Warning comparison with string literals results in unspecified behaviour

Question

I am starting a project of writing a simplified shell for linux in C  I am not at all proficient with C nor with Linux that s exactly the reason I decided it would be a good idea    Starting with the parser  I have already encountered some problems   The code should be straightforward that s why I didn t include any comments    I am getting a warning with gcc   comparison with string literals results in unspecified behaviour  at the lines commented with  WARNING HERE   see code below     I have no idea why this causes an warning  but the real problem is that even though I am comparing an   lt   to an   lt   is doesn t get inside the if     I am looking for an answer for the problem explained  however if there s something that you see in the code that should be improved please say so  Just take in mind I am not that proficient and that this is still a work in progress  or better yet  a work in start    Thanks in advance    include  lt stdio h gt   include  lt unistd h gt   include  lt string h gt   typedef enum  false  true  bool   typedef struct       char   arg      char  infile      char  outfile      int background    Command Info   int parse cmd char  cmd line  Command Info  cmd info        char  arg      char  args 100            int i   0      arg   strtok cmd line     n        while  arg    NULL            args i    arg          arg   strtok NULL     n            i               int num elems   i       cmd info- gt infile   NULL      cmd info- gt outfile   NULL      cmd info- gt background   0       int iarg   0      for  i   0  i  lt  num elems  i                  if  args i       amp      WARNING HERE             return -1                else if  args i       lt      WARNING HERE             if  args i 1     NULL                  cmd info- gt infile   args i 1               else                 return -1           else if  args i       gt      WARNING HERE             if  args i 1     NULL                  cmd info- gt outfile   args i 1               else                 return -1                     else              cmd info- gt arg iarg      args i              cmd info- gt arg iarg    NULL       return 0        void print cmd Command Info  cmd info        int i        for  i   0  cmd info- gt arg i     NULL  i            printf  arg  d     s   n   i  cmd info- gt arg i        printf  arg  d     s   n   i  cmd info- gt arg i            printf  infile    s   n   cmd info- gt infile       printf  outfile    s   n   cmd info- gt outfile       printf  background    d   n   cmd info- gt background      int main int argc  char  argv          char cmd line 100       Command Info cmd info       printf   gt  gt  gt           fgets cmd line  100  stdin        parse cmd cmd line   amp cmd info        print cmd  amp cmd info        return 0

User · Answer

You can t compare strings with    in C  For C  strings are just  zero-terminated  arrays  so you need to use string functions to compare them  See the man page for strcmp   and strncmp     If you want to compare a character you need to compare to a character  not a string   a  is the string a  which occupies two bytes  the a and the terminating null byte   while the character a is represented by  a  in C

User · Answer

You want to use strcmp      0 to compare strings instead of a simple     which will just compare if the pointers are the same  which they won t be in this case    args i  is a pointer to a string  a pointer to an array of chars null terminated   as is   amp   or   lt     The expression argc i       amp   checks if the two pointers are the same  point to the same memory location    The expression strcmp  argc i     amp       0 will check if the contents of the two strings are the same

User · Answer

There is a distinction between  a  and  a      a  means the value of the character a   a  means the address of the memory location where the string  a  is stored  which will generally be in the data section of your program s memory space   At that memory location  you will have two bytes -- the character  a  and the null terminator for the string

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clang has advantages in error reporting  amp  recovery     clang errors c errors c 36 21  warning  result of comparison against a string literal is unspecified  use strcmp instead          if  args i       amp      WARNING HERE                                        strcmp             0 errors c 38 26  warning  result of comparison against a string literal is unspecified  use strcmp instead          else if  args i       lt      WARNING HERE                                                  strcmp             0 errors c 44 26  warning  result of comparison against a string literal is unspecified  use strcmp instead          else if  args i       gt      WARNING HERE                                                  strcmp             0   It suggests to replace x    y by strcmp x y     0  gengetopt writes command-line option parser for you

