Passing a pointer to an array to another function C

Question

I am attempting to take a input file up to 2048 bytes and place it in its own array in layer4. I am also attempting to place the size of the array in spot [0] of the array for latter use. When layer4 finishes I am attempting to pass the pointer pointing to the array called code to transmit function where it will pass that value to layer3 and place the array in a structure. Currently when I compare the address of my pointer in layer4 and layer3 to each other, they match. However when i check for values in the array in layer3, they do not match the values of the array in my input file. This code is to be part of a bigger project. The various warnings I receive are at the bottom under my code:

#include <stdio.h>   
#include <stdlib.h>


main()
{
int *senddata;
senddata = layer4(); // get pointer address of input array
transmit(senddata); //put pointer value into transmit


}

int layer4(){
    FILE *file = fopen("sendtext.txt", "r"); //
    char *code;
    size_t n = 0;
    int c;
    if (file == NULL)
        return NULL; //could not open file

    code = malloc(2048); //allocate memory 

    while ((c = fgetc(file)) != EOF)
    {
    n++;       
    code[n] = (char) c;
    printf("%c", code[n]);
    }
    code[n] = '\0';
    n = n-1;  // for some reason the byte size is +1 for what it should be
    code[0] = n;  

  printf("Check Pointer Address in layer 4: %p \n", code); //test to see pointer address
  printf("Check to see value in pointer:%c \n", code[0]); //check to see if the byte size was placed in the array
  printf("Byte size:%zd\n", n); /// see array size


return code;
}

transmit(int* getdata){  //gets pointer value
int newdata = getdata;
int g = layer3(newdata); //puts pointer into new function
}

layer3(int b){
int x = b;
int w = &x;
char *MSS;
MSS = malloc(60); 


printf("Check to see value in pointer:%c \n", w);


printf("Check Pointer Address in layer 3:%p \n", x); //test to see pointer address

struct l3hdr {  
  char ver;
  char src_ipaddr[16];
  char dest_ipaddr[16];
  char reserved[7];
};

struct l3pdu {
// put array here
struct l3hdr hdr3; 


};
}

Output

Q sadfasd fsa asd fsadf sad f /// This is my input testfile
Check Pointer Address in layer 4: 0xa81250 
Check to see Byte size in array: 
Check to see first input character in array:Q 
Byte size:29
Check to see value in pointer:P 
Check Pointer Address in layer 3:0xa81250

Warnings

lab.c:20:9: warning: return makes integer from pointer without a cast [enabled by default]
         return NULL; //could not open file
         ^
lab.c:37:2: warning: format ‘%c’ expects argument of type ‘int’, but argument 2 has type ‘char *’ [-Wformat=]
  printf("Check to see value in pointer:%c \n", code);
  ^
lab.c:43:1: warning: return makes integer from pointer without a cast [enabled by default]
 return code;
 ^


lab.c: In function ‘transmit’:
lab.c:47:15: warning: initialization makes integer from pointer without a cast [enabled by default]
 int newdata = getdata;
               ^


lab.c: In function ‘layer3’:



lab.c:64:9: warning: initialization makes integer from pointer without a cast [enabled by default]
 int w = &x;
         ^


lab.c:72:1: warning: format ‘%p’ expects argument of type ‘void *’, but argument 2 has type ‘int’ [-Wformat=]
 printf("Check Pointer Address in layer 3:%p \n", x); //test to see pointer address
 ^

kaylum · Accepted Answer · 2015-04-15 19:58:09Z

You are mixing pointers and integers all over the place. That doesn't always cause problems but is bad practice at best. For your specific problem, the cause is likely to be this:

transmit(int* getdata){  //gets pointer value
    int newdata = getdata;
    int g = layer3(newdata); //puts pointer into new function
}

layer3(int b){
    int x = b;
    int w = &x;

You pass an int pointer to the layer3 call. But then you take it's address inside layer3. That's not what you want. Again, the mixing up of ints and pointers isn't the root cause here but does contribute to the confusion. Your code should be something like this:

transmit(int* getdata){  //gets pointer value
    int g = layer3(getdata); //puts pointer into new function
}

layer3(int *b){
    int *w = b;

That is, don't change pointers to integers. Just pass the pointer directly around (specifically into layer3). The compiler warnings already hint this to your strongly. A good sign that you are on the right track is if you get rid of all those warnings.

A few more tips:

You don't have explicit return types declared for your functions. Always good practice to declare that explicitly.
Format your code properly (consistent spacing, etc). Especially when posting on Stackoverflow but really even more so for yourself.

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