Insert a Substring into a C String

Inserting a substring means making room inside an existing character array and copying the new text into that position. C has no standard string_insert() function, so the operation is built from string length and memory-move operations.

Insertion Rules Top ↑

• The destination array must have enough capacity for both strings plus the null terminator.
• The insertion position must be between 0 and the current string length.
• memmove() is useful because the source and destination ranges may overlap.

Insert a Substring at a Position Top ↑

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

int insert_text(
    char destination[],
    size_t capacity,
    const char *insert,
    size_t position
) {
    size_t original_length = strlen(destination);
    size_t insert_length = strlen(insert);

    if (position > original_length) {
        return 0;
    }

    if (original_length + insert_length + 1 > capacity) {
        return 0;
    }

    memmove(
        destination + position + insert_length,
        destination + position,
        original_length - position + 1
    );

    memcpy(destination + position, insert, insert_length);
    return 1;
}

int main(void) {
    char text[80] = "plusnet";

    if (insert_text(text, sizeof text, "2", 4)) {
        printf("%s\n", text);
    } else {
        printf("Insertion failed.\n");
    }

    return 0;
}

Output

plus2net

Insert a Word into a Sentence Top ↑

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

int insert_text(
    char destination[],
    size_t capacity,
    const char *insert,
    size_t position
) {
    size_t original_length = strlen(destination);
    size_t insert_length = strlen(insert);

    if (position > original_length ||
        original_length + insert_length + 1 > capacity) {
        return 0;
    }

    memmove(
        destination + position + insert_length,
        destination + position,
        original_length - position + 1
    );

    memcpy(destination + position, insert, insert_length);
    return 1;
}

int main(void) {
    char sentence[100] = "C programming";
    const char *word = "language ";

    if (insert_text(sentence, sizeof sentence, word, 2)) {
        printf("%s\n", sentence);
    }

    return 0;
}

Output

C language programming

See additional character-position exercises and string-copy examples.

Subhendu Mohapatra

Author & Instructor at plus2net

I write and maintain practical tutorials on Python, PHP, SQL, JavaScript, HTML, jQuery, and web development at plus2net. The tutorials focus on clear explanations, working examples, and code that readers can test and adapt while learning.

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