Pass a char pointer array to a function in C? - c

Question

I have the following code:
int main(){
char **array;
char a[5];
int n = 5;
array = malloc(n *sizeof *array);
/*Some code to assign array values*/
test(a, array);
return 0;
}
int test(char s1, char **s2){
if(strcmp(s1, s2[0]) != 0)
return 1;
return 0;
}
I'm trying to pass char and char pointer array to a function, but the above code results in the following errors and warnings:
temp.c: In function ‘main’:
temp.c:6:5: warning: implicit declaration of function ‘malloc’ [-Wimplicit-function-declaration]
temp.c:6:13: warning: incompatible implicit declaration of built-in function ‘malloc’ [enabled by default]
temp.c:10:5: warning: implicit declaration of function ‘test’ [-Wimplicit-function-declaration]
temp.c: At top level:
temp.c:15:5: error: conflicting types for ‘test’
temp.c:15:1: note: an argument type that has a default promotion can’t match an empty parameter name list declaration
temp.c:10:5: note: previous implicit declaration of ‘test’ was here
temp.c: In function ‘test’:
temp.c:16:5: warning: implicit declaration of function ‘strcmp’ [-Wimplicit-function-declaration]
I'm trying to understand what the problem is.

Answer 1

First of all, you should include the necessary header files. For strcmp you need <string.h>, for malloc <malloc.h>. Also you need to at least declare test before main. If you do this you'll notice the following error:
temp.c: In function ‘test’:
temp.c:20:5: warning: passing argument 1 of ‘strcmp’ makes pointer from integer without a cast [enabled by default]
/usr/include/string.h:143:12: note: expected ‘const char *’ but argument is of type ‘char’
This indicates that test() should have a char * as first argument. All in all your code should look like this:
#include <string.h> /* for strcmp */
#include <malloc.h> /* for malloc */
int test(char*,char**); /* added declaration */
int main(){
char **array;
char a[5];
int n = 5;
array = malloc(sizeof(*array));
array[0] = malloc(n * sizeof(**array));
/*Some code to assign array values*/
test(a, array);
free(*array); /* free the not longer needed memory */
free(array);
return 0;
}
int test(char * s1, char **s2){ /* changed to char* */
if(strcmp(s1, s2[0]) != 0) /* have a look at the comment after the code */
return 1;
return 0;
}
Edit
Please notice that strcmp works with null-terminated byte strings. If neither s1 nor s2 contain a null byte the call in test will result in a segmentation fault:
[1] 14940 segmentation fault (core dumped) ./a.out
Either make sure that both contain a null byte '\0', or use strncmp and change the signature of test:
int test(char * s1, char **s2, unsigned count){
if(strncmp(s1, s2[0], count) != 0)
return 1;
return 0;
}
/* don' forget to change the declaration to
int test(char*,char**,unsigned)
and call it with test(a,array,min(sizeof(a),n))
*/
Also your allocation of memory is wrong. array is a char**. You allocate memory for *array which is itself a char*. You never allocate memory for this specific pointer, you're missing array[0] = malloc(n*sizeof(**array)):
array = malloc(sizeof(*array));
*array = malloc(n * sizeof(**array));

Answer 2

Error 1
temp.c:6:13: warning: incompatible implicit declaration of
built-in function ‘malloc’ [enabled by default]
Did you mean this?
array = malloc(n * sizeof(*array));
Error 2
temp.c:15:5: error: conflicting types for ‘test’
temp.c:15:1: note: an argument type that has a default promotion can’t
match an empty parameter name list declaration
temp.c:10:5: note: previous implicit declaration of ‘test’ was here
You are passing the address of the first element of an array a:
test(a, array);
So the function signature should be:
int test(char* s1, char** s2)

Answer 3

You have several problems. The first is that the prototype is wrong. The data type for a decays to a char pointer when passing to a function, so you need:
int test (char* s1, char** s2) { ... }
However, even when you fix this, the test declaration isn't in scope when you first use it. You should either provide a prototype:
int test (char* s1, char** s2);
before main, or simply move the whole definition (function) to before main.
In addition, don't forget to #include the string.h and stdlib.h headers so that the prototypes for strcmp and malloc are available as well.

Answer 4

When you pass an array of char to your function, the argument decays to a pointer. Change your function arguments to
int test(char* s1, char **s2);
^
^
and your code should at least compile