fwrite

[cs]

if i have
typedef struct{
unsigned a;
unsigned *b;
}tp;

i would like to ask if it is ok for the "C standard 89" doing
something like this

{
unsigned val=1;
tp e;
size_t k;
FILE fp;

e.a=1; e.b=&val;

if( (pf=fopen("file.dat", "w")) == 0 ) return 0;

k=fwrite(&e, sizeof(unsigned), 1, fp);
if(k<1) {fclose(fp); return 0;}
fwrite(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a) {fclose(fp); return 0;}

I would ask too if it goes ok in >98% cpu-oses. Thank you

 

[Michael Mair]

if i have
typedef struct{
unsigned a;
unsigned *b;
}tp;

i would like to ask if it is ok for the "C standard 89" doing
something like this

{
unsigned val=1;
tp e;
size_t k;
FILE fp;

Please provide code which can compile -- I do not know whether
the mistakes you make (FILE instead of FILE*, pf/fp etc) are just
because you were to lazy to provide a compiling minimal example
and _paste_ it unchanged or because you are not in the least
aware of what you are doing.

As you are trying to show us your question by pseudo-code, it
should be made clear what the question is.

e.a=1; e.b=&val;

if( (pf=fopen("file.dat", "w")) == 0 ) return 0;

Here your program goes wrong.

k=fwrite(&e, sizeof(unsigned), 1, fp);

Note:
k = fwrite(&e.a, sizeof e.a, 1, fp);
is much safer; if you change the type of e.a, the output will
automatically be correct.
And yes, e.a has the same address as e.

if(k<1) {fclose(fp); return 0;}
fwrite(e.b, sizeof(unsigned), e.a, fp);

You forgot to assign to k.
k = fwrite(e.b, sizeof *e.b, e.a, fp);
once more is better.

if(k<e.a) {fclose(fp); return 0;}

I would ask too if it goes ok in >98% cpu-oses. Thank you

What's a cpu-ose? Where can I find it in a C standard of your liking?
What was your actual question?
Leaving out main() and closing braces etc. will not work anywhere.
If you make a compiling program from all of it, then come again.

This time no fees for crystal ball use.


Cheers
Michael

 

[cs]

if i have
typedef struct{
unsigned a;
unsigned *b;
}tp;

i would like to ask if it is ok for the "C standard 89" doing
something like this

{
unsigned val=1;
tp e;
size_t k;
FILE fp;

e.a=1; e.b=&val;

if( (pf=fopen("file.dat", "w")) == 0 ) return 0;

k=fwrite(&e, sizeof(unsigned), 1, fp);
if(k<1) {fclose(fp); return 0;}
fwrite(e.b, sizeof(unsigned), e.a, fp);

k=fwrite(e.b, sizeof(unsigned), e.a, fp);

it is good to write
k=fwrite(&(e.a), sizeof(unsigned), 1, fp);
insteaf of
k=fwrite(&e, sizeof(unsigned), 1, fp);?
Thank you

 

[Richard Heathfield]

What's a cpu-ose?

It's the source for the standard input stream. Its purpose is to help the
machine keep all the points afloat.

 

[cs]

Please provide code which can compile -- I do not know whether
the mistakes you make (FILE instead of FILE*, pf/fp etc) are just
because you were to lazy to provide a compiling minimal example
and _paste_ it unchanged or because you are not in the least
aware of what you are doing.

As you are trying to show us your question by pseudo-code, it
should be made clear what the question is.

i don't know if this compile but
the question is why this program has the result down?

