Segmenting a subtring from a main string C - c

Question

Get specific content from a file and store it in a variable. So far I get that I can convert the file content into a string. But I'm not sure how can I 'extract' the content from the string I converted and would like some help.
The original file looks something like this:
XXXXXX
XXXXX
Addr = 12:23:34:45:45
XXX
XXX
I need to extract and store the Addr as a string. Want to look for the prefix Addr = and just copy it into a buffer. But I don't know how can I do it...
So far my code looks like below:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
//So far I'm looking for it using the MAC addr format
const char *get_mac_addr(char *str, char *dest) {
if (str == NULL)
return 0;
char *start = NULL;
int token_count = 0;
char *ptr = str;
if (*ptr && *(ptr + 1)) // skip two chars in the beginning of the string
ptr += 2;
else
return 0;
while (*ptr != '\0' && *ptr != '\n' && *ptr != '\r') {
if (token_count == 5)
break;
/* if ':' found and previous two characters are hexidecimal digits then
the substring could be part of MAC
*/
if (*ptr == ':' && isxdigit(*(ptr - 1)) && isxdigit(*(ptr - 2))) {
token_count++;
if (start == NULL)
start = ptr - 2;
int i = 0;
while (*ptr != '\0' && i++ < 3)
ptr++;
} else {
start = NULL;
token_count = 0;
ptr++;
}
}
strcpy(dest, start);
return dest;
}
const char *file2str(){
/* declare a file pointer */
FILE *infile;
char *buffer;
long numbytes;
char dest[18];
/* open an existing file for reading */
infile = fopen("~/Desktop/file.config", "r");
/* quit if the file does not exist */
//if (infile == NULL)
// return 1;
/* Get the number of bytes */
fseek(infile, 0L, SEEK_END);
numbytes = ftell(infile);
/* reset the file position indicator to
the beginning of the file */
fseek(infile, 0L, SEEK_SET);
/* grab sufficient memory for the
buffer to hold the text */
buffer = (char *)calloc(numbytes, sizeof(char));
/* memory error */
//if(buffer == NULL)
// return 1;
/* copy all the text into the buffer */
fread(buffer, sizeof(char), numbytes, infile);
fclose(infile);
/* confirm we have read the file by
outputing it to the console */
printf("The file called test.dat contains this text\n\n%s", buffer);
//memset(dest, '/0', sizeof(dest));
get_mac_addr(buffer, dest);
/* free the memory we used for the buffer */
//free(buffer);
printf("Dest is \n\n%s", dest);
return dest;
}
int main() {
printf(file2str);
return 0;
}

Answer 1

I really appreciate your help. Please bare with me as I'm not very good at c programming. I would like to convert the main function into one function so I can directly call it and return a string. I converted the main function as following, but I'm not sure why when I print it, there is nothing show up:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE * xfopen(const char *path, const char *mode);
const char *MACadd()
{
char buf[256];
char *addr = NULL;
FILE *in = xfopen("~Desktop/file.config", "r");
while( fgets(buf, sizeof buf, in) ){
addr = strstr(buf, "Addr = ");
if( addr && addr < buf + sizeof buf - ADDRLEN){
addr += strlen("Addr = ");
addr[ADDRLEN] = '\0';
break;
}
}
//printf("addr = %s\n", addr);
return addr;
}
FILE *xfopen(const char *path, const char *mode)
{
FILE *fp = fopen(path, mode);
if( fp == NULL ){
perror(path);
exit(EXIT_FAILURE);
}
return fp;
}
int main(){
printf("%s", MACadd());
return 0;
}

Answer 2

You could read the file line by line with the fgets function and use the sscanf function to extract the relevant portion like that:
#include <stdio.h>
#include <ctype.h>
#include <stdbool.h>
#define ISXDGT(c) isxdigit((unsigned char)(c))
static bool is_macaddr (const char *s)
{
return ISXDGT(s[0]) && ISXDGT(s[1]) && s[2] == ':'
&& ISXDGT(s[3]) && ISXDGT(s[4]) && s[5] == ':'
&& ISXDGT(s[6]) && ISXDGT(s[7]) && s[8] == ':'
&& ISXDGT(s[9]) && ISXDGT(s[10]) && s[11] == ':'
&& ISXDGT(s[12]) && ISXDGT(s[13]) && s[14] == ':'
&& ISXDGT(s[15]) && ISXDGT(s[16]);
}
bool get_macaddr_from_file (const char *filename, char *macaddr)
{
char line[4096];
bool done = false;
FILE *fp = fopen(filename, "r");
if (fp == NULL) {
fprintf(stderr, "Cannot open the file '%s'\n", filename);
return false;
}
while (fgets(line, sizeof line, fp) != NULL) {
/* Modify the prefix (" Addr = " here) at your convenience */
if (sscanf(line, " Addr = %17s", macaddr) == 1 && is_macaddr(macaddr)) {
done = true;
break;
}
}
fclose(fp);
return done;
}
int main (void)
{
char macaddr[18];
if (get_macaddr_from_file("file.conf", macaddr)) {
printf("MAC: %s\n", macaddr);
}
}

