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POreading characters line by line from a text file in c with multiprocessing
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copied!<p>hi there i'm facing with <code>null</code> character problem while i'm reading and try to printing characters of a file with using <code>fseek()</code> function in a multiprocess programme. here is my simple code,</p> <pre><code>#include <stdio.h> /* basic I/O routines. */ #include <unistd.h> /* define fork(), etc. */ #include <sys/types.h> /* define pid_t, etc. */ #include <sys/wait.h> /* define wait(), etc. */ #include <signal.h> /* define signal(), etc. */ #include <pthread.h> #include <time.h> void print_screen(int i); int counter=0; int main(int argc, char* argv[]) { FILE* fptr; fptr = fopen("sample.txt","w"); int counter = atoi(argv[1]); int i,k; int temp; pid_t child_pid; int child_status; char array[counter]; srand ( time(NULL) ); for(i=0; i<counter; i++){ temp = rand()%4; if( temp==0 ) { fprintf(fptr,"A\n"); array[i]='A'; } else if( temp==1 ) { fprintf(fptr,"C\n"); array[i]='C'; } else if( temp==2 ) { fprintf(fptr,"G\n"); array[i]='G'; } else if( temp==3 ) { fprintf(fptr,"T\n"); array[i]='T'; } } fclose(fptr); for(i=1; i<=counter; i++){ child_pid = fork(); switch(child_pid) { case -1: printf("Error occured with fork()\n"); exit(1); case 0: print_screen(i); /* Child Process */ exit(0); } } wait(&child_status); execl("/usr/bin/killall","killall","tail",(char *) 0); return 0; } void print_screen(int i){ char* str; FILE* fptr; fptr=fopen("sample.txt","r"); fseek(fptr,i,SEEK_SET); fscanf(fptr,"%s",str); printf("Process Number %d, Character = %s\n",i,str); sleep(1); fclose(fptr); return; } </code></pre> <p>Suppose that i enter <code>./sample 10</code> in to command line so programme will print 10 characters into sample.txt and then 10 child processes will be created, each of them try to pick a character and print onto the screen.moreover, as you can see i send <code>i</code> as a parameter to set the offset.but as i mentioned it prints null. this is the outlook the programme.</p> <pre><code>Process Number 7, Character = (null) Process Number 6, Character = (null) Process Number 5, Character = (null) Process Number 3, Character = (null) Process Number 8, Character = (null) Process Number 4, Character = (null) Process Number 9, Character = (null) Process Number 10, Character = (null) Process Number 2, Character = (null) Process Number 1, Character = (null) </code></pre> <p>and the txt file was like this.</p> <pre><code>G A A T G C C A A T </code></pre> <p>i will appreciated if you can help and thanks anyway.</p> <p>edit: i realized that i compile like <code>$ gcc sample.c -o sample -lpthread</code> it prints out null. On the other hand, i compile it without <code>-lpthread</code> it prints character but not properly for example like this is the text file.</p> <pre><code>T G G T G </code></pre> <p>and terminal gives the output like this.</p> <pre><code>Process Number 1, Character = G Process Number 2, Character = G Process Number 3, Character = G Process Number 4, Character = G Process Number 5, Character = T </code></pre>

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