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Defined in header <cstdio> | ||
long ftell(std::FILE* stream); | ||
Returns the current value of 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 tostd::fseek.
Contents |
| stream | - | file stream to examine |
File position indicator on success or-1L if failure occurs. Also setserrno on failure.
On Windows,_ftelli64 can be used to work with files larger than 2 GiB.
Demonstratesstd::ftell() with error checking. Writes then reads a few floating-point (FP) values to/from a file.
#include <cstdio>#include <cstdlib>#include <iostream> // If the condition is not met then exit the program with error message.void check(bool condition,constchar* func,int line){if(condition)return;std::perror(func);std::cerr<< func<<" failed in file "<< __FILE__<<" at line # "<< line-1<<'\n';std::exit(EXIT_FAILURE);} int main(){// Prepare an array of FP values.constexprint SIZE{5};double A[SIZE]={1.1,2.2,3.3,4.4,5.5}; // Write array to a file.constchar* fname="/tmp/test.bin"; FILE* file=std::fopen(fname,"wb"); check(file!=NULL,"fopen()", __LINE__); constint write_count=std::fwrite(A, sizeof(double), SIZE, file); check(write_count== SIZE,"fwrite()", __LINE__); std::fclose(file); // Read the FP values into array B.double B[SIZE]; file=std::fopen(fname,"rb"); check(file!=NULL,"fopen()", __LINE__); long pos= std::ftell(file);// position indicator at start of file check(pos!=-1L,"ftell()", __LINE__);std::cout<<"pos: "<< pos<<'\n'; constint read_count=std::fread(B, sizeof(double),1, file);// read one FP value check(read_count==1,"fread()", __LINE__); pos= std::ftell(file);// position indicator after reading one FP value check(pos!=-1L,"ftell()", __LINE__);std::cout<<"pos: "<< pos<<'\n';std::cout<<"B[0]: "<< B[0]<<'\n';// 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] | |
| returns the input position indicator (public member function of std::basic_istream<CharT,Traits>)[edit] | |
| returns the output position indicator (public member function of std::basic_ostream<CharT,Traits>)[edit] | |
C documentation forftell | |
