File modes in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C code when opening a file in "w" mode?

Consider the following C code snippet that opens a file in write mode and writes a string. What will be the content of the file after running this code?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("testfile.txt", "w");
    if (fp == NULL) return 1;
    fprintf(fp, "Hello, world!\n");
    fclose(fp);
    return 0;
}

A The file "testfile.txt" will contain the text "Hello, world!\n".

B The file "testfile.txt" will contain garbage data because the file was not opened in binary mode.

C The file "testfile.txt" will be empty because "w" mode does not write data.

D The program will crash because "w" mode cannot create files.

💡 Hint

Remember that "w" mode creates the file if it does not exist and truncates it if it does.

Opening a file in "w" mode creates the file if it does not exist or truncates it to zero length if it does. Then, writing to the file adds the given content. So the file will contain exactly the string written.

❓ Predict Output

intermediate

What happens when opening a file in "r" mode if the file does not exist?

What will the following C code print if "nofile.txt" does not exist?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("nofile.txt", "r");
    if (fp == NULL) {
        printf("File not found\n");
    } else {
        printf("File opened\n");
        fclose(fp);
    }
    return 0;
}

A The program crashes with segmentation fault

B File opened

C File not found

D The program prints nothing

💡 Hint

Opening a file in "r" mode requires the file to exist.

When opening a file in "r" mode, if the file does not exist, fopen returns NULL. So the program prints "File not found".

❓ Predict Output

advanced

What is the output of this code using "a" mode to append to a file?

Assuming "append.txt" initially contains "Line1\n", what will be the content of the file after running this code?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("append.txt", "a");
    if (fp == NULL) return 1;
    fprintf(fp, "Line2\n");
    fclose(fp);
    return 0;
}

A The file is empty

B "Line1\nLine2\n"

C "Line2\n"

D "Line1\n"

💡 Hint

The "a" mode appends data to the end of the file without deleting existing content.

Opening a file in "a" mode moves the write position to the end of the file. Writing adds new content after existing data. So the file will have the original content plus the new line appended.

❓ Predict Output

advanced

What error does this code raise when opening a file with mode "r+" if the file does not exist?

Consider this code snippet. What will happen if "data.txt" does not exist?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r+");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }
    fclose(fp);
    return 0;
}

A Error opening file: No such file or directory

B The file is created and opened successfully

C Segmentation fault

D No output, program runs silently

💡 Hint

The "r+" mode requires the file to exist for reading and writing.

Opening a file in "r+" mode requires the file to exist. If it does not, fopen returns NULL and perror prints the error message about missing file.

❓ Predict Output

expert

What is the content of the file after this code runs with mode "w+"?

Given the file "example.txt" initially contains "OldContent\n", what will be the content after running this code?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w+");
    if (fp == NULL) return 1;
    fprintf(fp, "NewContent\n");
    fseek(fp, 0, SEEK_SET);
    char buffer[20];
    fgets(buffer, sizeof(buffer), fp);
    printf("Read from file: %s", buffer);
    fclose(fp);
    return 0;
}

A The program crashes

B Read from file: OldContent\n

C Read from file: NewContent\n

D Read from file: (empty line)

💡 Hint

Opening with "w+" truncates the file, but reading immediately after writing requires a flush or reposition.

Opening a file with "w+" truncates it to zero length. Writing "NewContent\n" adds data, but the output buffer is not flushed before reading. The fseek resets the position, but the buffer may not be flushed, so fgets reads empty content. The printed line is empty after "Read from file:".