CIS241

System-Level Programming and Utilities

C - Memory and String Functions

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

New include for strings!

#include <string.h>

Clearing memory

Zero-ing out memory:

int* data = (int*)malloc(sizeof(int) * 32);
for(int i = 0; i < 32; i++){
    data[i] = 0
}

Alternative:

• memset(void* s, int c, size_t n);
• Fills n bytes of s with byte c

Does this work?

• memset(data, 1, 32 * sizeof(int));

Memory functions

Copy an array:

int* data = (int*)malloc(sizeof(int) * 32);
int* copy = (int*)malloc(sizeof(int) * 32);
for(int i = 0; i < 32; i++){
    copy[i] = data[i];
}

Or:

memcpy(void* dest, void* src, size_t n);

Copy n bytes:
memcpy(copy, data, 32 * sizeof(int));

Another alternative for copying an array

memmove(void* dest, void* src, size_t n);

Copy n bytes, allowing for overlapping buffers:

memmove(copy, data, 32 * sizeof(int));

Strings!

How are they different?

• \0 - null terminator!

String functions we'd like to have?

Concatenation

strcat(char *s1, char* s2);

Appends copy of s2 to s1

• What happens if s1 isn’t big enough?
• We are writing into arbitrary memory…

strncat(char* s1, char* s2, size_t n);

• Same, but will only copy n chars + terminator

Concatenation

Can also use string formatting

• sprintf(char *s1, char* format_str, ...);

• Same idea and formatting as printf

• Stores result in s1, adds terminator

• Returns new length of s1

sprintf(s, "x = %d", 5);

Copying

strcpy(char* dest, char* src);

• Copies src to dest, including '\0', too.
• Make sure dest has enough space!

strncpy(char* dest, char* src, size_t n);

• Copy up to n chars
• Will add '\0's to fill if src is too short
• Otherwise no terminators

Length

strlen(char* s);

• Returns length of string
• How does this work?
  • Returns number of characters before terminator!
  • (must be null-terminated!)

Comparison

int strcmp(char* s1, char* s2);

• Lexicographically compares strings
  • Return 0 if strings are the same
  • Returns negative num if s1 < s2
  • Returns positive num if s1 > s2

int strncmp(char* s1, char* s2, size_t n);

• Same, but limits to n chars (or to \0)

Searching

char* strchr(char *s, int c);

• Searches for first instance of char c in string
• Returns pointer to first instance
• Returns NULL if c not found

char* strrchr(char *s, int c);

• Returns last instance of c in s (or NULL if none)

Searching

char* strstr(char* s1, char* s2);

• Searches for substring s2 in s1.
• Returns pointer to first instance, or NULL if not found