Files

Write And Read

Opening a file with w+ lets the program write content, rewind, and read it back.

write mode `w+` creates a writable and readable file for this small example.
rewind After writing, `rewind` moves the stream position back to the beginning.

Write And Read

count
write_read.c
Replay: real traced execution (multi-file project)
#include <stdio.h>

int main(void) {
    int count = 2;
    FILE *file = fopen("write_read_demo.txt", "w+");

    if (file == 0) {
        return 1;
    }

    for (int i = 1; i <= count; i++) {
        fprintf(file, "%d\n", i);
    }

    rewind(file);

    int lines = 0;
    int value = 0;
    while (fscanf(file, "%d", &value) == 1) {
        lines++;
    }

    fclose(file);
    remove("write_read_demo.txt");

    printf("lines=%d\n", lines);
    return 0;
}

#include <stdio.h>

int main(void) {
    int count = 3;
    FILE *file = fopen("write_read_demo.txt", "w+");

    if (file == 0) {
        return 1;
    }

    for (int i = 1; i <= count; i++) {
        fprintf(file, "%d\n", i);
    }

    rewind(file);

    int lines = 0;
    int value = 0;
    while (fscanf(file, "%d", &value) == 1) {
        lines++;
    }

    fclose(file);
    remove("write_read_demo.txt");

    printf("lines=%d\n", lines);
    return 0;
}

#include <stdio.h>

int main(void) {
    int count = 4;
    FILE *file = fopen("write_read_demo.txt", "w+");

    if (file == 0) {
        return 1;
    }

    for (int i = 1; i <= count; i++) {
        fprintf(file, "%d\n", i);
    }

    rewind(file);

    int lines = 0;
    int value = 0;
    while (fscanf(file, "%d", &value) == 1) {
        lines++;
    }

    fclose(file);
    remove("write_read_demo.txt");

    printf("lines=%d\n", lines);
    return 0;
}

  1. count ← 2, file ← ⟨addr A⟩

    3int main(void) {4    int count→ 2 = 2; //@count=3, 45    FILE *file→ ⟨addr A⟩ = fopen("write_read_demo.txt", "w+");

  2. for (int i = 1; i <= count; i++)

    pass 1 of 2
    11for (int i1 = 1; i <= count2; i++) {12    fprintf(file⟨addr A⟩, "%d\n", i1);13}

  3. for (int i = 1; i <= count; i++)

    pass 2 of 2
    11for (int i2 = 1; i <= count2; i++) {12    fprintf(file⟨addr A⟩, "%d\n", i2);13}

  4. lines ← 0, value ← 0

    15rewind(file⟨addr A⟩);1617int lines→ 0 = 0;18int value→ 0 = 0;19while (fscanf(file, "%d", &value) == 1) {

  5. lines ← 1

    pass 1 of 2
    18int value = 0;19while (fscanf(file⟨addr A⟩, "%d", &value1) == 1) {20    lines→ 1++;21}

  6. lines ← 2

    pass 2 of 2
    18int value = 0;19while (fscanf(file⟨addr A⟩, "%d", &value2) == 1) {20    lines→ 2++;21}

  7. fclose(file);

    23    fclose(file⟨addr A⟩);24    remove("write_read_demo.txt");2526    printf("lines=%d\n", lines2);27    return 0;28}
    outputlines=2

  1. count ← 3, file ← ⟨addr A⟩

    3int main(void) {4    int count→ 3 = 3;5    FILE *file→ ⟨addr A⟩ = fopen("write_read_demo.txt", "w+");

  2. for (int i = 1; i <= count; i++)

    pass 1 of 3
    11for (int i1 = 1; i <= count3; i++) {12    fprintf(file⟨addr A⟩, "%d\n", i1);13}
    All 3 passes — pass 1 is the card above
    passi
    11
    22
    33

  3. lines ← 0, value ← 0

    15rewind(file⟨addr A⟩);1617int lines→ 0 = 0;18int value→ 0 = 0;19while (fscanf(file, "%d", &value) == 1) {

  4. lines ← 1

    pass 1 of 3
    18int value = 0;19while (fscanf(file⟨addr A⟩, "%d", &value1) == 1) {20    lines→ 1++;21}
    All 3 passes — pass 1 is the card above
    passvaluelines
    110 → 1
    221 → 2
    332 → 3

  5. fclose(file);

    23    fclose(file⟨addr A⟩);24    remove("write_read_demo.txt");2526    printf("lines=%d\n", lines3);27    return 0;28}
    outputlines=3

  1. count ← 4, file ← ⟨addr A⟩

    3int main(void) {4    int count→ 4 = 4;5    FILE *file→ ⟨addr A⟩ = fopen("write_read_demo.txt", "w+");

  2. for (int i = 1; i <= count; i++)

    pass 1 of 4
    11for (int i1 = 1; i <= count4; i++) {12    fprintf(file⟨addr A⟩, "%d\n", i1);13}
    All 4 passes — pass 1 is the card above
    passi
    11
    22
    33
    44

  3. lines ← 0, value ← 0

    15rewind(file⟨addr A⟩);1617int lines→ 0 = 0;18int value→ 0 = 0;19while (fscanf(file, "%d", &value) == 1) {

  4. lines ← 1

    pass 1 of 4
    18int value = 0;19while (fscanf(file⟨addr A⟩, "%d", &value1) == 1) {20    lines→ 1++;21}
    All 4 passes — pass 1 is the card above
    passvaluelines
    110 → 1
    221 → 2
    332 → 3
    443 → 4

  5. fclose(file);

    23    fclose(file⟨addr A⟩);24    remove("write_read_demo.txt");2526    printf("lines=%d\n", lines4);27    return 0;28}
    outputlines=4