fseek() in C: Random Access File Positioning with SEEK_SET, SEEK_CUR, SEEK_END

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fseek() in C: Random Access File Positioning with SEEK_SET, SEEK_CUR, SEEK_END

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Lesson format12 sections1 video4 code blocks1 practice itemUpdated Aug 27, 2026

fseek moves the file position indicator to an arbitrary byte offset. Three reference points: SEEK_SET (beginning of file), SEEK_CUR (current position), SEEK_END (end of file). Combined with ftell, which reports the current position, you can build index tables, skip headers, jump to records, and implement random access over binary data. This is how databases, file format parsers, and media players navigate files without reading them sequentially.

fseek() Function

#Jumping to a Specific Offset#link

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("notes.txt", "r");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }

    fseek(fp, 3, SEEK_SET);
    int ch = fgetc(fp);
    printf("Byte at offset 3: '%c' (ASCII %d)\n", ch, ch);

    fseek(fp, -1, SEEK_END);
    ch = fgetc(fp);
    printf("Last byte: '%c' (ASCII %d)\n", ch, ch);

    fseek(fp, 0, SEEK_SET);
    fseek(fp, 5, SEEK_CUR);
    ch = fgetc(fp);
    printf("Byte at offset 5: '%c' (ASCII %d)\n", ch, ch);

    fclose(fp);
    return 0;
}

Run fseek_demo

root@vulnarex:~#gcc -Wall fseek_demo.c -o fseek_demo && ./fseek_demo

#Getting File Size with fseek and ftell#link

A common idiom: seek to the end, ask for the position (which equals the file size), then seek back to the start. This gives you the file length without reading the entire contents.

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("notes.txt", "rb");
    if (fp == NULL) {
        perror("fopen");
        return 1;
    }

    fseek(fp, 0, SEEK_END);
    long file_size = ftell(fp);
    rewind(fp);

    printf("File size: %ld bytes\n", file_size);

    fclose(fp);
    return 0;
}

Run file_size demo

root@vulnarex:~#gcc -Wall file_size.c -o file_size && ./file_size

STRICT SECURE AUDIT RULE

ftell returns long, which is 32 bits on some platforms. Files larger than 2 GB overflow a long. For large files, use fseeko/ftello with off_t (POSIX) or _fseeki64/_ftelli64 (Windows). In new code targeting 64-bit systems, this is less of a concern, but embedded and legacy 32-bit targets still hit this wall.

★ 30 XP

quiz BLOCK (★ 30 XP)

What does fseek(fp, -10, SEEK_END) do?

Select your proof vectors above

fseek and ftell together give you random access. You can build file indexes, skip fixed-size headers, and jump directly to the record you need without scanning everything before it. This is the foundation of binary file format parsing. Next: binary file access, where we read and write raw structs and byte arrays.

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Jumping to a Specific Offset Getting File Size with fseek and ftell

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August 27, 2026

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Laboratory sanity code

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#include <stdio.h>

int main(void) {

FILE *fp = fopen("notes.txt", "r");

if (fp == NULL) {

perror("fopen");

return 1;

}

fseek(fp, 3, SEEK_SET);

int ch = fgetc(fp);

printf("Byte at offset 3: '%c' (ASCII %d)\n", ch, ch);

fseek(fp, -1, SEEK_END);

ch = fgetc(fp);

printf("Last byte: '%c' (ASCII %d)\n", ch, ch);

fseek(fp, 0, SEEK_SET);

fseek(fp, 5, SEEK_CUR);

ch = fgetc(fp);

printf("Byte at offset 5: '%c' (ASCII %d)\n", ch, ch);

fclose(fp);

return 0;

}

#include <stdio.h>

int main(void) {

FILE *fp = fopen("notes.txt", "rb");

if (fp == NULL) {

perror("fopen");

return 1;

}

fseek(fp, 0, SEEK_END);

long file_size = ftell(fp);

rewind(fp);

printf("File size: %ld bytes\n", file_size);

fclose(fp);

return 0;

}