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I need to write a program that asks the user to enter strings, each string ends when the user presses 'Enter'.

  • The program needs to receive the file name as a parameter, the file should be opened and closed for each operation and for every string entered, the program should append the string to the end of the file (on a new line).

This is my code so far:

 int is_file_exists(char *file_name) { FILE *file; if ((file = fopen(file_name,"r"))!=NULL) { /* file exists */ fclose(file); return 1; } else { //File not found, no memory leak since 'file' == NULL //fclose(file) would cause an error return 0; } } int main(int argc, char **argv) { char c; FILE *file; if (argc >= 2) { if (is_file_exists(argv[1])) { file = fopen(argv[1], "w"); } else { return 0; } } else { file = fopen("file.txt", "w"); } while ((c = getchar()) != EOF) { putc(c, file); } return 0; }

So far the code compiles and file is being created, but nothing is being written inside of it.

Edit: I also need some function pointers, see my comments on selected answer

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    Firstly c needs to be an int as EOF is an int. As it is, the loop will never end. Secondly, are you exiting the program and if so how? I ask because you do not close or flush the file. If you have not exited the program or if you force kill the program it may not correctly write the file. Commented Feb 28, 2021 at 4:15
  • have you checked the content of file.txt at the current location of your terminal? Commented Feb 28, 2021 at 4:22
  • @AntoninGAVREL yes. I'm re-checking by "ls" and "gedit" or "cat". Commented Feb 28, 2021 at 4:23
  • @kaylum I have changed it to int but it still does not write to the file. I'm exiting the program by CTRL+Z and later i'll need to add a special input of "exit" that will terminate the program if typed. Commented Feb 28, 2021 at 4:24
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    On linux? ctrl-z suspends the program. Try ctrl-d to send the EOF. Commented Feb 28, 2021 at 4:25

2 Answers 2

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I think one of the problem was that you were opening and closing a file, and then reopening it subsequently. It is better to just leave it open using a pointer while simultaneously testing that there were no issue to open the file. Another problem was that you were writing in the file, don't you prefer to append text to it? Well it's your decision. As for the code:

#include <stdio.h> #include <string.h> #include <stdlib.h> // exit typedef struct mystruct { char *exit_word; void (*exit_fptr)(int); // man exit int (*strcmp_fptr)(const char *, const char*); // man strcmp } t_mystruct; int is_file_exists(char *filename, FILE **file) { return (*file = fopen(filename,"a")) > 0; } #define BUFF_SIZE 1024 int main(int argc, char **argv) { char c; FILE *file; t_mystruct s = {.exit_word = "-exit", .exit_fptr = &exit, .strcmp_fptr = &strcmp}; if (argc >= 2) { if (!(is_file_exists(argv[1], &file))) return 0; } else file = fopen("file.txt", "a"); // open the file in append mode char buffer[BUFF_SIZE]; while (42) { int i = 0; memset(buffer, 0, BUFF_SIZE); while ((c = getchar()) != '\n') buffer[i++] = c; if (!s.strcmp_fptr(buffer,s.exit_word)) {// exit if user type exit, allow you to fclose the file fclose(file); s.exit_fptr(EXIT_SUCCESS); // better to use the define } buffer[i] = '\n'; fputs(buffer, file); } fclose(file); return 0; }
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8 Comments

Thanks! 1. the while(42) is just for it to run for ever right? You could write also while(1) and etc? 2. I actually need to create a struct that has 3 definitions inside of it: string ("-exit") and 2 function pointers. one pointer points to CMP function like strcmp that checks if the input of the user matches the string in the struct which is -exit. If it does, it will run the second function pointer, that will point to a function that will exit the program. Can you help me with that or that's too much for this post?
yes you can use while(1) or any number, sure let me edit
What is the purpose of memset here? can I replace it with something else?
to reset your buffer to 0; you can replace it by moving char buffer[BUFF_SIZE] at its location perhaps
the while ((c = getchar()) != '\n' && i < BUFF_SIZE) line, you don't open here a block of { ...... } why?
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your code can work

remember to press Ctrl+d when finished input. the file will have the content your expected

your code wait for EOF to quit the loop. Ctrl+d is a way to input EOF, or else the program never ends. putc will write to cache at first, then write to disk. this an optimization mechanism of File System. you can choose to avoid this by DirectIO when open file.

when program terminate normally, file will be closed automatically, then data in cache will be copy to disk;

but when program terminated abnormally, data in cache might be lost.

file should be closed

fclose is needed. open and close should be organized in pair just as malloc and free.

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