finsert, a function to insert data into the middle of a file...

finsert.c

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

int finsert(FILE *file, fpos_t position, void *object, size_t size) {
    unsigned char i[size], o[size];

    memcpy(o, object, size);
    if (fsetpos(file, &position)) {
        return -1;
    }

    while (size > 0) {
        size_t x = fread(i, 1, size, file);
        if (fsetpos(file, &position) || fwrite(o, 1, size, file) < size
                                     || fflush(file)
                                     || fgetpos(file, &position)) {
            return -1;
        }

        size = x;
        memcpy(o, i, size);
    }

    return 0;
}

int main(int argc, char **argv) {
    if (argc <= 3) {
        puts("Usage:  ./app filename position data");
        return 0;
    }

    FILE *file  = fopen(argv[1], "rb+");
    if (file == NULL) {
        puts("Error opening file...");
        return EXIT_FAILURE;
    }

    long offset = atoi(argv[2]);
    if (fseek(file, offset, SEEK_SET) == -1) {
        puts("Error seeking file...");
        return EXIT_FAILURE;
    }

    fpos_t pos;
    if (fgetpos(file, &pos)) {
        puts("Error saving seek state...");
        return EXIT_FAILURE;
    }
    
    size_t size = 0;
    for (size_t x = 3; x < argc; x++) {
        size += 1 + strlen(argv[x]);
    }

    unsigned char data[size];
    for (size_t x = 4, pos = sprintf(data, "%s", argv[3]); x < argc; x++) {
        pos += sprintf(data + pos, " %s", argv[x]);
    }

    if (finsert(file, pos, data, size)) {
        puts("Error modifying file! Possible file corruption...");
        return EXIT_FAILURE;
    }
}