Returning a C string from a function

Question

I am trying to return a C string from a function  but it s not working  Here is my code   char myFunction         return  My String       In main I am calling it like this   int main       printf   s   myFunction         I have also tried some other ways for myFunction  but they are not working  For example   char myFunction       char array      my string     return array      Note  I am not allowed to use pointers   Little background on this problem   There is function which is finding out which month it is  For example  if it s 1 then it returns January  etc   So when it s going to print  it s doing it like this  printf  Month   s  calculateMonth month     Now the problem is how to return that string from the calculateMonth function

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If you really can t use pointers  do something like this   char get string char int index        static char array      my string       return array index      int main         for  int i   0  i  lt  9    i          printf   c   get string char i        printf   n        return 0      The magic number 9 is awful  and this is not an example of good programming  But you get the point  Note that pointers and arrays are the same thing  kind of   so this is a bit cheating

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Your function prototype states your function will return a char   Thus  you can t return a string in your function

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Or how about this one   void print month int month        switch  month                case 0              printf  January                break          case 1              printf  february                break             etc              And call that with the month you compute somewhere else

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char  myFunction         return  My String       In C  string literals are arrays with the static constant memory class  so returning a pointer to this array is safe  More details are in Stack Overflow question  quot Life-time quot  of a string literal in C

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Your function return type is a single character  char   You should return a pointer to the first element of the character array  If you can t use pointers  then you are screwed

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Note this new function   const char  myFunction         static char array      my string       return array      I defined  array  as static  Otherwise when the function ends  the variable  and the pointer you are returning  gets out of scope  Since that memory is allocated on the stack  and it will get corrupted  The downside of this implementation is that the code is not reentrant and not threadsafe   Another alternative would be to use malloc to allocate the string in the heap  and then free on the correct locations of your code  This code will be reentrant and threadsafe   As noted in the comment  this is a very bad practice  since an attacker can then inject code to your application  he she needs to open the code using GDB  then make a breakpoint and modify the value of a returned variable to overflow and fun just gets started    It is much more recommended to let the caller handle about memory allocations  See this new example   char  myFunction char  output str  size t max len       const char  str    my string      size t l   strlen str      if  l 1  gt  max len          return NULL          strcpy str  str  l      return input      Note that the only content which can be modified is the one that the user  Another side effect - this code is now threadsafe  at least from the library point of view  The programmer calling this method should verify that the memory section used is threadsafe

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Your problem is with the return type of the function - it must be   char  myFunction        and then your original formulation will work   Note that you cannot have C strings without pointers being involved  somewhere along the line   Also  Turn up your compiler warnings  It should have warned you about that return line converting a char   to char without an explicit cast

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A char is only a single one-byte character  It can t store the string of characters  nor is it a pointer  which you apparently cannot have   Therefore you cannot solve your problem without using pointers  which char   is syntactic sugar for

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A C string is defined as a pointer to an array of characters   If you cannot have pointers  by definition you cannot have strings

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You can create the array in the caller  which is the main function  and pass the array to the callee which is your myFunction    Thus myFunction can fill the string into the array  However  you need to declare myFunction   as   char  myFunction char   buf  int buf len     strncpy buf   my string   buf len     return buf      And in main function  myFunction should be called in this way   char array 51   memset array  0  51      All bytes are set to   0     printf   s   myFunction array  50       The buf len argument  is 50  not 51  This is to make sure the string in buf is always null-terminated  array 50  is always   0        However  a pointer is still used

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Based on your newly-added backstory with the question  why not just return an integer from 1 to 12 for the month  and let the main   function use a switch statement or if-else ladder to decide what to print  It s certainly not the best way to go - char  would be - but in the context of a class like this I imagine it s probably the most elegant

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Return string from function  include  lt stdio h gt   const char  greet       return  quot Hello quot      int main void      printf  quot  s quot   greet

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Well  in your code you are trying to return a String  in C which is nothing but a null-terminated array of characters   but the return type of your function is char which is causing all the trouble for you  Instead you should write it this way   const char  myFunction          return  My String        And it s always good to qualify your type with const while assigning literals in C to pointers as literals in C aren t modifiable