#include <stdio.h>
#include <stdlib.h>

typedef struct{
unsigned a;
unsigned *b;
}tp;


int main(void)
{ int i;
unsigned val[90]={0};
tp e;
size_t k;
FILE *fp;
/**********************/
e.a=90; e.b=val;
for(i=0; i<e.a; ++i) e.b=rand();

if( (fp=fopen("file.dat", "w")) == 0 ) return 0;
k=fwrite(&e.a, sizeof(unsigned), 1, fp);
if(k<1) {fclose(fp); return 0;}

k=fwrite(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a) {fclose(fp); return 0;}
if( fclose(fp)==EOF ) printf("W Error\n");

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }
printf("\n");

if( (fp=fopen("file.dat", "r")) == 0 ) {printf("fopen"); goto la;}

k=fread(&e.a, sizeof(unsigned), 1, fp);

if(e.a>20000 || k<1 || ferror(fp))
{fclose(fp); return 0;}

k=fread(e.b, sizeof(unsigned), e.a, fp);
/* if(k<e.a || ferror(fp))
{fclose(fp);
printf("E0 k=%u e.a=%u\n", (unsigned) k, e.a);
return 0;
}
*/
if( fclose(fp)==EOF )
{la:;
printf("R Error\n");
}

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }
printf("\n");

return 0;
}

/* result */
e.b[0]=82 e.b[1]=2ae6 e.b[2]=442 e.b[3]=2d88 e.b[4]=1bcd e.b[5]=44bb
e.b[6]=190f e.b[7]=59a4 e.b[8]=7996 e.b[9]=232c e.b[10]=38de
e.b[11]=df3 e.b[12]=595f e.b[13]=483c e.b[14]=550 e.b[15]=1524
e.b[16]=6861 e.b[17]=57bf e.b[18]=61d7 e.b[19]=69ef e.b[20]=7ad1
e.b[21]=1c16 e.b[22]=36a1 e.b[23]=79ee e.b[24]=6b75 e.b[25]=762c
e.b[26]=67cb e.b[27]=39bb e.b[28]=4d68 e.b[29]=54b1 e.b[30]=4cc3
e.b[31]=464b e.b[32]=5c29 e.b[33]=e96 e.b[34]=33fe e.b[35]=f8b
e.b[36]=55eb e.b[37]=3cc9 e.b[38]=3edc e.b[39]=4839 e.b[40]=2c18
e.b[41]=6f32 e.b[42]=21d8 e.b[43]=1704 e.b[44]=3627 e.b[45]=58ee
e.b[46]=14f4 e.b[47]=44e7 e.b[48]=529f e.b[49]=4fcb e.b[50]=64
e.b[51]=64c3 e.b[52]=226c e.b[53]=3b04 e.b[54]=317a e.b[55]=303b
e.b[56]=4a62 e.b[57]=4d3e e.b[58]=23d1 e.b[59]=15d5 e.b[60]=670f
e.b[61]=25c2 e.b[62]=2abd e.b[63]=344e e.b[64]=1ec6 e.b[65]=34fd
e.b[66]=6f3 e.b[67]=3f3e e.b[68]=7de9 e.b[69]=35 e.b[70]=3475
e.b[71]=8ed e.b[72]=976 e.b[73]=208d e.b[74]=7cc1 e.b[75]=2
d74 e.b[76]=33d4 e.b[77]=193c e.b[78]=193a e.b[79]=1b18 e.b[80]=1fe0
e.b[81]=610a e.b[82]=71d6 e.b[83]=73e3 e.b[84]=5e33 e.b[85]=3b0c
e.b[86]=440a e.b[87]=484 e.b[88]=10b7 e.b[89]=3aa0
e.b[0]=82 e.b[1]=2ae6 e.b[2]=442 e.b[3]=2d88 e.b[4]=1bcd e.b[5]=44bb
e.b[6]=190f e.b[7]=59a4 e.b[8]=7996 e.b[9]=232c e.b[10]=38de
e.b[11]=5f0000f3 e.b[12]=3c000059 e.b[13]=50000048 e.b[14]=24000005
e.b[15]=61000015 e.b[16]=bf000068 e.b[17]=d7000057 e.b[18]=ef000061
e.b[19]=d1000069 e.b[20]=1600007a e.b[21]=a100001c e.b[22]=ee000036
e.b[23]=75000079 e.b[24]=2c00006b e.b[25]=cb000076 e.b[26]=bb00006
7 e.b[27]=68000039 e.b[28]=b100004d e.b[29]=c3000054 e.b[30]=4b00004c
e.b[31]=29000046 e.b[32]=9600005c e.b[33]=fe00000e e.b[34]=8b000033
e.b[35]=eb00000f e.b[36]=c9000055 e.b[37]=dc00003c e.b[38]=3900003e
e.b[39]=18000048 e.b[40]=3200002c
e.b[41]=d800006f e.b[42]=4000021 e.b[43]=27000017 e.b[44]=ee000036
e.b[45]=f4000058 e.b[46]=e7000014 e.b[47]=9f000044 e.b[48]=cb000052
e.b[49]=6400004f e.b[50]=c3000000 e.b[51]=6c000064 e.b[52]=4000022
e.b[53]=7a00003b e.b[54]=3b000031 e.b[55]=62000030 e.b[56]=3e00004a
e.b[57]=d100004d e.b[58]=d5000023 e.b[59]=f000015 e.b[60]=c2000067
e.b[61]=bd000025 e.b[62]=4e00002a e.b[63]=c6000034 e.b[64]=fd00001e
e.b[65]=f3000034 e.b[66]=3e000006 e.b[67]=e900003f e.b[68]=3500007d
e.b[69]=75000000 e.b[70]=ed000034 e.b[71]=76000008 e.b[72]=8d000009
e.b[73]=c1000020 e.b[74]=7400007c e.b[75]=d400002d e.b[76]=3c000033
e.b[77]=3a000019 e.b[78]=18000019 e.b[79]=e000001b e.b[80]=a00001f
e.b[81]=d6000061 e.b[82]=e3000071 e.b[83]=33000073 e.b[84]=c00005e
e.b[85]=a00003b e.b[86]=84000044 e.b[87]=b7000004 e.b[88]=a0000010
e.b[89]=3a

