How to cast a char ptr to an array (to prevent warning, it does work)

Owen_Townsend · 09-03-2008, 06:08 PM

/* testarray.c - to test compile warnings/errors */
/* "warning: passing arg1 from incompatible type" */
/* - by Owen Townsend, UV Software, Aug 25/2008 */
/* - original program was 20,000 lines (uvcopy.c) */
/* so I wrote this small test program to experiment*/

/* I was trying to pass a char ptr to a function which expects an array ptr*/
/* I was getting warning "passing arg1 from incompatible pointer type" */
/* note - program works OK, but I would like to eliminate the warning msg */
/* - I tried cast (char[][100]) but get "error: cast specifies array type" */

/* This test program written to investigate problem */
/* - here are warnings/errors from this test program */
/* 34: warning: passing arg1 of 'wordsep1' from incompatible pointer type*/
/* 36: error: cast specifies array type */
/* 38: error: cast specifies array type */
/* 40: warning: passing arg1 of 'wordsep1' from incompatible pointer type*/

char line[1000]; /* string to be split into words array */

char words[9][100]; /* words isolated from line */

char words2[1000]; /* words declared as an area vs array */

char *(words2p[9][100]); /* ptr to array of 100 elements */

/* declare prototype for wordsep1() function */
int wordsep1(char words[][100], char *input, int max, short bits);

int main (int argc, char *argv[])
{
wordsep1(words,line,9,0x03); /*32 <-- OK (no warning or error) */

wordsep1(words2,line,9,0x03); /*34 warning: arg1 incompatible type */

wordsep1((char[][100])words2,line,9,0x03); /*36 error: cast array type */

words2p = (char[][100])&(words2[0][0]); /*38 error: cast array type */

wordsep1(words2p,line,9,0x03); /*40 warning: arg1 incompatible type */

return(0);
}

/*-------------------------- wordsep1 ----------------------------*/
/* separate input string into array of words */

int wordsep1(char words[][100], char *input, int max, short bits)
{
return(1);
}

BrianK · 09-03-2008, 09:14 PM

first off, please put code inside code tags, it's really hard to read otherwise. Secondly, your question is easier to read when it's not in the code block.

That said, "char foo[]" is not the same as "char foo[][]" yet, you are trying to use them the same way. It may work because everything is static, but is actually wrong - hence the warnings/errors. (actually, I get all compile errors... what compiler are you using?)

There's no proper casting of a char** to a char* (or vice versa) - they simply aren't the same. You can create 2 different functions that take two sets of arguments even if they are called the same, i.e.:

Code:

#include <stdio.h>

void myfunc(int a) {
  printf("int version: %d.\n",a);
}

void myfunc(float a) {
  printf("float version: %f.\n",a);
}

int main() {
  int a = 4;
  float b = 4.4;
  myfunc(a);
  myfunc(b);
  return 0;
}

outputs:

Code:

int version: 4.
float version: 4.400000.

so you could make a func that takes a char* (or char[]) and one that takes a char** (or char[][]), call them the same thing, then your example may work. You can even make one call the other after correctly converting the data if that would make coding easier.

hope that helps.