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c 3/5 30 XP

String Functions

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Quick Reference

String functions (#include <string.h>):

Function Purpose Gotcha
strlen(s) Length (not counting '\0') Returns size_t, not int
strcmp(a, b) Compare: <0, 0, >0 Never use == on strings
strcpy(dest, src) Copy src into dest No bounds checking
strncpy(dest, src, n) Bounded copy May not null-terminate
strcat(dest, src) Append src to dest No bounds checking
strcspn(s, reject) Index of first char in reject Used for newline stripping

Character functions (#include <ctype.h>):

Function Tests/Converts
isalpha(c) Is it a letter?
isdigit(c) Is it a digit?
isupper(c) / islower(c) Case check
toupper(c) / tolower(c) Case conversion

Safe input pattern:

char buf[100];
fgets(buf, sizeof(buf), stdin);           // Safe read
buf[strcspn(buf, "\n")] = '\0';           // Strip newline

Java → C string comparison:

Java C
s.equals("hello") strcmp(s, "hello") == 0
s.length() strlen(s)
s.charAt(i) s[i]
s1 + s2 strcpy then strcat

Common Pitfalls

  • == compares addresses, not content — Always use strcmp. Two identical strings at different addresses → == says false.
  • Buffer overflow with strcpy/strcat/scanf — No bounds checking. Use bounded variants or fgets.
  • Off-by-one: forgetting '\0' — A 5-character string needs a 6-byte buffer. Always allocate strlen + 1.
  • fgets includes the newline — Strip it with buf[strcspn(buf, "\n")] = '\0';.
  • Never use gets() — Removed from C11. CWE-242. Use fgets().

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