Need help to extract comments from c file - c

Question

i just need help of how to extract comments from input text file and paste them in output file using C language in Unix command line. I don't need the code. Just give me favor of guiding me please. Here is what I want.
Input:
If the input file input_0.txt contains
/* This is a single-line C comment */
#include <stdio.h>
/******
* This is a nicely formatted
* multi-line comment.
******/
int main(int argc, char **argv)
{
// This is a C++ comment.
}
Output:
Then the execution of the program would be as follows.
$ ./Comments < input_0.txt
This is a single-line C comment
This is a nicely formatted
multi-line comment.
This is a C++ comment.
Here is my code which i modified from the Respected #David C. Rankin's code.
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define MAXC 1024
int main (int argc, char **argv) {
/* Variables for removing comments*/
int ch, i = 0, flag = 0, prev = '\0';
FILE *fp1, *fp2;
char fname[MAX], temp[] = "temp.txt";
/* Variables for removing comments*/
int inmulti = 0,
insingle = 0,
longline = 0;
char buf[MAXC] = "";
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
/* validate file open for reading */
if (!fp) {
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
/* open the temporary file in write mode */
fp2 = fopen(temp, "w");
/* error handling */
if (!fp2) {
printf("Unable to open temporary file!!\n");
return 0;
}
while (fgets (buf, MAXC, fp)) { /* read upto MAXC into buf */
char *p = buf; /* pointer to buf */
size_t len = strlen (buf); /* get length */
if (longline) { /* is this 2nd read of long line? */
if (insingle) { /* are we in a single comment? */
printf ("%s", buf); /* print it, get next buf */
continue;
}
else /* otherwise, reset insingle flag */
insingle = 0;
}
if (inmulti) { /* are we in a multi-line comment? */
/* (note: you need to check if quoted here) */
if (strstr (buf, "*/")) { /* does buf contain ending? */
inmulti = 0; /* reset inmulti comment */
}
printf ("%s", buf); /* print the line */
continue; /* (note: end can be before end of line) */
}
if (len && buf[len-1] != '\n') /* check if end of line read */
longline = 1; /* if not, set longline */
else
longline = 0; /* or, reset it */
while (*p && *p != '/') p++; /* find start (or end) of comment */
if (!*p) continue; /* none found, get next buf */
if (*(p + 1) == '/') { /* start of single line comment */
/* note: must make sure not part of path here */
insingle = 1; /* set single-line comment flag */
printf ("%s", buf); /* print line */
} /* note: can print from p for comment only */
else if (*(p + 1) == '*') { /* start of multiline comment */
if (!strstr (p + 2, "*/")) { /* check for ending */
inmulti = 1; /* set multiline flag */
}
printf ("%s", buf); /* print the line */
} /* note: can print from p for comment only */
else if (p > buf && *(p - 1) == '*') { /* was / end of multi? */
printf ("%s", buf); /* end of multi line comment */
inmulti = 0;
}
}
rewind(fp);
/* removes comments from the given input file */
prev = fgetc(fp);
while ((ch = fgetc(fp)) != EOF) {
/* flag is 1 - double slash comment */
if (flag == 1) {
/* skip the contents until you detect \n */
if (ch == '\n') {
flag = 0;
prev = fgetc(fp);
}
continue;
}
/* flag is 2 - slash arsterix comment */
if (flag == 2) {
/* skip the contents until you detect asterix slash */
if (ch == '/' && prev == '*') {
flag = 0;
prev = fgetc(fp);
}
continue;
}
/* checking for double slash comment */
if (ch == '/' && prev == '/') {
flag = 1;
} else if (prev == '/' && ch == '*') {
/* slash asterix comment */
flag = 2;
} else {
/* contents outside of comments */
fputc(prev, fp2);
}
prev = ch;
}
if (fp != stdin) fclose (fp); /* close file if not stdin */
/* closing the input file */
fclose(fp);
fclose(fp2);
return 0;
}

