# Writing to a file in C

The file `writehello.c` below represents a simple program in C for writing "Hello world" to a file, whose filename is specified as a command-line argument. The program includes error checking for the number of arguments, and the validity of the filename.

# `writehello.c`

```C
/* Write "Hello world" to a text file specified as a command-line argument

* Compile and run using:
gcc writehello.c -o writehello && writehello out.txt
*/

#include <stdio.h>

int main(int argc, char *argv[]) {
    // Checl the number of arguments
    if (argc != 2) {
        fprintf(stderr, "Error: You need to give 1 argument\n");
        return 1;
    }
    // Check the filename
    FILE* fout = fopen(argv[1], "w");
    if (fout == NULL) {
        fprintf(stderr, "Error: couldn't open file\n");
        return 2;
    }
    // Write to the file
    fprintf(fout, "Hello world");

    // Close and exit
    fclose(fout);
    return 0;
}
```