string.h
- C course
- Intermediate
- Lesson 83/116
- 4 min
Learn string functions in string.h.
string.h
The <string.h> header provides functions that work on C strings (char arrays): copy (strcpy), concatenate (strcat), length (strlen) and compare (strcmp).
In C a string is a null-terminated char array. strcpy(dst, src) copies src into dst. strcat(dst, src) appends src to the end of dst. strlen(s) returns the character count excluding null. strcmp(a, b) returns 0 if equal, negative if a is smaller, positive if larger. None of these check memory bounds; the destination array must be large enough. Modern C also has bounded variants like strncpy and strncat.
The strcpy and strcat example
To build a string from scratch, start with strcpy and add pieces with strcat. The destination array must be large enough.
Example: char buf[64]; strcpy(buf, "42"); strcat(buf, " + 58");. The first strcpy writes 42 into buf (3 bytes including null). strcat appends + 58; now buf holds 42 + 58. strlen(buf) returns 7 (excluding null). Output: 7 42 + 58. The 64-byte buf is plenty. If the destination were small, strcat would overflow and cause undefined behavior. Always check the destination size.
Practice Build a string
In main declare char buf[64];. Start with strcpy(buf, "42");. Append with strcat(buf, " + 58");. Print length and string with printf("%zu %s\n", strlen(buf), buf);.
Starter code
#include <stdio.h>
#include <string.h>
// main: char buf[64]; copy "42" with strcpy
// append " + 58" with strcat
// get length with strlen, print with printf
int main(void) {
// write here
return 0;
}Sample solution
#include <stdio.h>
#include <string.h>
int main(void) {
char buf[64];
strcpy(buf, "42");
strcat(buf, " + 58");
printf("%zu %s\n", strlen(buf), buf);
return 0;
}Expected output
7 42 + 58
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 string functions
| Term | Meaning |
|---|---|
strcpy |
copy |
strcat |
append |
strlen |
get length |
strcmp |
compare |
2 The strcat call
strcat(buf, ____);strcat(buf, " + 58");Explanation: strcat appends the second argument to the first.
3 What does strlen("42 + 58") return?
- 7 Correct answer
- 0
- 6
- 8
Explanation: strlen returns the character count excluding null: 7.