lseek() Trying to use with a byte file but pointer is of FILE type

Question

This is for a project for everyone's awareness. It's my first project in C and have a question regarding

lseek()

and moving the file pointer.

Right now I'm able to read the bitmap and DIB header of a bitmap file. I need to now traverse the pixels and manipulate them in certain ways. I have written out in pseudocode how I plan to tackle the manipulation. But I am having difficulty understanding how to properly use

lseek

, as I keep getting

incompatible pointer to integer conversion...

warnings with my code. I'll give a short example of my main method since this is for a project:

int main(int argc, const char * argv[]) {



    FILE *fp;



    if((fp = fopen(argv[1], "r+b")) == NULL){

        printf("Error:  Unable to open file.\n");

        exit(0);

    }



    fseek(fp, 0, SEEK_END);

    long fsize = ftell(fp);

    rewind(fp);



    char test;  // simply to test overwriting the current byte



    fread(&test, sizeof(char), 1, fp);

    test = ~test;

    lseek(fp, -1, SEEK_CUR); //produces error

    //also tried fseek prior to realizing I should be using lseek to move my pointer.

    fwrite(&test, 1, sizeof(char), fp);



    fclose(fp);

    return 0;

}

Again, not trying to provide too much of my code since this is a project. But I'd like help understanding how to properly use

lseek

please. I noticed that it returns an

int

value, so I know that it is because my file pointer is of type

FILE

. But what is the proper way to use this method? I saw one example that had opened the same file in both read and write mode, where write mode was using

lseek

. But for some reason I don't know if that is correct or not.

EDITED
Based on two members response, I changed my code above to:

rewind(fp);

lseek(fileno(fp), hdr.offset, SEEK_CUR); //hdr.offset == 54 bytes

printf("FD FILENO:  %d\n", fileno(fp)); // prints 3???

printf("CURRENT POS: %p\n", fp);  //prints 0x7fffe7eae0b0 (I understand it's an address)

fread(&test, sizeof(char), 1, fp);

lseek(fileno(fp), -1, SEEK_CUR);

fwrite(&test, 1, sizeof(char), fp);

printf("CURRENT POS: %p\n", fp);  //prints the same address as above?

What am I not getting other than everything to do with C?

Recommended for you: Get network issues from WhatsUp Gold. Not end users.

Answer 1

If you'd like to keep using FILE *fp, which is probably a good idea, you can get convert fp to its corresponding int file descriptor with int fileno(FILE *stream). In other words,

lseek(fileno(fp), -1, SEEK_CUR);

lseek() Trying to use with a byte file but pointer is of FILE type

Sign up