Answer 3

char *extract(const char *str, char *buff)
{
char *addr = strstr(str, "Addr");
if(addr)
{
addr += sizeof("Addr") - 1;
while(!isdigit((unsigned char)*addr))
{
if(*addr == '\n' || !*addr)
{
addr = NULL;
break;
}
addr++;
}
if(addr)
{
while(*addr && *addr != '\n' && (isdigit(*addr) || *addr == ':'))
{
*buff++ = *addr++;
}
*buff = 0;
}
}
return addr ? buff : NULL;
}
void main(int argc, char** argv)
{
char *str = "XXXXXX\nXXXXX\n\nAddr = 12:23:234:145:45 \nXXX\nXXX\n";
char mac[30];
if(extract(str,mac)) printf("Hurray!!! `%s`\n", mac);
else printf("MIsareble failure\n");
}
https://godbolt.org/z/6TjK8b

Answer 4

This gets a little tricky if you don't want to restrict yourself to a fixed maximum line length, but it's probably sufficient to do something like:
#define ADDRLEN 14
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE * xfopen(const char *path, const char *mode);
int
main(int argc, char **argv)
{
char buf[256];
char *addr = NULL;
FILE *in = xfopen(argc > 1 ? argv[1] : "-", "r");
while( fgets(buf, sizeof buf, in) ){
addr = strstr(buf, "Addr = ");
if( addr && addr < buf + sizeof buf - ADDRLEN){
addr += strlen("Addr = ");
addr[ADDRLEN] = '\0';
break;
}
}
printf("addr = %s\n", addr);
}
FILE *
xfopen(const char *path, const char *mode)
{
FILE *fp = path[0] != '-' || path[1] != '\0' ? fopen(path, mode) :
*mode == 'r' ? stdin : stdout;
if( fp == NULL ){
perror(path);
exit(EXIT_FAILURE);
}
return fp;
}
To break this into a function, you need to be a little bit careful. In your attempt, you've passed back references to local variables which cease to exist after the function returns. Perhaps you want something like:
#define ADDRLEN 14
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE * xfopen(const char *path, const char *mode);
const char *
MACadd(const char *path, char *buf, size_t s)
{
char *addr = NULL;
FILE *in = xfopen(path, "r");
while( fgets(buf, s, in) ){
addr = strstr(buf, "Addr = ");
if( addr && addr < buf + s - ADDRLEN){
addr += strlen("Addr = ");
addr[ADDRLEN] = '\0';
break;
}
}
return addr;
}
FILE *xfopen(const char *path, const char *mode)
{
FILE *fp = fopen(path, mode);
if( fp == NULL ){
perror(path);
exit(EXIT_FAILURE);
}
return fp;
}
int
main(void)
{
char buf[256];
printf("%s", MACadd("input", buf, sizeof buf));
return 0;
}

Answer 5

I think that is easier than you are doing, once you moved the file contnent in a string, use strstr() - here ther is the description http://www.cplusplus.com/reference/cstring/strstr/ - to find "addr = " and then get the string from there to the character "\n"
follow this example
#include<string.h>
#include<stdio.h>
#define endchrptr(ptr1, ptr2, ptr3) (ptr1 < ptr2 ? (ptr1<ptr3?ptr1:ptr3) : (ptr2<ptr3?ptr2:ptr3))
bool get_mac_addr(const char* source, char *dest) {
if(source!=NULL&&dest!=NULL) {
char* addr_pointer=strstr(source, "Addr = ")+7;//find where the address start
char* end_addr_pointer=endchrptr(strchr(addr_pointer, '\n'), strchr(addr_pointer, '\r'), strchr(addr_pointer, '\0'));//find where the address ends
if(end_addr!=NULL) {
for(int i=0; i<end_addr_pointer-addr_pointer; ++i) {//copy the address
dest[i]=addr_pointer[i];
}
dest[end_addr_pointer-addr_pointer],
}
else return false;
}
else return false;
}
int main()
{
char *str = "XXXXXX\nXXXXX\n\nAddr = 12:23:234:145:45 \nXXX\nXXX\n";
char mac[30];
get_mac_addr(str, mac);
printf("%s", mac);
}
I just tried in DevC++ and it works.
Let me know if it works.

Answer 6

There are multiple problems in the code:
fopen("~/Desktop/file.config", "r"); will fail because the ~ in the filename is not expanded to the home directory by fopen, it is a feature of the command shell. Use the full path instead, or take the filename as an argument.
you do not check for fopen() failure: passing a null stream pointer to fseek has undefined behavior and will probably crash the program.
printf(file2str); is a major mistake: you try to use the bytes from the function as a format string, you will get garbage output and possibly a crash because of undefined behavior. Use printf("%s\n", file2str()); instead.
there is no need to read the whole file in memory at once for this problem, just reading one line at a time is much simpler. Furthermore, you do not allocate enough memory for the null terminator, so you get undefined behavior with using buffer as a C string.
get_mac_addr is way too complicated: you could use strstr to locate the string "Addr = " and extract the following word.
Here is a simpler version:
#include <ctype.h>
#include <stdio.h>
#include <string.h>
const char *get_mac_address(const char *str, char *dest) {
if (!strncmp(buf, "Addr = ", 7)) {
const char *p = buf + 7;
for (i = 0; i < 17; i++) {
if (i % 3 == 2) {
if (p[i] != ':')
break;
} else {
if (!isxdigit((unsigned char)p[i]))
break;
}
}
if (i == 17 && !isalnum((unsigned char)p[i]) {
memcpy(dest, p, 17);
dest[17] = '\0';
return dest;
}
}
return NULL;
}
int main() {
char buf[256];
char address[20];
FILE *fp = fopen("/home/ImTrying/Desktop/file.config", "r");
if (fp != NULL) {
while (fgets(fp, buf, sizeof buf)) {
if (get_mac_address(buf, address)) {
printf("Dest is %s\n", address);
break;
}
}
fclose(fp);
}
return 0;
}