C Programming, Part 4: Strings and Structs

Strings, Structs, and Gotcha's

So what's a string?

In C we have Null Terminated strings rather than Length Prefixed for historical reasons. What that means for your average everyday programming is that you need to remember the null character! A string in C is defined as a bunch of bytes until you reach '\0' or the Null Byte.

Two places for strings

Whenever you define a constant string (ie one in the form char* str = "constant") That string is stored in the data or code segment that is read-only meaning that any attempt to modify the string will cause a segfault.

If one however malloc's space, one can change that string to be whatever they want.

Memory Mismanagement

One common gotcha is when you write the following

char* hello_string = malloc(14);
                       ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
// hello_string ----> | g | a | r | b | a | g | e | g | a | r | b | a | g | e |
                       ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾
hello_string = "Hello Bhuvan!";
// (constant string in the text segment)
// hello_string ----> [ "H" , "e" , "l" , "l" , "o" , " " , "B" , "h" , "u" , "v" , "a" , "n" , "!" , "\0" ]
                       ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
// memory_leak -----> | g | a | r | b | a | g | e | g | a | r | b | a | g | e |
                       ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾ ‾‾‾
hello_string[9] = 't'; //segfault!!

What did we do? We allocated space for 14 bytes, reassigned the pointer and successfully segfaulted! Remember to keep track of what your pointers are doing. What you probably wanted to do was use a string.h function strcpy.

strcpy(hello_string, "Hello Bhuvan!");

Remember the NULL byte!

Forgetting to NULL terminate a string is a big affect on the strings! Bounds checking

String Information/Comparison: strlen strcmp

String Alteration: strcpy strcat strdup

String Search: strchr strstr

String Tokenize: strtok

So what's a struct?

What do these offsets mean?