Question

I am trying to understand how a file position indicator moves after I read some bytes from a file. I have a file named “filename.dat” with a single line: “abcdefghijklmnopqrstuvwxyz” (without the quotes).

#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>


int main () {

    int fd = open("filename.dat", O_RDONLY);
    FILE* fp = fdopen(fd,"r");
    printf("ftell(fp): %ld, errno = %d\n", ftell(fp), errno);

    fseek(fp, 5, SEEK_SET); // advance 5 bytes from beginning of file
    printf("file position indicator: %ld, errno = %d\n", ftell(fp), errno);

    char buffer[100];
    int result = read(fd, buffer, 4); // read 4 bytes 
    printf("result = %d, buffer = %s, errno = %d\n", result, buffer, errno);
    printf("file position indicator: %ld, errno = %d\n", ftell(fp), errno);

    fseek(fp, 3, SEEK_CUR); // advance 3 bytes 
    printf("file position indicator: %ld, errno = %d\n", ftell(fp), errno);
    result = read(fd, buffer, 6);  // read 6 bytes 
    printf("result = %d, buffer = %s, errno = %d\n", result, buffer, errno);

    printf("file position indicator: %ld\n", ftell(fp));

    close(fd);
    return 0;
}


ftell(fp): 0, errno = 0
file position indicator: 5, errno = 0
result = 4, buffer = fghi, errno = 0
file position indicator: 5, errno = 0
file position indicator: 8, errno = 0
result = 0, buffer = fghi, errno = 0
file position indicator: 8

I do not understand why the second time I try to use read, I get no bytes from the file. Also, why does the file position indicator not move when I read contents from the file using read? On the second fseek, advancing 4 bytes instead of 3 did also not work. Any suggestions?

Answer 1

Use fseek and fread or lseek and read, but do not mix the two APIs, it won’t work.

A FILE* has its own internal buffer. fseek may or may not move the internal buffer pointer only. It is not guaranteed that the real file position indicator (one that lseek is responsible for) changes, and if it does, it is not known by how much.