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Top Forums Programming how to move file pointer to a particular line in c Post 302178615 by ramkrix on Wednesday 26th of March 2008 03:01:35 AM
Old 03-26-2008
CPU & Memory

Quote:
char data[80];
long cur_pos;
while( feof(fh) == 0){
cur_pos = ftell(fh);
printf( "cur_cursor = %ld \n", cur_pos);
fgets(data, 80, fh);
}
A simple change to this code may work:

char data[80];
int ct_str_length;
long new_byte_position;

long cur_pos;
while( feof(fh) == 0){
cur_pos = ftell(fh);
printf( "cur_cursor = %ld \n", cur_pos);
fgets(data, 80, fh);
ct_str_length=strlen(data);
new_byte_position=ct_str_length+1;
printf("new_byte_position or cursor position is %ld" ,new_byte_position);
fseek(fh,new_byte_position,SEEKSET);

}



we know that fgets() stops once it reaches newline characte/EOF.

Till EO of file is reached(feof(fp)==0), with the curent byte position as 0th byte or first byte of first line, we are reading the first line by fgets() till newline characte is reached. Once \n is reached, fgets() tops its function and the ead content is stored in data array. the string length in this aray is the total number of characters/bytes in first line. So adding 1 byte more than that, now the current byte position becomes the beginning byte of the next line. Check this.

-Ramkrix
 

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FGETS(3)						   BSD Library Functions Manual 						  FGETS(3)

NAME
fgets, gets -- get a line from a stream LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdio.h> char * fgets(char * restrict str, int size, FILE * restrict stream); char * gets(char *str); DESCRIPTION
The fgets() function reads at most one less than the number of characters specified by size from the given stream and stores them in the string str. Reading stops when a newline character is found, at end-of-file or error. The newline, if any, is retained. If any characters are read and there is no error, a '' character is appended to end the string. The gets() function is equivalent to fgets() with an infinite size and a stream of stdin, except that the newline character (if any) is not stored in the string. It is the caller's responsibility to ensure that the input line, if any, is sufficiently short to fit in the string. RETURN VALUES
Upon successful completion, fgets() and gets() return a pointer to the string. If end-of-file occurs before any characters are read, they return NULL and the buffer contents remain unchanged. If an error occurs, they return NULL and the buffer contents are indeterminate. The fgets() and gets() functions do not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred. ERRORS
[EBADF] The given stream is not a readable stream. The function fgets() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), read(2), or malloc(3). The function gets() may also fail and set errno for any of the errors specified for the routine getchar(3). SEE ALSO
feof(3), ferror(3), fgetln(3), fgetws(3), getline(3) STANDARDS
The functions fgets() and gets() conform to ISO/IEC 9899:1999 (``ISO C99''). SECURITY CONSIDERATIONS
The gets() function cannot be used securely. Because of its lack of bounds checking, and the inability for the calling program to reliably determine the length of the next incoming line, the use of this function enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack. It is strongly suggested that the fgets() function be used in all cases. BSD
May 5, 2012 BSD
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