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Use file descriptors directly instead of FILEs
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@shlevy
shlevy committed Aug 14, 2016
1 parent e9991b3 commit 3c5174b
Showing 1 changed file with 137 additions and 129 deletions.
266 changes: 137 additions & 129 deletions long-shebang.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,13 @@
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>

#ifndef O_CLOEXEC
#define O_CLOEXEC 0
#define NEED_FCNTL
#endif

typedef enum {
exit_usage = 1,
Expand All @@ -15,32 +22,56 @@ typedef enum {
exit_null = 6,
exit_unknown_escape = 7,
exit_exec = 8,
exit_no_shebang
exit_no_shebang = 9,
exit_stat = 10,
exit_no_args = 11,
exit_bad_script_arg = 12
} exit_code;

static void handle_eof(const char *script) {
if (errno) {
fprintf(stderr, "reading from %s: %s\n", script, strerror(errno));
typedef struct {
int fd;
char * buf;
size_t offset;
size_t fill;
size_t capacity;
char * filename;
} state;

static void read_more(state * st) {
if (st->fill == st->capacity) {
st->capacity *= 2;
if (st->fill >= st->capacity) {
if (st->fill == SIZE_MAX) {
fprintf(stderr, "argument buffer requires ridiculous amount of space (did you really hit this code path?)\n");
exit(exit_mem);
}
st->capacity = SIZE_MAX;
}
st->buf = realloc(st->buf, st->capacity);
if (!st->buf) {
perror("allocating argument buffer");
exit(exit_mem);
}
}

ssize_t nread = read(st->fd, st->buf + st->fill, st->capacity - st->fill);
if (nread == -1) {
fprintf(stderr, "reading from %s: %s\n", st->filename, strerror(errno));
exit(exit_reading);
} else if (nread == 0) {
fprintf(stderr, "unexpected EOF reading from %s\n", st->filename);
exit(exit_eof);
}
fprintf(stderr, "early EOF reading from %s", script);
exit(exit_eof);
st->fill += (size_t) nread;
}

static void increase_buffer(void ** buf, size_t * sz, size_t el_sz) {
if (*sz >= SIZE_MAX / el_sz) {
fprintf(stderr, "asking for ridiculously large buffer\n");
exit(exit_mem);
} else if (SIZE_MAX / 2 * el_sz < *sz) {
*sz = SIZE_MAX / el_sz;
} else {
*sz *= 2;
}
*buf = realloc(*buf, *sz * el_sz);
if (!*buf) {
perror("increasing buffer size");
exit(exit_mem);
static char next_char(state * st) {
if (st->offset == st->fill) {
st->fill = 0;
st->offset = 0;
read_more(st);
}
return st->buf[st->offset++];
}

int main(int argc, char ** argv) {
Expand All @@ -49,134 +80,111 @@ int main(int argc, char ** argv) {
return exit_usage;
}

FILE * script = fopen(argv[1], "r");
if (!script) {
fprintf(stderr, "opening %s for reading: %s\n", argv[1], strerror(errno));
state st;

st.filename = argv[1];

st.fd = open(st.filename, O_RDONLY | O_CLOEXEC);
if (st.fd == -1) {
fprintf(stderr, "opening %s for reading: %s\n", st.filename, strerror(errno));
return exit_open_script;
}
#ifdef NEED_FCNTL
fcntl(st.fd, F_SETFD, fcntl(st.fd, F_GETFD) | FD_CLOEXEC);
#endif

/* Randomly chosen starting size for number of args, should probably profile */
size_t args_size = 4 * sizeof(char *);
/* args[0] always used */
size_t args_used = 1;
char ** args = malloc(args_size);
if (!args) {
perror("allocating argument pointers");
return exit_mem;
struct stat buf;
if (fstat(st.fd, &buf) == -1) {
fprintf(stderr, "getting file status of %s: %s\n", st.filename, strerror(errno));
return exit_stat;
}

/* Randomly chosen starting size for the actual strings, should probably profile */
size_t args_string_size = 1024;
size_t args_string_used = 0;
char * args_string = malloc(args_string_size);
if (!args_string) {
st.capacity = buf.st_blksize;
st.fill = 0;
st.offset = 0;
st.buf = malloc(st.capacity);
if (!st.buf) {
perror("allocating argument buffer");
return exit_mem;
exit(exit_mem);
}

