C CLP: Strings and Character Handling

This domain measures how well you handle NUL-terminated byte strings: the difference between array size and string length, which library functions guarantee termination, and the pitfalls of the character-classification functions.

Representation

• A string is a char array ending with a NUL character. strlen counts characters before the NUL; sizeof reports the array's full size, including the terminator.
• A string literal has static storage duration and type char[N], and modifying it is undefined. Adjacent literals are concatenated during translation.
• Escape sequences: \n, \t, \\, \', \", \0, octal such as \101, hex such as \x41. A hex escape consumes as many hex digits as follow, so "\x41B" is one out-of-range character, not the two characters AB.

<string.h> essentials

• Copying: strcpy is unbounded; strncpy(dst, src, n) copies up to n bytes and does not terminate dst when src is at least n bytes long.
• Comparing: strcmp/strncmp return a negative value, zero, or a positive value - never rely on exactly 1 or -1. memcmp compares raw bytes and does not stop at a NUL.
• Overlap: memmove handles overlapping regions; memcpy on overlapping regions is undefined.
• Searching and splitting: strchr, strrchr, strstr, strspn, strcspn, and strtok, which writes into its input and is not reentrant.

Worked example

char s[] = "hello";
char *p = "hello";
printf("%zu %zu %zu\n", sizeof s, strlen(s), sizeof p);

s is a six-element array (h e l l o plus terminator), so sizeof s is 6. strlen(s) is 5. p is a pointer, so sizeof p is the pointer size (commonly 8), not 6. The two objects are distinct: s is writable, while p points at a literal that must not be modified.

Traps

• Passing a buffer with no terminator to any str* function causes an out-of-bounds read.
• Treating strncpy as a safe strcpy without manually writing dst[n-1] = '\0'.
• toupper, isalpha, and friends require an unsigned char value or EOF; a negative char is undefined.
• Confusing sizeof on an array with strlen, especially after the array decays to a pointer.
• Assuming snprintf's return value is what was written - it is the length that would have been written.