Create string from array of strings in C

There are two main ways you can do this:

• Do an initial pass over all the strings and do a sum of all the strlens
• Start with a NULL string and iterate over the strings. As you iterate you realloc the destination to accomodate the increase in size

Yes you had better to calculate all the space needed so you can malloc enough space for the new string.

After creating the space for all strings, just copy them with a space character maybe between them (but if you add spaces, don't forget to count them for the allocation ...)

There is a function that does this in the library over here: http://sourceforge.net/projects/libxc/

The actual function is documented here

(Yes, I am the owner of that project, no I don't make money of it)

You may want to consider the following commented code (live here on Ideone.com).

In summary:

1. Iterate through the input string array, computing the total length of the resulting string, summing the lengths of the input strings, using strlen().
2. Allocate memory for the resulting string, using malloc().
3. Iterate through the input array again, concatenating the input strings, using strcat().

#include <stdio.h>      /* For printf()             */
#include <stdlib.h>     /* For malloc(), free()     */
#include <string.h>     /* For strcat(), strlen()   */


/*
 * Concatenates input strings into a single string.
 * Returns the pointer to the resulting string.
 * The string memory is allocated with malloc(),
 * so the caller must release it using free().
 * The input string array must have NULL as last element.
 * If the input string array pointer is NULL, 
 * NULL is returned.
 * On memory allocation error, NULL is returned.
 */
char * ConcatenateStrings(const char** strings)
{
    int i = 0;              /* Loop index               */
    int count = 0;          /* Count of input strings   */
    char * result = NULL;   /* Result string            */
    int totalLength = 0;    /* Length of result string  */


    /* Check special case of NULL input pointer. */
    if (strings == NULL)
    {
        return NULL;
    }

    /* 
     * Iterate through the input string array,
     * calculating total required length for destination string.
     * Get the total string count, too.
     */
    while (strings[i] != NULL)
    {
        totalLength += strlen(strings[i]);
        i++;
    }
    count = i;
    totalLength++;  /* Consider NUL terminator. */

    /*
     * Allocate memory for the destination string.
     */
    result = malloc(sizeof(char) * totalLength);
    if (result == NULL) 
    {
        /* Memory allocation failed. */
        return NULL;
    }

    /*
     * Concatenate the input strings.
     */
    for (i = 0; i < count; i++) 
    {
        strcat(result, strings[i]);
    }

    return result;
}

/*
 * Tests the string concatenation function.
 */
int main(void)
{
    /* Result of string concatenation */
    char * result = NULL;

    /* Some test string array */
    const char * test[] = 
    {
        "Hello ",
        "world",
        "!",
        " ",
        "Ciao ",
        "mondo!",
        NULL                /* String array terminator */
    };

    /* Try string concatenation code. */
    result = ConcatenateStrings(test);

    /* Print result. */
    printf("%s\n", result);

    /* Release memory allocated by the concatenate function. */
    free(result);

    /* All right */
    return 0;
}