 

[Michael Mair]

It's the source for the standard input stream. Its purpose is to help the
machine keep all the points afloat.

Glad to hear that -- sounded potentially contagious to me, so I
was afraid to contract one without knowing what hit me.


Cheers
Michael

 

[cs]

i don't know if this compile but
the question is why this program has the result down?

i forgot "b"== binary mode

#include <stdio.h>
#include <stdlib.h>

typedef struct{
unsigned a;
unsigned *b;
}tp;



int main(void)
{ int i;
unsigned val[90]={0};
tp e;
size_t k;
FILE *fp;
/**********************/
e.a=90; e.b=val;
for(i=0; i<e.a; ++i) e.b=rand();

if( (fp=fopen("file.dat", "wb")) == 0 ) return 0;
k=fwrite(&e.a, sizeof(unsigned), 1, fp);
if(k<1) {fclose(fp); return 0;}

k=fwrite(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a) {fclose(fp); return 0;}
if( fclose(fp)==EOF ) printf("W Error\n");

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }
printf("\n");

if( (fp=fopen("file.dat", "rb")) == 0 ) {printf("fopen"); goto la;}

k=fread(&e.a, sizeof(unsigned), 1, fp);

if(e.a>20000 || k<1 || ferror(fp))
{fclose(fp); return 0;}

k=fread(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a || ferror(fp))
{fclose(fp);
printf("E0 k=%u e.a=%u\n", (unsigned) k, e.a);
return 0;
}
if( fclose(fp)==EOF )
{la:;
printf("R Error\n");
}

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }
printf("\n");

return 0;
}

 

[Michael Mair]

k=fwrite(e.b, sizeof(unsigned), e.a, fp);

it is good to write
k=fwrite(&(e.a), sizeof(unsigned), 1, fp);
insteaf of
k=fwrite(&e, sizeof(unsigned), 1, fp);?
Thank you

Ah, now it becomes clearer.

The address of a structure is the same as the address of its
first member but it is much safer to write what you mean --
imagine the position of a changes within the type tp; then you
would have to change all your clever &e to &e.a and almost certainly
miss one, which in turn would maybe give you funny bugs.
If you feel more comfortable with &(e.a), use it instead of &e.a;
it certainly does not hurt.
See also my initial reply which tells you among other things how to
use the sizeof operator in a more fail-safe way.

