c - Why am i getting this warning in "if (fd=fopen(fileName,"r") == NULL)"?

Question

FILE *fd;
if (fd=fopen(fileName,"r") == NULL)
{   
    printf("File failed to open");
    exit(1);
}

This is a code snippet. When I compile it with gcc, i get the following warning:-

warning: assignment makes pointer from integer without a cast

When I put fd=fopen(argv[2],"r") within brackets, the problem gets solved..

I am not able to understand where am i converting integer to pointer when the brackets are not put.

Answer 1

Due to operator precedence rules the condition is interpreted as fd=(fopen(fileName,"r") == NULL). The result of == is integer, fd is a pointer, thus the error message.

Consider the "extended" version of your code:

FILE *fd;
int ok;
fd = fopen(fileName, "r");
ok = fd == NULL;
// ...

Would you expect the last line to be interpreted as (ok = fd) == NULL, or ok = (fd == NULL)?