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UPDATE: This version of strcat_new() has a bug. This bug has been fixed in the newer version. The newer version is called str_join(). You can find the newer version here: str_join() function, not present in the standard C library

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

#include "strcat_new.h"

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(const char *delim, long num_args, ...)
{

    va_list valist;
    long i = 0;
    long j = 0;
    long iica = 0; // iica - index into character array
    long len = 0;
    long delim_len = 0;
    long total_len = 0;
    long num_delim_to_concat = -1;
    char *new_char_array = NULL;
    char *temp = NULL;
    
    if (num_args <= 0)
        return NULL;

    if (delim) {
        delim_len = strlen(delim);
    }
    
    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        total_len = total_len + strlen(temp);
        num_delim_to_concat = num_delim_to_concat + 1;
    }
    va_end(valist);

    if (total_len == 0)
        return NULL;

    total_len = total_len + (num_delim_to_concat) * (delim_len);
    total_len = total_len + 1; // 1 extra for terminating null byte

    new_char_array = malloc(total_len);
    if (!new_char_array)
        return NULL;

    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        len = strlen(temp);
        for (j = 0; j < len; j++) {
            new_char_array[iica] = temp[j];
            iica++;
        }
        if (num_delim_to_concat) {
            for (j = 0; j < delim_len; j++) {
                new_char_array[iica] = delim[j];
                iica++;
            }
            num_delim_to_concat = num_delim_to_concat - 1;
        }
    }
    va_end(valist);

    new_char_array[iica] = 0;

    return new_char_array;

} // end of strcat_new

#ifndef _STRCAT_NEW_H_
#define _STRCAT_NEW_H_

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(const char *delim, long num_args, ...);

#endif

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

#include "strcat_new.h"

int main(void)
{

    char *a = strcat_new(";:?", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 3, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 0, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, -1, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new("=", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new("{(=%$^%^&&(&)}", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, "abc", NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, "abc", NULL, NULL, "cde");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, NULL, NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 4, NULL, NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 4, NULL, "fdr", NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 1, NULL, "fdr", NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    printf("\n");

}
 

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

#include "strcat_new.h"

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(char *delim, long num_args, ...)
{

    va_list valist;
    long i = 0;
    long j = 0;
    long iica = 0; // iica - index into character array
    long len = 0;
    long delim_len = 0;
    long total_len = 0;
    char *new_char_array = NULL;
    char *temp = NULL;
    
    if (num_args <= 0)
        return NULL;

    if (delim) {
        delim_len = strlen(delim);
    }
    
    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        total_len = total_len + strlen(temp) + delim_len;
    }
    va_end(valist);

    total_len = total_len - delim_len; // remove the last delimiter

    total_len = total_len + 1; // 1 extra for terminating null byte

    new_char_array = malloc(total_len);
    if (!new_char_array)
        return NULL;

    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        len = strlen(temp);
        for (j = 0; j < len; j++) {
            new_char_array[iica] = temp[j];
            iica++;
        }
        if (i < (num_args - 1)) {
            for (j = 0; j < delim_len; j++) {
                new_char_array[iica] = delim[j];
                iica++;
            }
        }
    }
    va_end(valist);

    new_char_array[iica] = 0;

    return new_char_array;

} // end of strcat_new

#ifndef _STRCAT_NEW_H_
#define _STRCAT_NEW_H_

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(char *delim, long num_args, ...);

#endif

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

#include "strcat_new.h"

int main(void)
{

    char *a = strcat_new(";:?", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 3, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 0, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, -1, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new("=", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new("{(=%$^%^&&(&)}", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    printf("\n");

}

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

#include "strcat_new.h"

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(char *delim, long num_args, ...)
{

    va_list valist;
    long i = 0;
    long j = 0;
    long iica = 0; // iica - index into character array
    long len = 0;
    long delim_len = 0;
    long total_len = 0;
    char *new_char_array = NULL;
    char *temp = NULL;
    
    if (num_args <= 0)
        return NULL;

    if (delim) {
        delim_len = strlen(delim);
    }
    
    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        total_len = total_len + strlen(temp) + delim_len;
    }
    va_end(valist);

    total_len = total_len - delim_len; // remove the last delimiter

    total_len = total_len + 1; // 1 extra for terminating null byte

    new_char_array = malloc(total_len);
    if (!new_char_array)
        return NULL;