Cheers
Michael

 

[Michael Mair]

i don't know if this compile but
the question is why this program has the result down?

Yes, it compiles.
Sorry, you once more failed to tell what you _expected_.

Are you talking about the output? If yes: What did you expect
instead? Did you want to test rand() or your I/O procedure?

#include <stdio.h>
#include <stdlib.h>

typedef struct{
unsigned a;

a is used to denote the size of an array.
If you want to make these arrays to be able have >= 65536 elements,
do not use unsigned int as type for a but use long (up to (1L<<31)-1
elements), unsigned long (up to 2L*(1L<<31) - 1 elements) or, best,
size_t (as many elements as fit on your machine).
Note that these limits can be exceeded on your machine and your
implementation but these are what is _guaranteed_.

unsigned *b;
}tp;


int main(void)
{ int i;
unsigned val[90]={0};
tp e;
size_t k;
FILE *fp;
/**********************/
e.a=90; e.b=val;

Note: you are using a "magic number" (90). This may well bite
you later on if changing not all occurrences of 90 correctly.
Rather use symbolic constants or compute values, i.e.
#define NUM_VAL 90
....
unsigned val[NUM_VAL] = {0};
....
e.a = NUM_VAL;
or
e.a = sizeof val/sizeof val[0]; /* works only in the scope where
* the array val has been declared.*/

for(i=0; i<e.a; ++i) e.b=rand();

Why are you using rand()?
If you want to compare whether what has been written is read back
correctly, rather use values you know or, if you insist on rand(),
use a second array to read it in so you can compare directly or
at least write out the contents of val _immediately_
after setting them.

if( (fp=fopen("file.dat", "w")) == 0 ) return 0;

exit(EXIT_FAILURE);
preceded by an appropriate error message may be a better solution.

k=fwrite(&e.a, sizeof(unsigned), 1, fp);
if(k<1) {fclose(fp); return 0;}

k=fwrite(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a) {fclose(fp); return 0;}
if( fclose(fp)==EOF ) printf("W Error\n");

Note: If you open file.dat in mode "w+", it suffices to
rewind(fp) to start reading.

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }

Note: Use an iteration variable of the same type as e.a.
otherwise you may have the funny effect that e.a has a
value larger than the possible maximum value for i which
results in an overflow for i and thus undefined behaviour
(which may lead to an endless loop).

printf("\n");

if( (fp=fopen("file.dat", "r")) == 0 ) {printf("fopen"); goto la;}

Note: Most people will verbally hang, draw, disembowel and quarter you
happily for use of goto. Almost all will do so for such an unnecessary
and badly documented use of goto.
The la: label is rather hidden than made obvious.
It seems you were just too lazy to type the rest of the
error message a second time. Use goto only to make _clearer_ what
happens (which can be the case if you want a one-exit function or
have lenghty clean-up code) or if you can _prove_ that its use is
absolutely necessary to squeeze the _necessary_ bit of performance
out of some critical piece of code (which is the case in about one
in a million cases; some other cases may make do with a cleaner
construct, most improve with the use of a better algorithm).

k=fread(&e.a, sizeof(unsigned), 1, fp);

if(e.a>20000 || k<1 || ferror(fp))
{fclose(fp); return 0;}

Note that you throwing up on _very_ different things.
k<1 and ferror(fp) may go together as they document input
errors.
e.a>20000 (why not 90? -- Ah, the joys of magic numbers)
documents "wrong input value" which has nothing to do with
the rest.

