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Formatted I/O Done Right

printf/scanf format specifiers, and why scanf("%s") is a security hole.

Examples: printf, snprintf, fgets, %d %s %zu

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printf Specifiers

Match the specifier to the type exactly — mismatches are undefined behavior.

Example

example

printf("%d\n",   42);        // int
printf("%u\n",   42u);       // unsigned
printf("%ld\n",  42L);       // long
printf("%zu\n",  sizeof(int)); // size_t
printf("%.3f\n", 3.14159);   // 3.142
printf("%p\n",   (void*)&x); // pointer
printf("%-10s|\n", "left");  // width + left align

Output

42
42
42
4
3.142
0x7ffd…
left      |

Reading Input Safely

Never use gets(). Prefer fgets + explicit parsing.

Example

example

char line[128];
if (fgets(line, sizeof line, stdin)) {
    line[strcspn(line, "\n")] = '\0';  // trim newline
    int n = atoi(line);
}

WATCH OUT

scanf("%s", buf) has no length limit — it will overflow buf. Use "%127s" or fgets.

snprintf for Building Strings

Always bounded, always NUL-terminated, returns the length it wanted.

Example

example

char buf[32];
int need = snprintf(buf, sizeof buf, "user-%d", id);
if (need >= (int)sizeof buf) {
    // output was truncated
}

Quick Check

Test yourself on specifier mismatches.

Quick check

What is the correct specifier for a size_t value?