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Asked: 2026-06-17T07:31:19+00:00

I am trying to read in data from a file and am confused. The

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I am trying to read in data from a file and am confused. The file contains three columns of data. With fscanf it is giving the correct values.

FILE* fp = fopen("test.txt");
double buffer[3];
fscanf(fp,"%lf %lf %lf",&buffer[0],&buffer[1],&buffer[2]);

Now I am trying to read the same set of values using _read. This gives me all the wrong values. 

int fh;
char buffer[50];
_sopen_s( &fh,CStringA(PointFile),_O_RDONLY|_O_BINARY,_SH_DENYNO,0);
_read(fh,buffer,sizeof(double)*3);

It will be really helpful if someone can tell me what I am doing wrong.

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1 Answer

  1. Editorial Team

    The first code snippet reads textual representation of doubles and converts them. That’s what *scanf family of functions does.

    Example valid input:

    1.0 2.7182818 3.14159265358

    The second code snippet reads some (probably 24) bytes from a file, without parsing or conversion. You didn’t show how you extract doubles from your buffer. Something like *(double *)buffer (to get the first value) would be expected, and it would be right (modulo alignment issues, but it’s irrelevant for your platform) if you have a “dump” of 3 platform-specific doubles in your file, created with appropriate fwrite or _write.

    Example valid input (hex dump, assuming 64-bit IEEE floats):

    00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

    (If you were using POSIX read, here would be a warning on the possibility of short reads. However, _read and _s_open suggest Microsoft CRT, and it doesn’t do short reads in binary mode unless the end of file is reached).

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