Read file with comma separated hex values into array in C - c

Question

How I can read a text file with comma separated hex values e.g. 0x58,0xA9,0x00 and put its values as elements of an array e.g. LR0
I need to replace this hard-coded with reading from the file:
const unsigned char LR0[] = {0x58,0xA9,0x00}
Here is what I wrote so far. printf("%c", ch); shows me what I need but when I uncomment strcat(LR0, ch); it fails at run time with a segment fault. I don't know if I should use strcat or anything else to append the elements of this LR0 array.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
int ch;
FILE *fp;
//const unsigned char LR0 [1024] = "";
const unsigned char LR0 [] = {};
fp = fopen("test.txt", "r");
if (fp == NULL)
{
perror("Error while opening the file.\n");
exit(EXIT_FAILURE);
}
while((ch = fgetc(fp)) != EOF){
printf("%c", ch);
//strcat(LR0, (char) ch);
}
fclose(fp);
return 0;
}
Sorry so basic question not able to fix it by googling etc. I am not a C developer and I am using gcc on linux. My text file does not contain lines so I cannot use this solution

Answer 1

There are two problems in your code.
LR0 is declared as const with unspecified size which is just pointer, writing to it may result in UB.
strcat needs it arguments as char * type but your second argument is of char type (int ch;).
strcat(LR0, (char) ch)
You can use the fscanf with , as delimiter as below to read only the hex values discarding ,.
int main() {
FILE *fp;
unsigned char LR0 [1024] = {};
fp = fopen("test.txt", "r");
if (fp == NULL) {
perror("Error while opening the file.\n");
exit(EXIT_FAILURE);
}
int i = 0;
while(fscanf(fp, "%c,", &LR0[i]) == 1){
printf("%c", LR0[i++]);
}
fclose(fp);
return 0;
}

Answer 2

const unsigned char LR0 [] = {}; implies a zero length array - that is not standard C yet allowed with some compilers.
strcat(LR0, (char) ch); attempts to 1) write to a const array LR0 and 2) write outside the array - it is only 0 length. Both of the are undefined behavior (UB).
I don't know if I should use strcat
Using str...() functions will not well handle input which may contain many "0x00, 0x00, ...".
How I can read a text file with comma separated hex values e.g. 0x58,0xA9,0x00 and put its values as elements of an array (?)
Read the file to determine its length and contents. I suggest a pass for each.
The below is untested, yet hopefully enough to get OP started. It has little error detection.
// Parse a text file like "0x58,0xA9,0x00"
// Return byte count. Return 0 on error.
size_t read_comma_hex(FILE *f, unsigned char *dest, size_t num) {
rewind(f);
size_t i;
for (i = 0; i<num; i++) {
if (i) {
int ch = fgetc(f);
// Add to tolerate white space before the ',' or after the the last number
while (isspace(ch)) {
ch = fgetc(f);
}
if (ch == EOF) break; // no more data
if (ch != ',') return 0; // Fail, as ',' expected
}
unsigned char hex;
if (fscanf(f, "0x%hhx", &hex) != 1) return 0;
if (dest) dest[i] = hex;
}
return i;
}
void read_comma_hex_file(FILE *f) {
size_t n = read_comma_hex(f, NULL, SIZE_MAX);
if (n == 0) return; // no data or failure
// OP wants an array - research variable length array
const unsigned char LR0[n];
// Alternative: allocate memory instead of using an _array_. (not shown)
// Read data into the array
read_comma_hex(f, LR0, n);
// use LR0 and n some how
for (size_t i = 0; i<n; i++) {
printf("0x%02hhX%s", LR0[i], i > 0 ? "," : "");
}
}