Concatenating strings in C

Question

How to concatenate strings in C, not like 1 + 1 = 2 but like 1 + 1 = 11.

Solution 1

I think you need string concatenation:

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

int main() {
  char str1[50] = "Hello ";
  char str2[] = "World";

  strcat(str1, str2);

  printf("str1: %s\n", str1);

  return 0;
}

from: http://irc.essex.ac.uk/www.iota-six.co.uk/c/g6_strcat_strncat.asp

Solution 2

To concatenate more than two strings, you can use sprintf, e.g.

char buffer[101];
sprintf(buffer, "%s%s%s%s", "this", " is", " my", " story");

Solution 3

Try taking a look at the strcat API. With sufficient buffer space, you can add one string onto the end of another one.

char[50] buffer;
strcpy(buffer, "1");
printf("%s\n", buffer); // prints 1
strcat(buffer, "1");
printf("%s\n", buffer); // prints 11

Reference page for strcat

• http://www.cplusplus.com/reference/clibrary/cstring/strcat/

Solution 4

'strcat' is the answer, but thought there should be an example that touches on the buffer-size issue explicitly.

#include <string.h>
#include <stdlib.h>

/* str1 and str2 are the strings that you want to concatenate... */

/* result buffer needs to be one larger than the combined length */
/* of the two strings */
char *result = malloc((strlen(str1) + strlen(str2) + 1));
strcpy(result, str1);
strcat(result, str2);

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Comments

[Jonathan Leffler]

Note that strncat() (with the 'n') is extremely hard to use correctly - so don't use it.

[Vinko Vrsalovic]

Why is it harder than counting your buffers in strcat?

[Vinko Vrsalovic]

sizeof(char) is 1, always. drj11.wordpress.com/2007/04/08/sizeofchar-is-1

[Tal Pressman]

@Jonathan: Don't know about "extremely hard", but it's much safer.

[Thomas]

not much safer. There is a reason strlcat() exists. Although that's not a complete fix either. It's just a drop-in improvement. I'd use snprintf(). It has the best behavior.

[Erich Kitzmueller]

ynimous: snprintf is not available everywhere

[RBerteig]

This leaks memory by overwriting the pointer result, and needs at least one strcpy() as well. Also, it extends str1, but the implication is that you intended both str1 and str2 to be read only.

[RBerteig]

strncat() doesn't guarantee the result is nul terminated. strlcat() does, but may not be widely available. strcat() is blissfully ignorant of the size of the destination buffer. In short, the C standard library is burdened by a lot of history that gets in the way of preventing buffer overruns.

[Johannes Schaub - litb]

many people can only rely on c89. sweet snprintf isn't available for them :(

[aib]

malloc() may return NULL, causing UB.

[Volomike]

If I didn't use the [50] above but used just [], the code still works. But is that a memory leak?