I need to write data to a binary file using C's I/O functions. The following code causes a runtime exception :
#include "stdio.h"
int main(int argc,char* argv[]) { FILE *fp = fopen("path_to_file.bin","wb"); if(fp == NULL) { printf("error creating file"); return -1; } int val = 4; fwrite((const void*)val,sizeof(int),1,fp); fclose(fp); return 0;
}The code dies at the fwrite. Can you spot what I'm doing wrong? Apparently, I'm, trying to access data at 0x0000004 or something like that.
Thanks !
6 Answers
I think Neil's answer can be improved upon. I realize it's already accepted, so this is just to show some contrast (which is why I didn't just edit his).
fwrite(&val, sizeof val, 1, fp);Two improvements:
- No pointer casting, since it's not necessary in C and can hide errors.
- Use
sizeofdirectly on the object, since that is what you're passing a pointer to. Makes a lot of sense to me, and is safer than repeating yourself and using the type name.
fwrite((const void*)val,sizeof(int),1,fp);should be:
fwrite((const void*) & val,sizeof(int),1,fp);BTW, if you don't use the cast, you will get a sensible error message. Casts tend to be used by C (and C++) programmers far more often than they should be - a good rule of thumb is "if it needs a cast, it's probably wrong".
1Adding to Neil's answer: this works when you are reading and writing the file on the same platform. Things can become weird if you are reading/writing across platforms with different endianness.
2#include "stdio.h"
int main(int argc,char* argv[]) { FILE *fp = fopen("path_to_file.bin","wb"); if(fp == NULL) { printf("error creating file"); return -1; } int val = 4; fwrite((const void*)val,sizeof(int),1,fp);You should supply an address not integer itself.
Additionaly you should not use the integer in such way:
- It may differ in endianess on different computers (as mentioned)
- It may differ in size. On really old computers it may be 1 or 2 bytes. On most modern it will be 4 but it may be 8 as well (some 64-bit computers). On some strange architectures it may be even 36 bits.
int32_t val = 4; fwrite((const void *)val, 4, 1, fp)should solve the problem.
You may think that your software will never need to be ported. Well - many designers (software and hardware) made similar assumptions. Sometimes it is too costly to not make them - but in this case it is just a matter of few additional checks.
fclose(fp); return 0;
}I also faced this kind of problem. So this is my solution.
fwrite(val, sizeof(val[0], sizeof(val)/sizeof(val[0]), fp);
Apparently, I'm, trying to access data at 0x0000004 or something like that.
int val = 4There's the issue. fwrite is designed to work with strings, and as such its first input is a pointer, the location of a string in memory. You are passing the value of val directly (4) rather than the address of val to fwrite; however, the memory at 0x00000004 is not valid program memory and thus an error is given.
To fix this, change this:
fwrite((const void*)val,sizeof(int),1,fp);Into this:
fwrite((const void*)&val, sizeof(int), 1, fp);The "&" operator indicates the location of val. This would be a valid address in memory.
