Insert a string into another string

Limitations of getchar() and scanf() for reading strings

scanf("%s")

• Stops reading at the first space, tab or newline, so a string with spaces ("I love Nepal") cannot be read in one call (only "I" is read).
• Does not check the array size; a long input overflows the array (undefined behaviour) unless a width such as %49s is given.
• Leaves the newline in the input buffer, which can disturb the next input (gets, getchar).

getchar()

• Reads only one character at a time, so a loop is needed until the newline or EOF.
• The programmer must add the '\0' at the end and also check the array limit.
• Characters typed after the loop ends stay in the buffer.

fgets() reads a whole line with spaces and checks the size, so it is used below.

Program

insertString(str1, str2, pos, res) copies the first pos characters of str1, then all of str2, then the rest of str1 into res, and returns the resulting string. main() displays the returned string.

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

/* insert str2 into str1 at position pos (0-based); returns the result */
char *insertString(char str1[], char str2[], int pos, char res[])
{
    int i, j = 0, k;
    for (i = 0; i < pos; i++)              /* part before position */
        res[j++] = str1[i];
    for (k = 0; str2[k] != '\0'; k++)      /* the new string */
        res[j++] = str2[k];
    for (; str1[i] != '\0'; i++)           /* rest of str1 */
        res[j++] = str1[i];
    res[j] = '\0';
    return res;
}

int main(void)
{
    char s1[100], s2[50], res[150];
    int pos;
    printf("Enter main string: ");
    fgets(s1, sizeof s1, stdin);
    s1[strcspn(s1, "\n")] = '\0';
    printf("Enter string to insert: ");
    fgets(s2, sizeof s2, stdin);
    s2[strcspn(s2, "\n")] = '\0';
    printf("Enter position (0 to %d): ", (int)strlen(s1));
    scanf("%d", &pos);
    if (pos < 0 || pos > (int)strlen(s1)) {
        printf("Invalid position\n");
        return 1;
    }
    printf("Resulting string: %s\n", insertString(s1, s2, pos, res));
    return 0;
}

Sample run (insert "really " at position 7)

Input (three lines): I love Nepal, then really (with a trailing space), then 7

Enter main string: Enter string to insert: Enter position (0 to 12): Resulting string: I love really Nepal