Can't print an int to a file - c

Question

I'm trying to print an int array on a file but it is giving me an error.
write_error:Bad address.
My int array is something like :
1 2 3 4 5 6 7 8 9
And I want to print it on a file as follows :
1 2 3
4 5 6
7 8 9
This is the code I'm using :
void printToFile(int listBoard[]) {
int file_write;
char buffer[100];
char buffer2[10];
if ((file_write = open("./board.txt", O_WRONLY | O_CREAT | O_TRUNC, 0700)) < 0) {
err_sys("error output file");
}
if (write(file_write, snprintf(buffer2, 10, "%d",listBoard[2]), 18) != 18){
err_sys("write_error");
}
if (close(file_write) != 0){
err_sys("error close write");
}
}
I also tried (char) but it converts the int to the ascii char.
Could you tell me what I'm missing?
Edit
My file output is :
3#\00\00펈㰙\00ጠ
Code edited :
void printToFile(int listBoard[]){
int file_write;
char buffer[100];
int value;
int cont = 0;
char buffer2[10];
if ((file_write = open("./board.txt", O_WRONLY | O_CREAT | O_TRUNC, 0700)) < 0) {
err_sys("error output file");
}
int len = snprintf(buffer2, 10, "%d", listBoard[2]);
if (write(file_write, buffer2, len) != len) {
err_sys("write_error");
}
if (close(file_write) != 0) {
err_sys("error close write");
}
}
Output file :
3
Last edit
This is how I did now to print it as the output that I want
if ((file_write = open("./board.txt", O_WRONLY | O_CREAT | O_TRUNC, 0700)) < 0) {
err_sys("error output file");
}
for (int index = 0; index < SIZE_BOARD; index++) {
if (cont == 3) {
int len = snprintf(buffer2, 1, "%d \n", ' ');
if(write(file_write, buffer2, len) != len) {
err_sys("write_error");
cont=0;
}
}
int len = snprintf(buffer2, 10, "%d ", listBoard[index]);
if (write(file_write, buffer2, len) != len) {
err_sys("write_error");
cont++;
}
}
if(close(file_write) != 0) {
err_sys("error close write");
}
But it is not doing the \n correctly. What I'm missing?

Answer 1

The definition of write is as follows:
ssize_t write(int fd, const void *buf, size_t count);
The definition of snprintf is as follows:
int snprintf(char *str, size_t size, const char *format, ...);
Now you're feeding snprintf to where write is expecting void *buf. i.e. the address to where the data is.
So you'll have to do
snprintf(buffer2, 10, "%d", listBoard[2]);
then
write(file_write, buffer2, 18);
EDIT:
That will fix the syntax error. Now semantically you can see that you're writing 10 bytes to buffer2 but you're writing 18 to the file. This will fill the file with junk. So i suggest.
int len = snprintf(buffer2, 10, "%d", listBoard[2]);
write(file_write, buffer2, len);
EDIT 2:
int len = snprintf(buffer2, 1, "%d \n", ' ');
First off your format specifier has 3 bytes while you're only writing 1 byte to the buffer2. Second, why is your argument ' ' when you're supposed to give the listBoard[n]?

Answer 2

Use dprintf() to write formatted strings to files:
dprintf(file_write, "%d", listBoard[2]);
dprintf will handle all the write stuff for you, so this is the only call you need with open and close.
dprintf is the fprintf for file descriptors. More informations here: https://linux.die.net/man/3/dprintf

Answer 3

The snprintf (and family) functions returns an integer.
You use this integer in the write call as a pointer to a buffer. That will lead to undefined behavior.
Call snprintf separately, and then pass buffer2 as the second argument. And use strlen to get the actual length of the string in buffer2, not hard-code the magic number 18 which will lead to reading from out-of-bounds and another case of UB.

Answer 4

First of all snprintf returns an integer that is the number of chars that would have been written if the string were not truncated (as you can read on the snprintf manual)
you cannot then use it in the write command. You can then use:
if ( snprintf(buffer2, 10,"%d",listBoard[2]) >= 10 ) {
/* handle truncation error*/
}
if(write(file_write,buffer2,18)!=18){
err_sys("write_error");
Anyway since buffer2 is maximum 10 characters why do you expect to write 18 bytes?
Other possible error: buffer2 should be at least of 10 + 1 byte for null termination.

Answer 5

You never specify that you must use raw file descriptors, so let's port your code up one level to C's standard library's FILE abstraction. It's easier to work with.
void printToFile(const int listBoard[]) {
FILE * const out = fopen("board.txt", "wt");
if (out == NULL) {
err_sys("error output file");
}
for(int j = 0; j < 3; ++j) {
for (int i = 0; i < 3; ++i)
fprintf(out, "%d ", listBoard[3 * j + i]);
fprintf(out, "\n");
}
fclose(out);
}
Note that this uses loops to iterate over the elements of the board array, this seemed to be missing from your original code and part of the problem you had with it.
This prints:
1 2 3
4 5 6
7 8 9
There's a trailing space on each line due to me being lazy, but hopefully that's fine. Also I omitted some inner error checking.