strcpy and strcat

Learn to copy with strcpy and concatenate with strcat.

Copying strings with strcpy

In C, you cannot copy strings later with =; you must use the strcpy function from <string.h>.

char src[] = "Hello";
char dst[20];
strcpy(dst, src);
/* dst: "Hello" */

The destination array must be at least one element larger than the source string (for the null). Otherwise, overflow occurs and behavior becomes undefined.

Joining strings with strcat

strcat appends the source to the end of the destination. The destination array must be large enough to hold the result.

char greeting[20] = "Hello, ";
strcat(greeting, "Anna");
/* greeting: "Hello, Anna" */

strcat finds the null in the destination, overwrites it, and places a new null at the end. If the destination is too small, overflow occurs; sizing matters.

Exercise Join first and last name

Define char full[40] and copy "Anna" into it with strcpy. Then append " Smith" with strcat. Print with printf %s; output should be Anna Smith.

Show example solution

#include <stdio.h>
#include <string.h>

int main(void) {
    char full[40];
    strcpy(full, "Anna");
    strcat(full, " Smith");
    printf("%s\n", full);
    return 0;
}

Test yourself

Which of these assignments causes a compile error?
- char a[] = "Hello";
- char b[20]; strcpy(b, a);
- char c[20] = a;
- char d[] = {'A','l','i','\0'};
Show answer

char c[20] = a; — You cannot copy between arrays with =; strcpy is needed.

strcpy call

strcpy(dst, ____);  /* copy from src to dst */

Show answer

strcpy(dst, src);  /* copy from src to dst */

Match strcpy and strcat

strcpy

copy
strcat

append
dst

destination array
src

source array

Next lesson: strcmp

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