c - Assigning a char array to a char* -

i'm trying write function prefixes string length. can't seem assign char[] char *. mysteriously, if print out debugging code before assignment, works.

char *prefixmsgwlength(char *msg){   char *msgwlength;   int msglength = strlen(msg);    if (msglength == 0){     msgwlength = "2|";   }   else{      int ndigits = floor(log10(abs(msglength))) + 1;     int ndigits2 = floor(log10(abs(msglength + ndigits + 1))) + 1;      if (ndigits2 > ndigits){       ndigits = ndigits2;     }      msglength += ndigits + 1;      char prefix[msglength];     sprintf(prefix, "%d|", msglength);      strcat(prefix, msg);     // if uncomment below, msgwlength returned correctly     // printf("msg: %s\n", prefix);     msgwlength = prefix;   }   return msgwlength; } 

the problem in code is

 msgwlength = prefix; 

here, you're assigning address of local variable (prefix) pointer , try return it.

once function finishes execution, local variables go out of scope , returned pointer invalid.

you need make prefix pointer , allocate memory dynamically, if want retain it's existence after returning function.