Modify a binary file in C

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Modify a binary file in C

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#1 by (0 votes)

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I need to read and modify a binary file, I am using this code:

FILE *fp; t_data dat; fp=fopen("file.bin","r+b"); 
fread(&dat,sizeof(t_data),1,fp); 
while(!feof(fp))
{
    if(dat.aaaa==2018)
    {
        int pos = ftell(fp)-sizeof(t_data);
        dat.prec=2;
        fseek(fp,pos,SEEK_SET);
        fwrite(&dat,sizeof(t_data),1,fp);
    }
    fread(&dat, sizeof(t_data),1,fp);
}
fclose(fp);

the program enters a loop apart from not modifying the data with which I want to work, I do not understand what I'm doing wrong, what can it be? thanks

c archivo

asked by Sebastardo 11.10.2018 в 16:12

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1 answer

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score 0 · Answer 1

ok, I found the reason why the loop was made

FILE *fp; t_data dat; fp=fopen("file.bin","r+b"); 
fread(&dat,sizeof(t_data),1,fp); 
while(!feof(fp))
{
    if(dat.aaaa==2018)
    {
        int pos = ftell(fp)-sizeof(t_data);
        dat.prec=2;
        fseek(fp,pos,SEEK_SET);
        fwrite(&dat,sizeof(t_data),1,fp); // faltaba esta linea
    }
    fread(&dat, sizeof(t_data),1,fp);
}
fclose(fp);

you must call fseek or rewind between writing and reading to reset the read / write state before switching back to reading mode