in the strcmp Open Group specification      The sign of a non-zero return value shall be determined by the sign of the difference between the values of the first pair of bytes  both interpreted as type unsigned char  that differ in the strings being compared    Note that the return value may not be equal to this difference  but it will carry the same sign

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Here is my version  written for small microcontroller applications  MISRA-C compliant  The main aim with this code was to write readable code  instead of the one-line goo found in most compiler libs   int8 t strcmp  const uint8 t  s1  const uint8 t  s2      while     s1      0    amp  amp    s1     s2            s1         s2           return  int8 t    int16 t  s1 -  int16 t  s2        Note  the code assumes 16 bit int type

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I found this on web   http   www opensource apple com source Libc Libc-262 ppc gen strcmp c  int strcmp const char  s1  const char  s2        for      s1     s2  s1    s2            if   s1      0               return 0      return     unsigned char   s1  lt    unsigned char   s2    -1    1

[c] How does strcmp() work?

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