Whats the difference between this lseek, fseek, read, fread?

Question

I am calling these functions:

unsigned blah[5];
lseek(0, 100, SEEK_CUR);
read(0, blah, sizeof(blah));

and

FILE *fr;
fr = fopen(         


        

Answer 1

I'm not a 100% sure about the difference, but it seems to be related to pipe vs stdio/file. To illustrate this, I made these 2 different test programs:

exam.c

#include 
#include 
#include 
#include 
#include 

int main(int argc, char **argv)
{
        char blah[5] = {0,};

        /* we're using stdin instead of pipeline  :( */
        lseek(0, 100, SEEK_CUR);
        read(0, blah, sizeof(blah));

        printf("%c%c%c%c%c\n", blah[0], blah[1], blah[2], blah[3], blah[4]);

        return 0;
}

test.c

#include 
#include 
#include 
#include 
#include 

void testA(const char *s)
{
        int f = -1;
        char blah[5] = {0,};

        f = open(s, O_RDONLY);

        if (-1 < f)
        {
                lseek(f, 100, SEEK_CUR);
                read(f, blah, sizeof(blah));
        }

        printf("%c%c%c%c%c\n", blah[0], blah[1], blah[2], blah[3], blah[4]);
}

void testB(const char *s)
{
        FILE *fp = NULL;
        char blah[5] = {0,};

        fp = fopen(s, "r");

        if (fp)
        {
                fseek(fp, 100, SEEK_CUR);
                fread(blah, 1, sizeof(blah), fp);
        }

        printf("%c%c%c%c%c\n", blah[0], blah[1], blah[2], blah[3], blah[4]);
}

int main(int argc, char **argv)
{
        testA(argv[1]);
        testB(argv[1]);

        return 0;
}

Then I created some test data.

data.txt

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

When I executed cat ./data.txt | ./exam, the output is

01234

When I executed ./test ./data.txt, I got

bcdef
bcdef

Just FYI, the results remain unchanged even if we replace SEEK_CUR with SEEK_SET.

However, ./exam <./data.txt results in

bcdef

which is legitimate.

I know this is not an acceptable answer without knowing why '0' brings up the data file's contents, but I hope it helps somehow.