C language random access to files
C language To implement random access to files, two functions fseek (), ftell () are required. Below the programs that reverse display the specified files to introduce the use of these two functions.
RESERVE.C:
#include
#include
#define cntl_z '/ 032' // DOS text file end tag
#define Slen 50
Int main (int Argc, char * argv [])
{
Char file [slen];
CHAR CH;
File * fp;
Long count, Last;
PUTS ("Enter the name of the file to be processed:");
Gets (file);
IF ((fp = fopen (file, "rb")) == null) // read-only and binary mode
{
Printf ("Reverse CAN't open% S / N", File);
Exit (1);
}
FSeek (fp, 0l, seek_set); // Positioning at the beginning of the file
Last = FTELL (FP);
Printf ("FSEEK (FP, 0L, Seek_set), FTEEL (P):% D / N", Last);
FSeek (fp, 0l, seek_ek_ek); // Located at the end of the file
Last = FTELL (FP);
Printf ("FSeek (FP, 0L, Seek_ek_ely), FTEEL (P):% D / N", LAST);
For (count = 1L; count <= last; count )
{
FSeek (fp, -count, seek_ek_ek);
CH = Getc (FP);
IF (ch! = cntl_z && ch! = '/ r')
{
PUTCHAR (CH);
}
}
PUTCHAR ('/ n');
Fclose (fp);
System ("pause");
Return 0;
}
Assume that a file Test.txt content is:
1234567890
1234567890
1234567890
1111111112
2222222223
3333333334
Perform reserve to reverse display:
ENTER The name of the file to be processed:
Test.txt
FSEEK (FP, 0L, Seek_set), Fteel (p): 0
FSeek (fp, 0l, seek_ek, fteel (p): 72
4333333333
3222222222
2111111111
0987654321
0987654321
0987654321
Below, let's explain how fseek () and ftell () work.
l fseek () function
FSEEK (reading and writing location of mobile file stream)
related functions
Rewind, Ftell, Fgetpos, FSETPOS, LSEEK
Header file
#include
Definition function
Int fseek (file * stream, long offset, int imce);
Function description
FSeek () used to move the read and write location of the file stream. The parameter stream is the opened file pointer, and the parameter OFFSET is the number of displacements that move the read-write position based on the parameter imines. parameter
The use is one of the following: Seek_set from the beginning OFFSET displacement of the file as a new read and write location. Seek_cur increases OFFSET unloading after the current read and write position. Seek_end points the read-write position to the end of the file and increases OFFSET. When the WHENCE value is seek_cur or seek_ek_ek_ek_end, the parameter OFFSET allows the emergence of negative values. The following is a more special way: 1) When you move the read / write position to the beginning of the file: FSeek (File * Stream, 0, Seek_set); 2) When you move the read / write location to the file: FSeek (File * stream 0, 0seek_end);
return value
When the call is successful, it returns 0. If there is an error, return -1, Errno will save the error code.
Additional information
FSeek () is unlike LSeek () will return a read-write location, so you must use ftell () to achieve the current read and write location.
l ftell () function
Ftell (acquired the reading location of the file stream)
related functions
FSeek, Rewind, Fgetpos, FSETPOS
Header file
#include
Definition function
Long Ftell (File * Stream);
Function description
ftell () is used to obtain the current read and write location of the file stream. The parameter stream is the open file pointer.
return value
When the call is successful, return to the current read and write position. If there is an error, return -1, errno will save the error code.
error code
Ebadf parameter stream is invalid or removable of file streams for read-write locations.
example
Refer to FSeek ().
Through FSeek (), fTell () two functions, we can access any position of the file at will, think about it seems to be used as an operation file, so there is no more to say. Yes, FSeek () and ftell () There is a potential question to limit the size of the file only within the long-term representation of the long, that is, in this way, only open 2,000,000 bytes of files, but In most cases, it seems to have been enough. If you need to open a larger file, you need to use the fgetpos (), the fsetpos () function, that is another proposition.
