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Defined in header <stdio.h> | ||
long ftell(FILE* stream); | ||
Returns the file position indicator for the file streamstream.
If the stream is open in binary mode, the value obtained by this function is the number of bytes from the beginning of the file.
If the stream is open in text mode, the value returned by this function is unspecified and is only meaningful as the input tofseek().
Contents |
| stream | - | file stream to examine |
File position indicator on success or-1L if failure occurs.
On error, theerrno variable is set to implementation-defined positive value.
On Windows,_ftelli64 can be used to work with files larger than 2 GiB.
Demonstratesftell() with error checking. Writes then reads a few floating-point (FP) values to/from a file.
#include <stdio.h>#include <stdlib.h> /* If the condition is not met then exit the program with error message. */void check(_Bool condition,constchar* func,int line){if(condition)return;perror(func);fprintf(stderr,"%s failed in file %s at line # %d\n", func, __FILE__, line-1);exit(EXIT_FAILURE);} int main(void){/* Prepare an array of FP values. */#define SIZE 5double A[SIZE]={1.1,2.0,3.0,4.0,5.0}; /* Write array to a file. */constchar* fname="/tmp/test.bin";FILE* file=fopen(fname,"wb"); check(file!=NULL,"fopen()", __LINE__); constint write_count=fwrite(A,sizeof(double), SIZE, file); check(write_count== SIZE,"fwrite()", __LINE__); fclose(file); /* Read the FP values into array B. */double B[SIZE]; file=fopen(fname,"rb"); check(file!=NULL,"fopen()", __LINE__); longint pos= ftell(file);/* position indicator at start of file */ check(pos!=-1L,"ftell()", __LINE__);printf("pos: %ld\n", pos); constint read_count=fread(B,sizeof(double),1, file);/* read one FP value */ check(read_count==1,"fread()", __LINE__); pos= ftell(file);/* position indicator after reading one FP value */ check(pos!=-1L,"ftell()", __LINE__);printf("pos: %ld\n", pos);printf("B[0]: %.1f\n", B[0]);/* print one FP value */ returnEXIT_SUCCESS;}
Possible output:
pos: 0pos: 8B[0]: 1.1
| gets the file position indicator (function)[edit] | |
| moves the file position indicator to a specific location in a file (function)[edit] | |
| moves the file position indicator to a specific location in a file (function)[edit] | |
C++ documentation forftell | |
