Writing a file to users HOME directory in C

Question

I am trying to write a .txt file to the user's HOME directory.

I have tried:

char str[] = "TEST";
char* file = strcat(getenv("HOME"), "/dataNumbers.txt"); //I am concatenating the user's home directory and the filename to be created.
myFile = fopen(file,"w");
fwrite(str , 1 , sizeof(str) , myFile );

However, this does not create the file.

Answer 1

You are using strcat incorrectly.

 char *file;
 char *fileName = "/dataNumbers.txt";

 file = malloc(strlen(getenv("HOME") + strlen(fileName) + 1); // to account for NULL terminator
 strcpy(file, getenv("HOME"));
 strcat(file, fileName);

file will now contain the concatenated path and filename.

Obviously, this could be written much more cleanly. I'm just trying to be very straightforward.

Answer 2

You can't strcat() to an environment variable. You need another buffer:

char file[256]; // or whatever, I think there is a #define for this, something like PATH_MAX
strcat(strcpy(file, getenv("HOME")), "/dataNumbers.txt");
myFile = fopen(file,"w");

EDIT To address one of the comments below, you should first ensure that the data to be concatenated doesn't overflow the file buffer, or allocate it dynamically - not forgetting to free it afterwards.

Answer 3

Strings in C are not like other strings in other high-level languages; that is, You have to allocate all the space for them yourself. When using c-string functions, you need to ensure that you have enough memory allocated for the resultant strings, by either reserving enough space on the stack (i.e. char filename[HOPEFULLY_ENOUGH]) or via the heap (malloc() and friends). You cannot count on getenv to return a string with enough space for you to concatenate to;

Here's a complete program from start to finish that does what you want to do (minus some pedantic error checking):

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(void)
{
    char *filename = "/dataNumbers.txt";
    char *writeString = "TEST";
    char *home_dir = getenv("HOME");
    char *filepath = malloc(strlen(home_dir) + strlen(filename) + 1);
    strncpy(filepath, home_dir, strlen(home_dir) + 1);
    strncat(filepath, filename, strlen(filename) + 1);
    printf("%s\n", filepath);

    FILE *myFile = fopen(filepath, "w");
    fwrite(writeString, 1, strlen(writeString), myFile);
    fclose(myFile);
    free(filepath);
    return 0;
}

strncpy will copy the string plus the null terminator (that's the reason for the +1). strncat will begin concatenation at the null terminator and the resultant concatenated string itself must be null-terminated (hence the +1 in the third argument to strncat).

I use malloc and free because it's hard to know exactly the length of the full path to your file. You could go with something enormous (like char[4096]) but that seems wasteful, doesn't it? malloc() allows you to reserve exactly the amount of memory you need. No more, and no less.

You can also do the string concatenation using the snprintf function. I include this only for your personal enrichment, and sometimes it's nice to have multiple ways to do the same thing, even though strcpy and strcat will make your intentions much more clear to readers of your code:

char *filename = "/dataNumbers.txt";
char *writeString = "TEST";
char *home_dir = getenv("HOME");
size_t size = strlen(home_dir) + strlen(filename) + 1;
char *filepath = malloc(size);
snprintf(filepath, size, "%s%s", home_dir, filename);

Hope this helps!