Question

How come when I use the following method, to be used to convert all the characters in a string to uppercase,

while (*postcode) {
    *postcode = toupper(*postcode);

    postcode++;
}

Using the following argument works,

char wrong[20];
strcpy(wrong, "la1 4yt");

But the following, doesn’t, despite them being the same?

char* wrong = "la1 4yt";

My program crashes in an attempt to write to an illegal address (a segfault, I presume). Is it an issue with not mallocing? Not being null-terimanted? It shouldn’t be…

Through debugging I notice it crashes on the attempt to assign the first character as its uppercase.

Any help appreciated!

Answer 1

char* wrong = "la1 4yt";

This declares a pointer to a string constant. The constant cannot be modified, which is why your code crashes. If you wrote the more pedantic

const char* wrong = "la1 4yt"; // Better

then the compiler would catch the mistake. You should probably do this any time you declare a pointer to a string literal rather than creating an array.

This, on the other hand, allocates read/write storage for twenty characters so writing to the space is fine.

char wrong[20];

If you wanted to initialize it to the string above you could do so and then would be allowed to change it.

char wrong[20] = "la1 4yt"; // Can be modified
char wrong[]   = "la1 4yt"; // Can be modified; only as large as required