# Converting fixed-width text file to CSV in C

I'm trying to get reacquainted with C after years of higher-level languages.

I wrote a small fixed-width to CSV converter. It works (for ASCII at least) and it's quite fast, but I wonder if I missed anything or if it can be made even faster.

I'm reading the file line by line and process the desired columns (some can be skipped), placing them into an output 8K buffer. I'm using one function to do the substring, appending and trimming trailing space (just the space, in this case I don't want to bother testing for other whitespace characters).

#include <ctype.h>

// number of columns to process
#define COLS 3
#define LINE_SIZE 256
#define BUFFER_SIZE 8192

#define INFILE "in.txt"
#define OUTFILE "out.csv"

size_t RANGES[COLS][2] = {{0, 6}, {6, 20}, {29, 3}};

/*
* Copy from source to destination, up to len chars, trimming trailing spaces
* Returns number of chars actually copied
*/
int trimcpy(char *destination, char *source, size_t len) {
// trim spaces from the end - we only care about the space char
while (len>0 && source[len-1]==' ')
len--;

int i = 0;
while (i<len && *source != '\0') {
*destination++ = *source++;
i++;
}
*destination = '\0';

return i;
}

int main(void) {
FILE *rfp;
FILE *wfp;

char line[LINE_SIZE];
char out[BUFFER_SIZE];

rfp = fopen(INFILE, "r");
if (rfp == NULL)
exit(EXIT_FAILURE);

wfp = fopen(OUTFILE, "w");
if (wfp == NULL)
exit(EXIT_FAILURE);

int p = 0;

// fgets is 4x faster than getline!
while (fgets(line, LINE_SIZE, rfp) != NULL) {
// write buffer if almost full (largest column is 20 chars)
if (p > BUFFER_SIZE - 20) {
fputs(out, wfp);
p = 0;
}

// go through the columns
for (int i=0; i<COLS; i++) {
p += trimcpy(out+p, line+RANGES[i][0], RANGES[i][1]);
p += trimcpy(out+p, i<COLS-1 ? "," : "\n", 1);
}
}

// write any remaining data in buffer
fputs(out, wfp);

fclose(rfp);
fclose(wfp);

exit(EXIT_SUCCESS);
}


Consider const char *

const char *source allows the function to be used with const strings, better conveys code's functionality and more readily allows for optimizations.

// int trimcpy(char *destination, char *source, size_t len) {
int trimcpy(char *destination, const char *source, size_t len) {


Undefined behavior

trimcpy() begins by examining later elements of source[] even though they are not known to have been assigned. The string may not be as long as len.

int trimcpy_alt(char *destination, char *source, size_t len) {
// Suggest memchr() here rather than strlen() to not look too far.
char *null_character_pos = memchr(source, '\0', len);
if (null_character_pos) len = null_character_pos - source;
...


Undefined behavior 2

In a selective case of an empty file, the first call to fgets() returns NULL and then the following fputs(out, wfp); is UB as out contents are not initialized. Add initialization or assignment.

char out[BUFFER_SIZE];


Not trimming end spaces before \n

trimcpy() does not trim spaces just before '\n'. I suspect this in not in line with OP's goals.

Avoid redundant information

Redundant information takes more work to maintain. Consider dropping the 3

// #define COLS 3
size_t RANGES[][2] = {{0, 6}, {6, 20}, {29, 3}};
#define COLS (sizeof RANGES/sizeof RANGES[0])


Minor things:

Mixing types

Little reason to mix size_t and int types here for array indexing and sizing. Recommend to use just one: size_t (my preference) or 2) int, but not both.

// From
int trimcpy(char *destination, char *source, size_t len) {
...
int i = 0;
while (i<len && *source != '\0') {

// To
size_t trimcpy(char *destination, char *source, size_t len) {
...
size_t i = 0;
while (i<len && *source != '\0') {


main()

  // int p = 0;
size_t p = 0;


Subtraction with unsigned types

If above size_t employed, consider the 2 below: Which works well should a later version of code surprisingly define #define BUFFER_SIZE 19?

if (p > BUFFER_SIZE - 20) {
if (p + 20 > BUFFER_SIZE) {


The first is the same as if (p > SIZE_MAX) { or if (0) {

• @Armand null_character_pos - source subtracts two char * pointers. The difference is the length from source to null_character_pos, the length of the string. – chux - Reinstate Monica Dec 27 '18 at 19:10

This code:

int i = 0;
while (i<len && *source != '\0') {
*destination++ = *source++;
i++;
}
*destination = '\0';


shouldn't be doing a byte-by-byte copy. (It should also be a for-loop instead of a while loop, but that's beside the point.) Instead, you should probably just call memcpy:

memcpy(destination, source, len);
destination[len] = '\0';


The reference for fopen says that:

Upon successful completion, fopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.

rfp = fopen(INFILE, "r");
if (rfp == NULL)
exit(EXIT_FAILURE);


is throwing away the error information. In the failure block, you should be calling perror to see why exactly the call failed.

This:

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


assumes that NULL only happens if an EOF is encountered, but that isn't necessarily the case. You need to check feof, and if it isn't an EOF, then something bad has happened and you need to again call perror and bail.

A note about the fgets documentation described in the POSIX standard. The "CX" in this text:

... [CX] and shall set errno to indicate the error.

indicates that support for setting errno is in an extension; however, from the same standard:

The functionality described is an extension to the ISO C standard. Application developers may make use of an extension as it is supported on all POSIX.1-2017-conforming systems.

So as long as you're targeting a system that doesn't violate POSIX, you should be able to use it. Even if a system violated POSIX and didn't set errno, you should still be checking feof; the condition where fgets returns NULL and errno is set to an error would just never be seen. The worst that would happen is a perror indicating that the system doesn't know what the error is, but you still know that there's an error.

Lastly: do some light reading here - https://stackoverflow.com/questions/461449/return-statement-vs-exit-in-main

I don't recommend calling exit at the end of main; simply return.

• On fopen() error, The C spec says "If the open operation fails, fopen returns a null pointer.". There is no spec concerning errno with that function - unlike the extension in your reference. OP's code is portable because of that. Using "you should be calling perror()" can well be un-informative. – chux - Reinstate Monica Dec 26 '18 at 9:00
• The similar advice about "You need to check feof, ... and you need to again call perror and bail." similarly relies on an extension. – chux - Reinstate Monica Dec 26 '18 at 9:06
• @chux Edited; I stand by my recommendation but I've added more detail about the standard. – Reinderien Dec 26 '18 at 15:03