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As the previous strcat/strncat/strlcat examples show, C can be very clumsy when it comes to concatening strings (or any kind of string manipulation, actually).
Using asprintf(3) is another, more convenient solution which is considered secure, and automatically allocates as much memory as you need.
The only thing you have to care about is to free the used memory when finished.
The drawback is that asprintf(3) isn't available on all platforms.
The source code below includes its own version of asprintf() if it can't be found in the system library (i.e. in stdio.h) on your operating system.

Tip: If you are new to pointers and memory allocation, you may want to read a pointer tutorial and a memory allocation tutorial.
You may also read the malloc(3)/free(3) man page.

concat-strings-asprintf.c:

/* Include unistd.h if not on MS Windows. */
#ifndef _WIN32
#include <unistd.h>
#endif

#include <errno.h>
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "asprintf.c"

/* Compile: cc -g -ansi -pedantic -Wall -O2 -o concat-strings-asprintf concat-strings-asprintf.c */
/* Run: ./concat-strings-asprintf */

int
main(int argc, char *argv[])
{
    char **pointer_to_full_name = NULL;
    char *full_name = NULL;
    int number_of_chars_printed = 0;
    
    /* Command line argument check. */
    if (argc != 3)
    {
        printf("Usage: %s <first_name> <last_name>\n", argv[0]);
        return 1;
    }
    /* Allocate enough memory for pointer-to-pointer to hold a pointer to char.  */
    if ((pointer_to_full_name = (char **)malloc(sizeof(pointer_to_full_name))) == NULL)
    {
        printf("malloc() error: %s\n", strerror(errno));
        return -1;
    }
    /* Good practice: always initialize *pointer_to_full_name. */
    *pointer_to_full_name = "\0";
    /* Concat strings using asprintf(). */
    if ((number_of_chars_printed = asprintf(pointer_to_full_name, "%s %s", argv[1], argv[2])) < 0)
    {
        printf("asprintf() error: %s\n", strerror(errno));
        free(pointer_to_full_name);
        return -1;
    }
    full_name = *pointer_to_full_name;
    printf("Your name is '%s'.\n", full_name);
    /* free() allocated memory. */
    free(full_name);
    free(pointer_to_full_name);

    return 0;
}

Copy the file below to the same directory as the file above, and it will be included if not present on your operating system.

asprintf.c:

/*
 * Copyright (c) 2004 Darren Tucker.
 *
 * Based originally on asprintf.c from OpenBSD:
 * Copyright (c) 1997 Todd C. Miller <Todd.Miller AT courtesan.com>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

extern int vsnprintf();

/* Include vasprintf() if not on your OS. */
#ifndef HAVE_VASPRINTF

#include <errno.h>
#include <limits.h>
#include <stdarg.h>
#include <stdlib.h>

#ifndef VA_COPY
# ifdef HAVE_VA_COPY
#  define VA_COPY(dest, src) va_copy(dest, src)
# else
#  ifdef HAVE___VA_COPY
#   define VA_COPY(dest, src) __va_copy(dest, src)
#  else
#   define VA_COPY(dest, src) (dest) = (src)
#  endif
# endif
#endif

#define INIT_SZ 128

int
vasprintf(char **str, const char *fmt, va_list ap)
{
        int ret = -1;
        va_list ap2;
        char *string, *newstr;
        size_t len;

        VA_COPY(ap2, ap);
        if ((string = malloc(INIT_SZ)) == NULL)
                goto fail;

        ret = vsnprintf(string, INIT_SZ, fmt, ap2);
        if (ret >= 0 && ret < INIT_SZ) { /* succeeded with initial alloc */
                *str = string;
        } else if (ret == INT_MAX || ret < 0) { /* Bad length */
                goto fail;
        } else {        /* bigger than initial, realloc allowing for nul */
                len = (size_t)ret + 1;
                if ((newstr = realloc(string, len)) == NULL) {
                        free(string);
                        goto fail;
                } else {
                        va_end(ap2);
                        VA_COPY(ap2, ap);
                        ret = vsnprintf(newstr, len, fmt, ap2);
                        if (ret >= 0 && (size_t)ret < len) {
                                *str = newstr;
                        } else { /* failed with realloc'ed string, give up */
                                free(newstr);
                                goto fail;
                        }
                }
        }
        va_end(ap2);
        return (ret);

fail:
        *str = NULL;
        errno = ENOMEM;
        va_end(ap2);
        return (-1);
}
#endif

/* Include asprintf() if not on your OS. */
#ifndef HAVE_ASPRINTF
int asprintf(char **str, const char *fmt, ...)
{
        va_list ap;
        int ret;
        
        *str = NULL;
        va_start(ap, fmt);
        ret = vasprintf(str, fmt, ap);
        va_end(ap);

        return ret;
}
#endif

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Last modified: Tue Dec 18 12:55:06 Romance Standard Time 2007