User · Answer

I ran across this issue today working with a clients program  The program works FINE in VS6 0 using the following   I ve changed it slightly         This is the one include file that every user-written Nextest programs needs     Patcom-generated files will also look for this file      include  stdio h   define IS NONE  a key         a key     none     a key     N A      TRUE   FALSE          Note in my environment we have output   which is printf which adds  n at the end    main       char  psNameNone    none       char  psNameNA      N A       char  psNameCAT     CAT        if  IS NONE psNameNone              output  psNameNone Matches NONE            output   s psNameNoneAddr 0x x    none   addr 0x X               psNameNone psNameNone               none          else           output  psNameNone Does Not Match None            output   s psNameNoneAddr 0x x    none   addr 0x X               psNameNone psNameNone               none               if  IS NONE psNameNA              output  psNameNA Matches N A            output   s psNameNA 0x x    N A   addr 0x X           psNameNA psNameNA           N A          else           output  psNameNone Does Not Match N A            output   s psNameNA 0x x    N A   addr 0x X           psNameNA psNameNA           N A              if  IS NONE psNameCAT             output  psNameNA Matches CAT            output   s psNameNA 0x x    CAT   addr 0x X           psNameNone psNameNone           CAT          else           output  psNameNA does not match CAT            output   s psNameNA 0x x    CAT   addr 0x X           psNameNone psNameNone           CAT              If built in VS6 0 with Program Database with Edit and Continue  The compares APPEAR to work   With this setting STRING pooling is enabled  and the compiler optimizes all STRING pointers to POINT TO THE SAME ADDRESSS  so this can work   Any strings created on the fly after compile time will have DIFFERENT addresses so will fail the compare   Changing the setting to Program Database only will build the program so that it will fail

User · Answer

This an old question  but I have had to explain it to someone recently and I thought recording the answer here would be helpful at least in understanding how C works   String literals like    a    or    This is a string    are put in the text or data segments of your program   A string in C is actually a pointer to a char  and the string is understood to be the subsequent chars in memory up until a NUL char is encountered   That is  C doesn t really know about strings   So if I have  char  s1    This is a string     then s1 is a pointer to the first byte of the string   Now  if I have   char  s2    This is a string     this is also a pointer to the same first byte of that string in the text or data segment of the program   But if I have  char  s3   malloc  17    strcpy s3   This is a string      then s3 is a pointer to another place in memory into which I copy all the bytes of the other strings   Illustrative examples   Although  as your compiler rightly points out  you shouldn t do this  the following will evaluate to true   s1    s2    True  we are comparing two pointers that contain the same address   but the following will evaluate to false  s1    s3    False  Comparing two pointers that don t hold the same address    And although it might be tempting to have something like this   struct Vehicle      char  type         other stuff    if  type     Car         blah1 else if  type     Motorcycle        blah2   You shouldn t do it because it s not something that is guarantied to work   Even if you know that type will always be set using a string literal   I have tested it and it works   If I do   A type    Car     then blah1 gets executed and similarly for  Motorcycle    And you d be able to do things like   if  A type    B type     but this is just terrible   I m writing about it because I think it s interesting to know why it works  and it helps understand why you shouldn t do it   Solutions   In your case  what you want to do is use strcmp a b     0 to replace a    b  In the case of my example  you should use an enum   enum type  CAR   0  MOTORCYCLE   1    The preceding thing with string was useful because you could print the type  so you might have an array like this  char  types       Car    Motorcycle      And now that I think about it  this is error prone since one must be careful to maintain the same order in the types array   Therefore it might be better to do  char  getTypeString int type        switch type      case CAR  return  Car       case MOTORCYCLE  return  Motorcycle      default  return NULL

User · Answer

if  args i       amp      Ok  let s disect what this does    args is an array of pointers  So  here you are comparing args i   a pointer  to   amp    also a pointer   Well  the only way this will every be true is if somewhere you have args i    amp   and even then    amp   is not guaranteed to point to the same place everywhere    I believe what you are actually looking for is either strcmp to compare the entire string or your wanting to do if   args i       amp    to compare the first character of the args i  string to the  amp  character

[c] Warning: comparison with string literals results in unspecified behaviour

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