How to write using fwrite without affecting existing content of file?

Basel · 10-30-2010, 11:11 AM

How can I write to a file multiple times using fwrite without affecting the previous writes?

The method shown below accepts a file name, buffer and offset. The method opens the file in reading/writing mode and writes the content of the buffer at offset.

Code:

int writeData(char *filename, char *buffer, int offset){
    FILE *fp;
    fp = fopen(filename, "w+"); // this truncates the file :(
    fseek(fp, offset, SEEK_SET);
    fwrite(buffer, sizeof(char), strlen(buffer), fp);
    int pos = ftell(fp);
    fclose(fp);
    return pos;
}

void main(){
    writeData("alpha", "ABCDEFGH", 0);
    writeData("alpha", "WXYZ", 2);
}

// content of file: [^@^@WXYZ]

However, the intended behavior should be "ABWXYZGH." Using the "r+" mode gives the intended behavior if the file exists, which is not always the case. As a workaround, I try to open in "r+" first and create the file if it does not exist:

Code:

int writeData(char *filename, char *buffer, int offset){
    FILE *fp;
    fp = fopen(filename, "r+");
    if (fp == NULL) {
        fp = fopen(filename, "w");
        if (fp) {
            fclose(fp);
            fp = fopen(filename, "r+");
        } else // do not have sufficient permission
            return -1;
    }
    fseek(fp, offset, SEEK_SET);
    fwrite(buffer, sizeof(char), strlen(buffer), fp);
    int pos = ftell(fp);
    fclose(fp);
    return pos;
}

I am not sure that's the right way of doing it.

eSelix · 10-30-2010, 11:13 AM

Use fopen(filename, "a") for appending at the end or replacing somewhere in the middle. But if you want to insert some bytes in the middle, you must read everything from insertion point to the buffer and then seek to insert position and write new bytes and buffer.