C Language Interview: File Operations and Memory Management

In the world of C programming, file operations and memory management are two crucial topics. Whether you are handling data, saving application states, or optimizing memory usage in high-performance computing, these two concepts are fundamental and indispensable. In this article, we will provide a detailed overview of these two topics, accompanied by code demonstrations to help basic users gain a deeper understanding.

1. File Operations

1. File Pointers

C uses the FILE structure to represent files, and we manipulate it through file pointers. The standard library <stdio.h> provides various functions to open, read, write, and close files.

#include <stdio.h>
int main() {    FILE *file; // Define a FILE pointer
    file = fopen("example.txt", "w"); // Open or create a text file for writing    if (file == NULL) {        printf("Cannot open or create file!\n");        return 1;    }
    fprintf(file, "Hello, World!\n"); // Write content to the file    fclose(file); // Close the file
    return 0;}

File Mode Descriptions:

"r": Read-only mode, opens an existing text file.
"w": Write mode, creates a new text file, clearing the content if it already exists.
"a": Append mode, adds new content after existing content.
"rb", "wb", "ab", etc.: Used for binary mode.

2. File Reading Example

The following code demonstrates how to read data from a text file:

#include <stdio.h>
int main() {    FILE *file;    char buffer[100];
    file = fopen("example.txt", "r"); // Open the text file for reading    if (file == NULL) {        printf("Cannot open the document!\n");        return 1;    }
    while (fgets(buffer, sizeof(buffer), file)) { // Read line by line from the beginning        printf("%s", buffer);    }
    fclose(file); // Close the document after reading
    return 0;}

2. Memory Management

In C, you need to manually allocate and free memory, which is a significant characteristic compared to other high-level programming languages. All dynamic memory operations are included in the <stdlib.h> library.

Examples of malloc and free functions:

malloc() is used to dynamically allocate a block of memory of a specified size, returning a void pointer that needs to be cast to the target type.
free() is used to release previously allocated memory to avoid memory leaks.

#include <stdio.h>
#include <stdlib.h>
int main() {    int n, i;
   printf("Please enter the number of integers: ");   scanf("%d", &n);
   int *arr = (int *)malloc(n * sizeof(int)); // Dynamically allocate an integer array
   if (arr == NULL) {        printf("Memory allocation failed!\n");       return -1;    }
   for(i = 0; i < n; i++) {       arr[i] = i + 1;   }
   for(i = 0; i < n; i++) {       printf("%d ", arr[i]);   }
   free(arr); // Free the allocated object
   return 0;}

3. Precautions

Memory Leak Detection:

Every time you use malloc or calloc to allocate dynamic memory, you must use free() to release it to avoid wasting resources.

Safety:

Always verify the success of input and output operations to ensure the program does not crash during execution. Additionally, prevent security risks such as buffer overflows by using fgets() instead of lower-level character arrays (scanf()). 

Conclusion

Familiarity with the basic knowledge of process-oriented and flexible control in C can effectively solve many practical problems in various environments. Future program development and maintenance will also become easier.

I hope this article helps you further understand and master the knowledge related to file operations and memory management in C, preparing you for upcoming interviews.