Answer 1

Note, to do this correctly, there are many more conditions that need to be checked for (such as "//", "/*" or "*/" appearing as part of a path, or within a string). Making use of regular expressions is also another way to approach this.
If I understand you correctly, and you a looking to parse comment lines of a source file using basic C, then the following is a quick example of reading all lines in a file (which is provided as the 1st argument, or on stdin) and looking for single-line or multi-line comment delimiters.
This is not intended to be complete and cover all corner-cases or cases where the delimiters appear within literals, defines, etc.., but some care has been taken to note where additional code should be added to address those concerns.
The basic approach is to read a line in MAXC (1024 byte chunks) and keeping track of 3 flags. longline meaning the line exceeds MAXC chars and you have read the 2nd (or 3rd, or 4th...) buffer full. inmulti tracking whether you are in a multi-line comment. Finally insingle where you are within a single-line comment that may exceed MAXC chars. The read loops checks and acts based upon the state of the flags, while looking for the end of a multi-line comment (if within one). The code also checks for multi-line comment start and end -- all within one line.
Given those qualifications, you could start with something like:
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#define MAXC 1024
int main (int argc, char **argv) {
int inmulti = 0,
insingle = 0,
longline = 0;
char buf[MAXC] = "";
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
while (fgets (buf, MAXC, fp)) { /* read upto MAXC into buf */
char *p = buf; /* pointer to buf */
size_t len = strlen (buf); /* get length */
if (longline) { /* is this 2nd read of long line? */
if (insingle) { /* are we in a single comment? */
printf ("%s", buf); /* print it, get next buf */
continue;
}
else /* otherwise, reset insingle flag */
insingle = 0;
}
if (inmulti) { /* are we in a multi-line comment? */
/* (note: you need to check if quoted here) */
if (strstr (buf, "*/")) { /* does buf contain ending? */
inmulti = 0; /* reset inmulti comment */
}
printf ("%s", buf); /* print the line */
continue; /* (note: end can be before end of line) */
}
if (len && buf[len-1] != '\n') /* check if end of line read */
longline = 1; /* if not, set longline */
else
longline = 0; /* or, reset it */
while (*p && *p != '/') p++; /* find start (or end) of comment */
if (!*p) continue; /* none found, get next buf */
if (*(p + 1) == '/') { /* start of single line comment */
/* note: must make sure not part of path here */
insingle = 1; /* set single-line comment flag */
printf ("%s", buf); /* print line */
} /* note: can print from p for comment only */
else if (*(p + 1) == '*') { /* start of multiline comment */
if (!strstr (p + 2, "*/")) { /* check for ending */
inmulti = 1; /* set multiline flag */
}
printf ("%s", buf); /* print the line */
} /* note: can print from p for comment only */
else if (p > buf && *(p - 1) == '*') { /* was / end of multi? */
printf ("%s", buf); /* end of multi line comment */
inmulti = 0;
}
}
if (fp != stdin) fclose (fp); /* close file if not stdin */
return 0;
}
Example Input File
$ cat dat/comments.txt
/* This is a single-line C comment */
#include <stdio.h>
/******
* This is a nicely formatted
* multi-line comment.
******/
int main(int argc, char **argv)
{
// This is a C++ comment.
}
Example Use/Output
$ ./bin/comments <dat/comments.txt
/* This is a single-line C comment */
/******
* This is a nicely formatted
* multi-line comment.
******/
// This is a C++ comment.
note: the worth in an exercise such as this is in the learning value stepping though a long string identifying certain individual characters, as well as in handling various flags and program states as you loop though a file.
Reading Character-by-Character
To switch from a line-oriented approach to a character-oriented approach (and adding a couple of states noted in the comments from chux), you will read the first character (save it), and then read the remaining characters in the file. This provides a way of comparing the previous to current to determine if you are within or without a single-line comment, a multi-line comment or single or double quotes.
Again, this isn't intended to catch every corner case, but the output was updated to not print the opening or closing comment delimiters. (you will need to tweak the printing of * within multi-line comments and quotes within comments to your taste).
Changing from reading with fgets to fgetc, you could do something similar to the following:
#include <stdio.h>
int main (int argc, char **argv) {
int inmulti = 0, /* in multi-line comment flag */
insingle = 0, /* in single-line comment flag */
insquo = 0, /* within single-quotes */
indquo = 0, /* within double-quotes */
c, prev = 0;
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
fprintf (stderr, "error: file open failed '%s'.\n", argv[1]);
return 1;
}
if ((prev = fgetc(fp)) == EOF) /* read 1st char */
return 1;
while ((c = fgetc(fp)) != EOF) { /* read remaining */
switch (c) { /* switch on c */
case '/':
if (prev == '/' && !(insquo | indquo))
insingle = 1;
if (prev == '*' && !(insquo | indquo))
inmulti = 0;
break;
case '*':
if (prev == '/' && !(insquo | indquo))
inmulti = 1;
break;
case '\n':
insingle = 0;
if (insingle || inmulti)
putchar (c);
break;
case '\'':
insquo = insquo ? 0 : 1;
break;
case '"':
indquo = indquo ? 0 : 1;
break;
default:
if ((insingle || inmulti) && !(insquo | indquo))
putchar (c);
break;
}
prev = c;
}
if (fp != stdin) fclose (fp); /* close file if not stdin */
putchar ('\n'); /* tidy up with newline */
return 0;
}
Example Use/Output
$ ./bin/commentsfgetc <dat/comments.txt
This is a single-line C comment
This is a nicely formatted
multi-line comment.
This is a C++ comment.
Look things over and let me know if you have questions over how characters are being identified or the code controlled to locate the start and end of the comment blocks.

Answer 2

You can use this shell script to do that, and save to a file comments.txt
cat generic.c | awk '/\/\// {print $0}; /\/\*/ {aux=1}; {if(aux) print $0}; /\*\// {aux=0}' > comments.txt
Good lucky