/* Skip real shebang line */
errno = 0;
while (1) {
switch (fgetc(script)) {
case EOF:
handle_eof(argv[1]);
case '\n':
goto done_first_line;
}
}
done_first_line:

switch (fgetc(script)) {
case EOF:
handle_eof(argv[1]);
case '#':
break;
default:
fprintf(stderr, "second line of %s doesn't begin with shebang line\n", argv[1]);
return exit_no_shebang;
}
/* Skip first line */
while (next_char(&st) != '\n');

switch (fgetc(script)) {
case EOF:
handle_eof(argv[1]);
case '!':
break;
default:
fprintf(stderr, "second line of %s doesn't begin with shebang line\n", argv[1]);
return exit_no_shebang;
/* Ensure we start with a shebang */
if (next_char(&st) != '#' || next_char(&st) != '!') {
fprintf(stderr, "second line of %s doesn't begin with #!\n", st.filename);
return exit_no_shebang;
}

/* Skip leading spaces */
while (next_char(&st) == ' ');

st.fill = st.fill - (st.offset - 1);
memmove(st.buf, st.buf + st.offset - 1, st.fill);
st.offset = 0;

size_t arg_count = 0;
/* Read long shebang line */
while (1) {
int res = fgetc(script);
switch (res) {
case EOF:
handle_eof(argv[1]);
case ' ':
continue;
default:
ungetc(res, script);
break;
for (; st.offset < st.fill; ++st.offset) {
switch (st.buf[st.offset]) {
case '\0':
fprintf(stderr, "unexpected null character in long shebang line of %s\n", st.filename);
return exit_null;
case '\n':
st.buf[st.offset] = '\0';
arg_count++;
goto args_done;
case ' ':
st.buf[st.offset] = '\0';
arg_count++;
/* arg_count overflow would require st.capacity overflow, which we already check for */
break;
case '\\':
++st.offset;
if (st.offset == st.fill)
read_more(&st);
switch (st.buf[st.offset]) {
case 'n':
st.buf[st.offset] = '\n';
case '\\':
case ' ':
memmove(st.buf + st.offset - 1, st.buf + st.offset, st.fill - st.offset);
st.fill--;
st.offset--;
break;
default:
fprintf(stderr, "unknown escape %c in long shebang line of %s\n", st.buf[st.offset], st.filename);
return exit_unknown_escape;
}
break;
}
}
break;
read_more(&st);
}
args_done:

/* Read second line */
errno = 0;
int done = 0;
while (!done) {
int res = fgetc(script);
switch (res) {
case EOF:
handle_eof(argv[1]);
case '\0':
fprintf(stderr, "unexpected null character in long shebang line of %s\n", argv[1]);
return exit_null;
case '\n':
done = 1;
if ((args_used + 1) * sizeof(char *) >= args_size)
increase_buffer((void**) &args, &args_size, sizeof(char *));
args[args_used++] = argv[1];
args[args_used++] = NULL;
res = '\0';
break;
case ' ':
res = '\0';
if (args_used * sizeof(char *) == args_size)
increase_buffer((void **) &args, &args_size, sizeof(char *));
args[args_used++] = args_string + args_string_used + 1;
break;
case '\\':
switch (fgetc(script)) {
case EOF:
handle_eof(argv[1]);
case '\\':
break;
case 'n':
res = '\n';
break;
case ' ':
res = ' ';
break;
default:
fprintf(stderr, "unknown escape %c in long shebang line of %s\n", res, argv[1]);
return exit_unknown_escape;
}
break;
}
args_string[args_string_used++] = res;
if (args_string_used == args_string_size && !done) {
char * old_args_string = args_string;
increase_buffer((void **) &args_string, &args_string_size, sizeof(char));
if (old_args_string != args_string) {
/* Update pointers... */
for (size_t i = 1; i < args_used; ++i)
args[i] = args_string + (args[i] - old_args_string);
}
}
if (arg_count == 0) {
fprintf(stderr, "no arguments in long shebang line of %s\n", st.filename);
return exit_no_args;
}

args[0] = args_string;
char ** args = malloc(sizeof(char**) * (arg_count + 2 /* script, trailing null */));
if (!args) {
perror("allocating arg pointer buffer");
return exit_mem;
}

args[0] = st.buf;
for (size_t i = 1; i < arg_count; ++i) {
args[i] = args[i-1] + 1;
while (*args[i] != '\0')
args[i]++;
args[i]++;
}
args[arg_count] = st.filename;
args[arg_count + 1] = NULL;

execvp(args[0], args);
fprintf(stderr, "executing %s: %s\n", args[0], strerror(errno));
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