char **

[Prodiga1]

I'm working with code that grabs strings from a file and then works with said strings in different ways.

I would like to check if the first character in a char* is a #:

FILE* file = file_open(filename);
char* fileline = malloc(160);
file_getline(fileline,file);
printf("Test: %s\n", &fileline[0]);

Where the two methods are:

/***************
 * These functions are just some handy file functions.
 * We have not yet covered opening and reading from files in C,
 * so we're saving you the pain of dealing with it, for now.
 *******/
FILE* file_open(char* filename) {
        FILE* fp = fopen(filename, "r");
        if(fp == NULL) {
                fprintf(stderr, "Error opening file %s, abort.\n", filename);
                exit(1);
        }
        return fp;
}

/*******************
 * To use this function properly, create a char* and malloc
 * 160 bytes for it. Then pass that char* in as the argument
 ******************/
char* file_getline(char* buffer, FILE* fp) {
        buffer = fgets(buffer, 160, fp);
        return buffer;
}

But when I reference the text with &fileline[0], it prints out the entire string.

Moreover, why is this string suddenly a char** when I attempt to print it? If i just do printf("Test: %s\n", filename) the code works, but if i try printf("Test: %s\n", filename[0]) it yells at me.

What am I missing here?

[anduril462]

char *fileline = malloc(160)

This is a pointer to a char. It points to a contiguous block of 160 chars, that can be treated as an array through indexing, like so: filename[i].

&filename[0] is the address of the first (zeroth) element in the array, which is synonymous with filename.

filename[0] is the first element in the array, which is just a plain char. %s requires a char * pointing to the beginning of a null-terminated array of chars. Use %c for printing an individual character like fileline[0].

[itsme86]

That's because fileline[0] is only a char. The %s format specifier tells printf() to print a string. If you want to just print the first character of a string, then use %c:

printf("%c\n", fileline[0]);

Or...

if(fileline[0] == '#')  // Equivalent to if(*fileline == '#')
  // Do something

Likewise with the filename. filename[0] is a char, not a char *, so you should be using %c to print it instead of %s.

[Prodiga1]

Thanks!

[CommonTater]

It's a bit of C screwyness...

filename.... is the address of the array holding the string.

filename[10] is the 11th character in the string and returns only 1 character by value.

Essentially the [] notation dereferences the address + offset for you.

Sooo...
printf("%s", filename); is correct.
printf("%c",filename[1]); is correct.
printf("%s", &filename[1]); is correct.

I know it's confusing... I trip over it all the time...

For example... testing for # as the first character:

if (filename[0] == '#')
  DoMyFancyThingHere();