multithreading - C: fgets error during multiple pthread -

my overall goal of program read data (float or letter) file , change global constant using mutex. (i have not applied until now)

but before that, thought of creating basic program read entire content of file , print screen. currently, program unable so. reads first character of file , exits file.
providing code , error. assistance helpful.

#include <stdio.h> #include <pthread.h> #include <string.h>  pthread_mutex_t mutex = pthread_mutex_initializer; char *files[] = {"one.in", "two.in", "three.in", "four.in", "five.in"};  void* thread( void * arg ) {     char * file = (char *) arg;     // open file , read content     file *fp = fopen(file,"r");     char line[1024];     int len;      printf("thread id %s enter\n",file);     if( !fp)     {         printf("%s file open failed\n", file);         return 0;     }     else         printf("%s file open success %p %d\n", file, fp, ftell(fp));     // dump file content thread id (file name)     while (fgets(line,len, fp))     {         printf("%s %s", file, line);     }     printf("thread id %s %d exit\n",file, ftell(fp));     fclose(fp);     return 0; }  int main( void ) {     int = 0;      if (pthread_mutex_init(&mutex, null) != 0)     {         printf("\n mutex init failed\n");         return 1;     }     for(i = 4; >= 0; i--)     {         pthread_t  id;         pthread_create( &id, null, &thread, files[i] );         pthread_detach(id);     }     printf("main thread exit\n");     pthread_exit(0);     printf("main thread real exit\n");     return 0; } 

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errors

thread id five.in enter five.in file open success 0x7fff7a2e7070 0 thread id five.in 0 exit thread id four.in enter four.in file open success 0x7fff7a2e7070 0 thread id four.in 0 exit thread id three.in enter three.in file open success 0x7fff7a2e7070 0 thread id three.in 0 exit thread id two.in enter two.in file open success 0x7fff7a2e7070 0 thread id two.in 0 exit thread id one.in enter one.in file open success 0x7fff7a2e7070 0 thread id one.in 0 exit main thread exit 

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file format

r 1 2 3 -4 -5 4 w 

the problem call fgets():

while (fgets(line,len, fp)) 

len uninitialized. thi technically undefined behaviour.

what want use size of line:

while (fgets(line, sizeof line, fp))