Lære Opening Files with fopen | Introduction and File Streams

The fopen function is used in C to open files for reading, writing, or appending. It is part of the standard library and allows you to create a connection between your program and a file on disk. When you call fopen, you specify both the name of the file and the mode in which you want to open it, such as reading or writing. The function returns a pointer of type FILE *, which you then use with other file handling functions to perform operations on the file.

main.c


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#include <stdio.h>

int main() {
    FILE *file; 
    file = fopen("data.txt", "r");

    if (file == NULL) {
        fprintf(stderr, "Failed to open file.\n");
        return 1;
    }

    printf("File opened successfully.\n");
    return 0;
}

"r"

Open a file for reading. The file must exist;

"w"

Open a file for writing. If the file exists, its contents are erased. If it does not exist, a new file is created;

"a"

Open a file for appending. Data is added to the end of the file if it exists, or a new file is created;

"rb"

Open a file for reading in binary mode. The file must exist;

"wb"

Open a file for writing in binary mode. If the file exists, its contents are erased, or a new file is created;

"ab"

Open a file for appending in binary mode. Data is added to the end of the file, or a new file is created.

Check fopen Return Value

Always check if fopen returns NULL before performing any file operations. If fopen fails to open the file, it returns NULL, which can prevent your program from crashing or corrupting data.

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