c - Return argument doesn't work — gives me weird error -

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this simple program should create substring string, should return substring can printed out. it's exercise , substring function can changed. problem can't find return type doesn't spark kinds of warnings , errors.

how should change return type? 

static void panic(const char *serror) {     printf("%s", serror);     exit(1); }  static void *xmalloc(size_t size) {     void *ptr;     if (size == 0)         panic("size 0!\n");     ptr = malloc(size);     if (!ptr)         panic("no mem left!\n");     return ptr; }  static char *substring(const char *str, off_t pos, size_t len) {     char out [len];     int index;      for(index = 0; index < (pos + len); index++)     {         if(index >= pos && index < (pos + len))         {         out[index - pos] = str[index];         }     }      return out; }  int main(int argc, char **argv) {     char *foo = "nicht\n";     char *bar = substring(foo, 2, 3);     printf("%s", bar);     free(bar);     return 0; }

you invoked 2 undefine behavior by

  • dereferencing pointer bar points @ vanished local variable.
  • passing non-null pointer doesn't point @ buffer allocated via malloc(), calloc() or realloc().

also note that

  • you have terminate string adding null character.
  • your loop not efficient.

corrected code:

static char *substring(const char *str, off_t pos, size_t len) {     char *out = xmalloc(len + 1);     int index;      for(index = pos; index < (pos + len); index++)     {         out[index - pos] = str[index];     }     out[len] = '\0';      return out; }

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