如何在C的csv文件的行尾写入新的字符串

Question

I am writing a C code which need to add the new information in the end of the line if the condition match.

    FILE *file = fopen(fileName, "r+");

    //read every line if line!=NULL
    while (fgets(line, LINE_SIZE, file)!= NULL){
        //split the line by sep
        split(line, ',', fields);       
        fprintf(file,",%d,%d,%d",D,F,G);
   }

It is the code I try to write the matched information. However, it will write the information in the beginning of the line. Is that possible I can move the "file" pointer to the end of the line?

Answer 1

Probably you need to follow couple more steps

Open a temporary file along with existing file

FILE *file = fopen(fileName, "r");
FILE *tmpFile = fopen(tempFileName, "a+" );

get a line from existing file, trim Next line append your fields after comma, probably create comma separated string of extra fields

while (fgets(line, LINE_SIZE, file)!= NULL){

Remove New line and add your paremeters

    if( line[ strlen(line) - 1  ] == '\n' )
        line[ strlen(line) - 1  ] = '\0';

    fprintf(tmpFile,"%s,%s\n",line, your_field); // Write to temporary file
}

Delete old file unlink is probably useful

man -a unlink

and then rename temp file to original file

man -a rename

Answer 2

There's no way of doing what you want. You can't insert bytes in the middle of a file.

However, if you have spaces or something similar that allows bytes to be written into the file without overwriting data, you could accomplish something close to what you want. Say you have the lines

"1,2,3,            "
"4,5,6,            "

(note the space) you could insert text there. I would use two file pointers:

• get the position from read file pointer when the line starts
• read the line using that file pointer
• add the number of bytes until the first space to the position
• seek on the other file pointer to this position
• write the new data and make sure there's enough space

The first file pointer would be in "read" mode, the second in "write" mode and none in "truncate" because that would screw up your file.

Or use a much better database format. ;)