How do I concatenate const/literal strings in C?

In C, "strings" are just plain char arrays. Therefore, you can't directly concatenate them with other "strings".

You can use the strcat function, which appends the string pointed to by src to the end of the string pointed to by dest:

char *strcat(char *dest, const char *src);

Here is an example from cplusplus.com:

char str[80];
strcpy(str, "these ");
strcat(str, "strings ");
strcat(str, "are ");
strcat(str, "concatenated.");

For the first parameter, you need to provide the destination buffer itself. The destination buffer must be a char array buffer. E.g.: char buffer[1024];

Make sure that the first parameter has enough space to store what you're trying to copy into it. If available to you, it is safer to use functions like: strcpy_s and strcat_s where you explicitly have to specify the size of the destination buffer.

Note: A string literal cannot be used as a buffer, since it is a constant. Thus, you always have to allocate a char array for the buffer.

The return value of strcat can simply be ignored, it merely returns the same pointer as was passed in as the first argument. It is there for convenience, and allows you to chain the calls into one line of code:

strcat(strcat(str, foo), bar);

So your problem could be solved as follows:

char *foo = "foo";
char *bar = "bar";
char str[80];
strcpy(str, "TEXT ");
strcat(str, foo);
strcat(str, bar);

Avoid using strcat in C code. The cleanest and, most importantly, the safest way is to use snprintf:

char buf[256];
snprintf(buf, sizeof(buf), "%s%s%s%s", str1, str2, str3, str4);

Some commenters raised an issue that the number of arguments may not match the format string and the code will still compile, but most compilers already issue a warning if this is the case.

Folks, use strncpy(), strncat(), or snprintf().
Exceeding your buffer space will trash whatever else follows in memory!
(And remember to allow space for the trailing null '\0' character!)

Also malloc and realloc are useful if you don't know ahead of time how many strings are being concatenated.

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

void example(const char *header, const char **words, size_t num_words)
{
    size_t message_len = strlen(header) + 1; /* + 1 for terminating NULL */
    char *message = (char*) malloc(message_len);
    strncat(message, header, message_len);

    for(int i = 0; i < num_words; ++i)
    {
       message_len += 1 + strlen(words[i]); /* 1 + for separator ';' */
       message = (char*) realloc(message, message_len);
       strncat(strncat(message, ";", message_len), words[i], message_len);
    }

    puts(message);

    free(message);
}

It is undefined behaviour to attempt to modify string literals, which is what something like:

strcat ("Hello, ", name);

will attempt to do. It will try to tack on the name string to the end of the string literal "Hello, ", which is not well defined.

Try something this. It achieves what you appear to be trying to do:

char message[1000];
strcpy (message, "TEXT ");
strcat (message, var);

This creates a buffer area that is allowed to be modified and then copies both the string literal and other text to it. Just be careful with buffer overflows. If you control the input data (or check it before-hand), it's fine to use fixed length buffers like I have.

Otherwise, you should use mitigation strategies such as allocating enough memory from the heap to ensure you can handle it. In other words, something like:

const static char TEXT[] = "TEXT ";

// Make *sure* you have enough space.

char *message = malloc (sizeof(TEXT) + strlen(var) + 1);
if (message == NULL)
     handleOutOfMemoryIntelligently();
strcpy (message, TEXT);
strcat (message, var);

// Need to free message at some point after you're done with it.

If you have experience in C you will notice that strings are only char arrays where the last character is a null character.

Now that is quite inconvenient as you have to find the last character in order to append something. strcat will do that for you.

So strcat searches through the first argument for a null character. Then it will replace this with the second argument's content (until that ends in a null).

Now let's go through your code:

message = strcat("TEXT " + var);

Here you are adding something to the pointer to the text "TEXT" (the type of "TEXT" is const char*. A pointer.).

That will usually not work. Also modifying the "TEXT" array will not work as it is usually placed in a constant segment.

message2 = strcat(strcat("TEXT ", foo), strcat(" TEXT ", bar));

That might work better, except that you are again trying to modify static texts. strcat is not allocating new memory for the result.

I would propose to do something like this instead:

sprintf(message2, "TEXT %s TEXT %s", foo, bar);

Read the documentation of sprintf to check for it's options.

And now an important point:

Ensure that the buffer has enough space to hold the text AND the null character. There are a couple of functions that can help you, e.g., strncat and special versions of printf that allocate the buffer for you. Not ensuring the buffer size will lead to memory corruption and remotely exploitable bugs.

Do not forget to initialize the output buffer. The first argument to strcat must be a null terminated string with enough extra space allocated for the resulting string:

char out[1024] = ""; // must be initialized
strcat( out, null_terminated_string ); 
// null_terminated_string has less than 1023 chars

The first argument of strcat() needs to be able to hold enough space for the concatenated string. So allocate a buffer with enough space to receive the result.

char bigEnough[64] = "";

strcat(bigEnough, "TEXT");
strcat(bigEnough, foo);

/* and so on */

strcat() will concatenate the second argument with the first argument, and store the result in the first argument, the returned char* is simply this first argument, and only for your convenience.

You do not get a newly allocated string with the first and second argument concatenated, which I'd guess you expected based on your code.

You are trying to copy a string into an address that is statically allocated. You need to cat into a buffer.

Specifically:

...snip...

destination

Pointer to the destination array, which should contain a C string, and be large enough to contain the concatenated resulting string.

...snip...

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

There's an example here as well.