Consider writing an errMsgExit() function which outputs
a string to stderr and exit(EXIT_FAILURE)s.

k=fread(e.b, sizeof(unsigned), e.a, fp);
/* if(k<e.a || ferror(fp))
{fclose(fp);
printf("E0 k=%u e.a=%u\n", (unsigned) k, e.a);
return 0;
}
*/
if( fclose(fp)==EOF )
{la:;
printf("R Error\n");
}

If you _ever_ _need_ a goto, at least document the labels as
good as you can. e.g.
if( fclose(fp)==EOF )
{
la:
printf("R Error\n");
}
Do add empty statements only if necessary (which is hardly ever
the case in C89).

for(i=0; i<e.a; ++i)
{printf("e.b[%d]=%x ", i, e.b); e.b=0; }

Why are you resetting e.b without need?

printf("\n");

return 0;
}

/* result */
e.b[0]=82 e.b[1]=2ae6 e.b[2]=442 e.b[3]=2d88 e.b[4]=1bcd e.b[5]=44bb
e.b[6]=190f e.b[7]=59a4 e.b[8]=7996 e.b[9]=232c e.b[10]=38de
e.b[11]=df3 e.b[12]=595f e.b[13]=483c e.b[14]=550 e.b[15]=1524
e.b[16]=6861 e.b[17]=57bf e.b[18]=61d7 e.b[19]=69ef e.b[20]=7ad1
e.b[21]=1c16 e.b[22]=36a1 e.b[23]=79ee e.b[24]=6b75 e.b[25]=762c
e.b[26]=67cb e.b[27]=39bb e.b[28]=4d68 e.b[29]=54b1 e.b[30]=4cc3
e.b[31]=464b e.b[32]=5c29 e.b[33]=e96 e.b[34]=33fe e.b[35]=f8b
e.b[36]=55eb e.b[37]=3cc9 e.b[38]=3edc e.b[39]=4839 e.b[40]=2c18
e.b[41]=6f32 e.b[42]=21d8 e.b[43]=1704 e.b[44]=3627 e.b[45]=58ee
e.b[46]=14f4 e.b[47]=44e7 e.b[48]=529f e.b[49]=4fcb e.b[50]=64
e.b[51]=64c3 e.b[52]=226c e.b[53]=3b04 e.b[54]=317a e.b[55]=303b
e.b[56]=4a62 e.b[57]=4d3e e.b[58]=23d1 e.b[59]=15d5 e.b[60]=670f
e.b[61]=25c2 e.b[62]=2abd e.b[63]=344e e.b[64]=1ec6 e.b[65]=34fd
e.b[66]=6f3 e.b[67]=3f3e e.b[68]=7de9 e.b[69]=35 e.b[70]=3475
e.b[71]=8ed e.b[72]=976 e.b[73]=208d e.b[74]=7cc1 e.b[75]=2
d74 e.b[76]=33d4 e.b[77]=193c e.b[78]=193a e.b[79]=1b18 e.b[80]=1fe0
e.b[81]=610a e.b[82]=71d6 e.b[83]=73e3 e.b[84]=5e33 e.b[85]=3b0c
e.b[86]=440a e.b[87]=484 e.b[88]=10b7 e.b[89]=3aa0
e.b[0]=82 e.b[1]=2ae6 e.b[2]=442 e.b[3]=2d88 e.b[4]=1bcd e.b[5]=44bb
e.b[6]=190f e.b[7]=59a4 e.b[8]=7996 e.b[9]=232c e.b[10]=38de
e.b[11]=5f0000f3 e.b[12]=3c000059 e.b[13]=50000048 e.b[14]=24000005
e.b[15]=61000015 e.b[16]=bf000068 e.b[17]=d7000057 e.b[18]=ef000061
e.b[19]=d1000069 e.b[20]=1600007a e.b[21]=a100001c e.b[22]=ee000036
e.b[23]=75000079 e.b[24]=2c00006b e.b[25]=cb000076 e.b[26]=bb00006
7 e.b[27]=68000039 e.b[28]=b100004d e.b[29]=c3000054 e.b[30]=4b00004c
e.b[31]=29000046 e.b[32]=9600005c e.b[33]=fe00000e e.b[34]=8b000033
e.b[35]=eb00000f e.b[36]=c9000055 e.b[37]=dc00003c e.b[38]=3900003e
e.b[39]=18000048 e.b[40]=3200002c
e.b[41]=d800006f e.b[42]=4000021 e.b[43]=27000017 e.b[44]=ee000036
e.b[45]=f4000058 e.b[46]=e7000014 e.b[47]=9f000044 e.b[48]=cb000052
e.b[49]=6400004f e.b[50]=c3000000 e.b[51]=6c000064 e.b[52]=4000022
e.b[53]=7a00003b e.b[54]=3b000031 e.b[55]=62000030 e.b[56]=3e00004a
e.b[57]=d100004d e.b[58]=d5000023 e.b[59]=f000015 e.b[60]=c2000067
e.b[61]=bd000025 e.b[62]=4e00002a e.b[63]=c6000034 e.b[64]=fd00001e
e.b[65]=f3000034 e.b[66]=3e000006 e.b[67]=e900003f e.b[68]=3500007d
e.b[69]=75000000 e.b[70]=ed000034 e.b[71]=76000008 e.b[72]=8d000009
e.b[73]=c1000020 e.b[74]=7400007c e.b[75]=d400002d e.b[76]=3c000033
e.b[77]=3a000019 e.b[78]=18000019 e.b[79]=e000001b e.b[80]=a00001f
e.b[81]=d6000061 e.b[82]=e3000071 e.b[83]=33000073 e.b[84]=c00005e
e.b[85]=a00003b e.b[86]=84000044 e.b[87]=b7000004 e.b[88]=a0000010
e.b[89]=3a

