Fseek on C problem

Question

I'm testing this code, but doesn't work, it always says that an error occurred :S

int main(int argc, char **argv) {
    FILE *file_pointer;
    file_pointer = fopen("text.txt","r");
    if(fseek(file_pointer, 0, -1)) {
        puts("An error occurred");
    }
    else {
        char buffer[100];
        fgets(buffer, 100, file_pointer);
        puts("The first line of the file is:");
        puts(buffer);
    }
    fclose(file_pointer);
    return 0;
}

Answer 1

Have you checked that the file opened correctly?
ie if file_pointer is null?

Typical C usage would be something like

FILE *file_pointer;
if ( !(file_pointer=fopen("text.txt","r")) ) {
    puts("Error opening file");
    puts(strerror(errno)); /* prints the system error message */
    return 1; /* returning non-zero exits the program as failed */
}

if(fseek(file_pointer, 0, -1)) {
    puts("An error occurred");
}

ps You should use the macros SEEK_SET,SEEK_CUR,SEEK_END in fseek rather than the -1

Answer 2

fseek() returns 0 if it succeeds.

Answer 3

Why do you use -1 for the third parameter of fseek? It should be any of SEEK_SET, SEEK_CUR or SEEK_END.

As per your code, this should be SEEK_SET, that seeks to the beginning of the file, but using 0 as in your case leaves the pointer just at the beginning of the file.

Answer 4

The problem was here:

 if(fseek(file_pointer, 0, 1))

how can i, using fgets, know when the line it's over??

i try this:

       while(fp=='\n'){
                       printf("%s\n", firstline);
                      }

but doesn't work

Answer 5

You are opening you file in text mode (since you haven't specified and explicit b flag for fopen). For files opened in text mode the functionality of fseek is limited. The last parameter can only be SEEK_SET and nothing else. The position, if specified by a user-constructed value, must be 0 and nothing else.

You obviously satisfied the latter requirement, but what is that -1 doing there is not clear.

7.19.9.2 The fseek function
4 For a text stream, either offset shall be zero, or offset shall be a value returned by an earlier successful call to the ftell function on a stream associated with the same file and whence shall be SEEK_SET.