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Your function signature needs to be   const char   myFunction         return  My String       Background   It s so fundamental to C  amp  C    but little more discussion should be in order   In C   amp  C   for that matter   a string is just an array of bytes terminated with a zero byte - hence the term  string-zero  is used to represent this particular flavour of string  There are other kinds of strings  but in C   amp  C     this flavour is inherently understood by the language itself  Other languages  Java  Pascal  etc   use different methodologies to understand  my string    If you ever use the Windows API  which is in C     you ll see quite regularly function parameters like   LPCSTR lpszName   The  sz  part represents this notion of  string-zero   an array of bytes with a null   zero  terminator   Clarification   For the sake of this  intro   I use the word  bytes  and  characters  interchangeably  because it s easier to learn this way  Be aware that there are other methods  wide-characters  and multi-byte character systems  mbcs   that are used to cope with international characters  UTF-8 is an example of an mbcs  For the sake of intro  I quietly  skip over  all of this   Memory   This means that a string like  my string  actually uses 9 1   10   bytes  This is important to know when you finally get around to allocating strings dynamically   So  without this  terminating zero   you don t have a string  You have an array of characters  also called a buffer  hanging around in memory   Longevity of data   The use of the function this way   const char   myFunction         return  My String      int main         const char  szSomeString   myFunction       Fraught with problems     printf   s   szSomeString           will generally land you with random unhandled-exceptions segment faults and the like  especially  down the road    In short  although my answer is correct - 9 times out of 10 you ll end up with a program that crashes if you use it that way  especially if you think it s  good practice  to do it that way  In short  It s generally not   For example  imagine some time in the future  the string now needs to be manipulated in some way  Generally  a coder will  take the easy path  and  try to  write code like this   const char   myFunction const char  name        char szBuffer 255       snprintf szBuffer  sizeof szBuffer    Hi  s   name       return szBuffer      That is  your program will crash because the compiler  may may not  have released the memory used by szBuffer by the time the printf   in main   is called   Your compiler should also warn you of such problems beforehand    There are two ways to return strings that won t barf so readily    returning buffers  static or dynamically allocated  that live for a while  In C   use  helper classes   for example  std  string  to handle the longevity of data  which requires changing the function s return value   or pass a buffer to the function that gets filled in with information    Note that it is impossible to use strings without using pointers in C  As I have shown  they are synonymous  Even in C   with template classes  there are always buffers  that is  pointers  being used in the background   So  to better answer the  now modified question    There are sure to be a variety of  other answers  that can be provided    Safer Answers   Example 1  using statically allocated strings   const char  calculateMonth int month        static char  months       Jan    Feb    Mar              static char badFood      Unknown       if  month lt 1    month gt 12          return badFood     Choose whatever is appropriate for bad input  Crashing is never appropriate however      else         return months month-1      int main         printf   s   calculateMonth 2       Prints  Feb      What the  static  does here  many programmers do not like this type of  allocation   is that the strings get put into the data segment of the program  That is  it s permanently allocated   If you move over to C   you ll use similar strategies   class Foo       char  someData 12   public      const char  someFunction   const          The final  const  is to let the compiler know that nothing is changed in the class when this function is called          return  someData                but it s probably easier to use helper classes  such as std  string  if you re writing the code for your own use  and not part of a library to be shared with others    Example 2  using caller-defined buffers   This is the more  foolproof  way of passing strings around  The data returned isn t subject to manipulation by the calling party  That is  example 1 can easily be abused by a calling party and expose you to application faults  This way  it s much safer  albeit uses more lines of code    void calculateMonth int month  char  pszMonth  int buffersize        const char  months       Jan    Feb    Mar             Allocated dynamically during the function call   Can be inefficient with a bad compiler      if   pszMonth    buffersize lt 1          return     Bad input  Let junk deal with junk data      if  month lt 1    month gt 12                 pszMonth     0      Return an  empty  string            OR  strncpy pszMonth   Bad Month   buffersize-1             else               strncpy pszMonth  months month-1   buffersize-1             pszMonth buffersize-1      0      Ensure a valid terminating zero  Many people forget this     int main         char month 16      16 bytes allocated here on the stack      calculateMonth 3  month  sizeof month        printf   s   month      Prints  Mar      There are lots of reasons why the second method is better  particularly if you re writing a library to be used by others  you don t need to lock into a particular allocation deallocation scheme  third parties can t break your code  and you don t need to link to a specific memory management library   but like all code  it s up to you on what you like best  For that reason  most people opt for example 1 until they ve been burnt so many times that they refuse to write it that way anymore     Disclaimer   I retired several years back and my C is a bit rusty now  This demo code should all compile properly with C  it is OK for any C   compiler though

[c] Returning a C string from a function

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