    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        len = strlen(temp);
        for (j = 0; j < len; j++) {
            new_char_array[iica] = temp[j];
            iica++;
        }
        if (i < (num_args - 1)) {
            for (j = 0; j < delim_len; j++) {
                new_char_array[iica] = delim[j];
                iica++;
            }
        }
    }
    va_end(valist);

    new_char_array[iica] = 0;

    return new_char_array;

} // end of strcat_new

#ifndef _STRCAT_NEW_H_
#define _STRCAT_NEW_H_

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(char *delim, long num_args, ...);

#endif

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

#include "strcat_new.h"

int main(void)
{

    char *a = strcat_new(";:?", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 3, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 0, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, -1, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new("=", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new("{(=%$^%^&&(&)}", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    printf("\n");

}

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

#include "strcat_new.h"

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(const char *delim, long num_args, ...)
{

    va_list valist;
    long i = 0;
    long j = 0;
    long iica = 0; // iica - index into character array
    long len = 0;
    long delim_len = 0;
    long total_len = 0;
    long num_delim_to_concat = -1;
    char *new_char_array = NULL;
    char *temp = NULL;
    
    if (num_args <= 0)
        return NULL;

    if (delim) {
        delim_len = strlen(delim);
    }
    
    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        total_len = total_len + strlen(temp);
        num_delim_to_concat = num_delim_to_concat + 1;
    }
    va_end(valist);

    if (total_len == 0)
        return NULL;

    total_len = total_len + (num_delim_to_concat) * (delim_len);
    total_len = total_len + 1; // 1 extra for terminating null byte

    new_char_array = malloc(total_len);
    if (!new_char_array)
        return NULL;

    va_start(valist, num_args);
    for (i = 0; i < num_args; i++) {
        temp = va_arg(valist, char *);
        if (!temp) continue;
        len = strlen(temp);
        for (j = 0; j < len; j++) {
            new_char_array[iica] = temp[j];
            iica++;
        }
        if (num_delim_to_concat) {
            for (j = 0; j < delim_len; j++) {
                new_char_array[iica] = delim[j];
                iica++;
            }
            num_delim_to_concat = num_delim_to_concat - 1;
        }
    }
    va_end(valist);

    new_char_array[iica] = 0;

    return new_char_array;

} // end of strcat_new

#ifndef _STRCAT_NEW_H_
#define _STRCAT_NEW_H_

/* 
 * strcat_new:
 *
 * Parameters:
 *      num_args: number of variable arguments that are passed to this function
 *                excluding the 'delim' string.
 *      ...: Variable number of pointers to character arrays.
 *
 * Description:
 *      strcat_new concatenates all the strings/character arrays passed to it. If
 *      'delim' is not NULL then after every string, the 'delim' string is concatenated.
 *      It allocates a new character array whose size is equal to the sum of the
 *      lengths of all strings passed to it plus 1 (extra 1 for terminating null byte).
 *      It then concatenates all the strings passed to it separated by 'delim' string
 *      into the newly allocated character array and then returns the pointer to
 *      the newly allocated character array. If memory allocation fails then NULL is returned.
 *
 *      It is the responsibility of the caller to free the allocated memory.
 */
char *strcat_new(const char *delim, long num_args, ...);

#endif

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

#include "strcat_new.h"

int main(void)
{

    char *a = strcat_new(";:?", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 3, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(NULL, 0, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, -1, "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new("=", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new("{(=%$^%^&&(&)}", 4, "abc", "123", "xyz", "455");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, "abc", NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, "abc", NULL, NULL, "cde");
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, strlen(a));
    free(a);

    a = strcat_new(";;;", 4, NULL, NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 4, NULL, NULL, NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 4, NULL, "fdr", NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    a = strcat_new(NULL, 1, NULL, "fdr", NULL, NULL);
    printf("\n");
    printf("a = %s, strlen(a) = %lu\n", a, a?strlen(a):0);
    free(a);

    printf("\n");

}