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?

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); 

Added

All works fine with fseek.

#include <stdio.h>

int main(int argc, const char *argv[]) {
  if (argc < 2) {
    fprintf(stderr, "Usage: %s FILENAME\n", argv[0]);
    return 1;
  }
  FILE *fp;
  if((fp = fopen(argv[1], "r+b")) == NULL){
    fprintf(stderr, "Error:  Unable to open fle '%s'.\n", argv[1]);
    return 1;
  }
  char buf[17];
  size_t n_read = fread(buf, sizeof(char), sizeof buf - 1, fp);
  buf[n_read] = '\0';
  printf("Initial contents: %s\n", buf);

  fseek(fp, 0, SEEK_END);
  long fsize = ftell(fp);
  printf("Position at end of file: %ld\n", fsize);

  // Read first character in file.
  rewind(fp);
  char test;
  fread(&test, sizeof(char), 1, fp);

  ++test;
  fseek(fp, -1, SEEK_CUR);
  fwrite(&test, sizeof(char), 1, fp);

  rewind(fp);
  n_read = fread(buf, sizeof(char), sizeof buf - 1, fp);
  buf[n_read] = '\0';
  printf("Final contents: %s\n", buf);

  fclose(fp);
  return 0;
}

And then

$ gcc foo.c -o foo
$ ./foo foo.c
Initial contents: #include <stdio.
Position at end of file: 881
Final contents: $include <stdio.

Answer 2

Here fp is type of FILE *, but lseek take an int as first parameter.
So use open instead fopen:

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

int fp;

// Open return the file descriptor
if((fp = open(argv[1], O_RDWR | O_WRONLY)) == -1){
    printf("Error:  Unable to open file.\n");
    exit(0);
}

// Rest of the function

lseek(fp, -1, SEEK_CUR);    

---

[EDIT]

Another issue is to use fileno() function.
fileno() take a FILE * as parameter and return the file_descriptor (int) that you need for lseek() function.
You can use it like this:

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

FILE *fp;
int  fd;

// Open return the file descriptor
if((fp = fopen(argv[1], "r+b")) == NULL){
    printf("Error:  Unable to open file.\n");
    exit(0);
}
if ((fd = fileno(fp)) == -1){
    printf("Fileno Error\n");
    exit(0);
}

// Rest of the function

lseek(fd, -1, SEEK_CUR);

Here is the doc => https://linux.die.net/man/3/fileno