ftell

ftell

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Obtains the current file access position

 #include <stdio.h> long ftell ( FILE *fp  ); 

The ftell( ) function returns the current access position in the file controlled by the FILE pointer argument. If the function fails to obtain the file position, it returns the value -1 and sets the errno variable to an appropriate positive value.

To save the access position in a multibyte stream, use the fgetpos( ) function, which also saves the stream's multibyte parsing state.

Example

This example searches in a file, whose name is the second command-line argument, for a string, which the user can specify in the first command-line argument.

 #define MAX_LINE 256 FILE *fp; long lOffset = 0L; char sLine[MAX_LINE] = ""; char *result = NULL; int lineno = 0; /* ... */ if ((fp = fopen(argv[2], "r")) == NULL) {   fprintf(stderr, "Unable to open file %s\n", argv[2]);   exit( -1 ); } do {   lOffset = ftell( fp );    // Bookmark the beginning of                             // the line we're about to read.   if ( -1L == lOffset )     fprintf( stderr, "Unable to obtain offset in %s\n", argv[2] );   else     lineno++;   if ( ! fgets( sLine, MAX_LINE, fp ) )   // Read next line from file.   {     fprintf( stderr, "Unable to read from %s\n", argv[2] );     break;   } } while ( strstr( sLine, argv[1] ) == NULL ); // Test for argument in sLine. /* Dropped out of loop: Found search keyword or EOF */ if ( feof( fp ))   {     fprintf( stderr, "Unable to find \"%s\" in %s\n", argv[1], argv[2] );       rewind( fp );   } else   {     printf( "%s (%d): %s\n", argv[2], lineno, sLine );     fseek( fp, lOffset, 0 );    // Set file pointer at beginning of                                 // the line containing the keyword   } 

The following example runs this program on its own source file, searching for a line containing the word "the". As you can see, the first occurrence of "the" is in line 22. The program finds that line and displays it:

 tony@luna:~/ch17$ ./ftell the ftell.c ftell.c (22):      lOffset = ftell(fp);    // Bookmark the beginning of 

See Also

fgetpos( ), fsetpos( ), fseek( ), rewind( )