On my machine, everything looks well.
Does the value of e.b[11] also go wrong for NUM_VAL 12 or is
this keyed to 90?


Cheers
Michael

Have a look at

#include <stdio.h>
#include <stdlib.h>

typedef struct{
size_t a;
unsigned *b;
}tp;

#define MAX_VAL 90UL

static void errMsgExit(const char *msg, FILE *fp)
{
(void)fclose(fp);
if (msg)
(void)fputs(msg, stderr);
exit(EXIT_FAILURE);
}

int main(void)
{
size_t i;
unsigned val[MAX_VAL]={0};
unsigned rval[MAX_VAL]={0};
tp e = {0,0};
tp r = {0,0};
size_t k;
FILE *fp;
/**********************/
e.a = MAX_VAL; e.b = val;
r.b = rval;

for(i=0; i<e.a; ++i) e.b=rand();

if( (fp=fopen("file.dat", "w+")) == NULL )
errMsgExit("W Error on opening file\n", NULL);
k=fwrite(&e.a, sizeof(unsigned), 1, fp);
if(k<1)
errMsgExit("W Error on writing size\n", fp);

k=fwrite(e.b, sizeof(unsigned), e.a, fp);
if(k<e.a)
errMsgExit("W Error on writing array contents\n", fp);

/*
for(i=0; i<e.a; ++i)
printf("e.b[%lu]=%x ", (unsigned long)i, e.b);
printf("\n");
*/
rewind(fp);

k=fread(&r.a, sizeof r.a, 1, fp);

if(k<1 || ferror(fp)!=0)
errMsgExit("R Error on reading size\n", fp);
if (r.a != e.a)
errMsgExit("R Error wrong size\n", fp);

k=fread(r.b, sizeof *r.b, r.a, fp);
if(k<r.a || ferror(fp)!=0)
errMsgExit("R Error on reading array\n", fp);

if( fclose(fp)==EOF )
{
errMsgExit("R Error closing file\n", NULL);
}

for(i=0; i<e.a; ++i)
if (e.b != r.b)
printf("e.b[%lu]=%x r.b[%lu]=%x\n",
(unsigned long)i, e.b,
(unsigned long)i, r.b);
printf("\n");

return 0;
}

 

[Michael Mair]

i forgot "b"== binary mode

That certainly explains it. Completely missed that, sorry.

Cheers
Michael

 

[cs]

for(i=0; i<e.a; ++i) e.b=rand();

Why are you using rand()?
If you want to compare whether what has been written is read back
correctly, rather use values you know or, if you insist on rand(),
use a second array to read it in so you can compare directly or
at least write out the contents of val _immediately_
after setting them.

the rand seed is the same every call; rand() it is supposed has the
same output so i know what values they are; then this way to use
random had saved me from many errors
Thank you