multithreading - Multiple threads reading from the same file sharing one handler -

c++11, multiplatform code, threads std::thread. have 1 open file , multiple threads have read it.

premise: @ first had 1 string held path file, each new thread opened file, read it, closed it. worked @ point realized maybe bit slow purpose. have file* opened in class constructor , closed in destroy, , multiple threads reading same file. reading, no writing.

i know has been long discussed in other questions, haven't found answer specific questions, are:

1) safe so? 2) faster have multiple threads accessing same handler rather having each thread doing open/close tasks? 3) can o.s. interfere in way threads?

some example code:

before (thread opens/closes file):

void continuepreload(int sn, unsigned long bytesize) {     // open file     file *fpfile = fopen(openfile, "rb");     // seek position     fseek(fpfile, wavepointers[sn] + preloadbytesize[sn], seek_set);     // read portion     fread(&sampledata[sn][preloadbytesize[sn] / sizeof(short)], 1, bytesize - preloadbytesize[sn], fpfile);     // close file     fclose(fpfile); } 

now (thread accesses open file):

void continuepreload(int sn, unsigned long bytesize) {     // no need open/close file again, seek position...     fseek(openfp, wavepointers[sn] + preloadbytesize[sn], seek_set);     // , read desired data     fread(&sampledata[sn][preloadbytesize[sn] / sizeof(short)], 1, bytesize - preloadbytesize[sn], openfp); } 

the code wrong. have race multiple threads accessing same file handler , moving "current pointer" concurrently without synchronisation. being correct need protect file descriptor mutex. locking / unlocking make slower opening file once per thread.

your first solution better (correct , fast).