segmentation fault (c language) - c

Question

While trying to recover jpgs from a raw file in week4 of CS50 course , I got this seg fault error which I can't understand why or how it is caused, all I can tell is that according to valgrind line 110 causes this issue
I am fairly new to c language
int main(int argc, char *argv[])
{
// check if one command arg is passed
if (argc > 2 )
{
printf("Usage: ./recover image\n");
return 1;
}
// try to open file to read from
FILE *file = fopen(argv[1], "r");
if (!file)
{
return 1;
}
BYTE bytes[512];
int x =0 ;
char num[10];
FILE* output =NULL;
while (fread(&bytes, 512, 1, file) == 1)
{
// first step create file name
sprintf(num, "%03i.jpg",x);
// check for start of jpg file
if (bytes[0]==0xff && bytes[1] ==0xd8 && bytes[2]== 0xff && (bytes[3] & 0xf0) == 0xe0 )
{
printf("Found file {%i}", x);
output = fopen(num , "w");
fwrite(&bytes ,512 ,1 ,output);
x++;
}
else
{
fwrite(&bytes , 512 , 1 , output);
}
}
return 0;
}
output of valgrind
==12934== Invalid read of size 4
==12934== at 0x4A1C521: fwrite (iofwrite.c:37)
==12934== by 0x4014CA: main (recover.c:110)
==12934== Address 0x0 is not stack'd, malloc'd or (recently) free'd
==12934==
==12934==
==12934== Process terminating with default action of signal 11 (SIGSEGV)
==12934== Access not within mapped region at address 0x0
==12934== at 0x4A1C521: fwrite (iofwrite.c:37)
==12934== by 0x4014CA: main (recover.c:110)
==12934== If you believe this happened as a result of a stack
==12934== overflow in your program's main thread (unlikely but
==12934== possible), you can try to increase the size of the
==12934== main thread stack using the --main-stacksize= flag.
==12934== The main thread stack size used in this run was 8388608.
==12934==
==12934== HEAP SUMMARY:
==12934== in use at exit: 472 bytes in 1 blocks
==12934== total heap usage: 2 allocs, 1 frees, 4,568 bytes allocated
==12934==
==12934== LEAK SUMMARY:
==12934== definitely lost: 0 bytes in 0 blocks
==12934== indirectly lost: 0 bytes in 0 blocks
==12934== possibly lost: 0 bytes in 0 blocks
==12934== still reachable: 472 bytes in 1 blocks
==12934== suppressed: 0 bytes in 0 blocks
==12934== Rerun with --leak-check=full to see details of leaked memory
==12934==
==12934== For lists of detected and suppressed errors, rerun with: -s
==12934== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
zsh: segmentation fault valgrind ./recover card.raw

Answer 1

take this one
int main(int argc, char *argv[])
{
// check if one command arg is passed
if (argc > 2 )
{
printf("Usage: ./recover image\n");
return 1;
}
// try to open file to read from
FILE *file = fopen(argv[1], "r");
if (!file)
{
return 1;
}
BYTE bytes[512];
int x =0 ;
char num[10];
FILE* output =NULL;
while (fread(bytes, 512, 1, file) == 1)
{
// check for file start
if (bytes[0]==0xff && bytes[1] ==0xd8 && bytes[2]== 0xff && (bytes[3] & 0xf0) == 0xe0 )
{
if (output)
{
fclose(output);
}
// create file name
sprintf(num, "%03i.jpg",x);
printf("Found file {%i}\n", x);
output = fopen(num , "w");
fwrite(&bytes ,512 ,1 ,output);
x++;
}
else if (output)
{
fwrite(bytes , 512 , 1 , output);
}
}
if(output)
{
fclose(output);
}
return 0;
}
notice the use of fclose(output); to close file descriptors, as noted by Marek R

Answer 2

changing
else
{
fwrite(&bytes , 512 , 1 , output);
}
to
else if (output)
{
fwrite(&bytes , 512 , 1 , output);
}
generates the pictures
thanks to Marek R for mentioning the if file is opened comment