How to peek ahead when processing an input file (processing 2 lines) - c

Question

When going through a text file line by line I would like to be able to look ahead to the next line and inspect it while working on the current line. I am working in the C language. I believe that fseek() or other similar functions would help me with this but I'm unsure and don't know how to use them. I would want to achieve something to the effect of:
fp = fopen("test-seeking.txt", "r");
while((fgets(line, BUFMAX, fp))) {
// Peek over to next line
nextline = ...;
printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n",
line, nextline);
}
I appreciate any help.

Answer 1

Indeed you can use fseek and try something like this:
fp = fopen("test-seeking.txt", "r");
while ((fgets(line, BUFMAX, fp))) {
// Get the next line
fgets(nextline, BUFMAX, fp);
// Get the length of nextline
int nextline_len = strlen(nextline);
// Move the file index back to the previous line
fseek(fp, -nextline_len, SEEK_CUR); // Notice the - before nextline_len!
printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}
Another way is to use fgetpos and fsetpos, like this:
fp = fopen("test-seeking.txt", "r");
while ((fgets(line, BUFMAX, fp))) {
// pos contains the information needed from
// the stream's position indicator to restore
// the stream to its current position.
fpos_t pos;
// Get the current position
fgetpos(fp, &pos);
// Get the next line
fgets(nextline, BUFMAX, fp);
// Restore the position
fsetpos(fp, &pos);
printf("Current line starts with: %-3.3s / Next line starts with %-3.3s\n", line, nextline);
}

Answer 2

The following code is inspired by #Jean-François Fabre comment. It will use a 2D character array lineBuffer which is used to hold the lines. The first read line is written to index 0 lineBuffer[0] and the second line to lineBuffer[1]. After that the writings are alternating between index 0 and 1 with the help of a toggle variable lineSel. As last step the curLine pointer will be set to nextLine.
As result you can use curLine and nextLine within the loop.
If you have a file that consists of:
line 1
line 2
line 3
...
You will work with:
curLine = "line 1\n"
nextLine = "line 2\n"
curLine = "line 2\n"
nextLine = "line 3\n"
...
See live example with stdin instead of a file on ideone.
Code:
#include <stdio.h>
#define BUFMAX 256
#define CURLINE 0
#define NEXTLINE 1
#define TOGGLELINE (CURLINE ^ NEXTLINE)
int main ()
{
FILE* fp = fopen("test-seeking.txt", "r");
char lineBuffer[2][BUFMAX];
char* curLine;
char* nextLine;
int lineSel;
if (fp != NULL)
{
if ((curLine = fgets(lineBuffer[CURLINE], BUFMAX, fp)))
{
for (lineSel = NEXTLINE;
(nextLine = fgets(lineBuffer[lineSel], BUFMAX, fp));
lineSel ^= TOGGLELINE)
{
printf("Current line: \"%s\" / Next line \"%s\"\n",
curLine, nextLine);
curLine = nextLine;
}
}
fclose(fp);
}
return 0;
}