/* Postprocess module symbol versions * * Copyright 2003 Kai Germaschewski * Copyright 2002-2004 Rusty Russell, IBM Corporation * * Based in part on module-init-tools/depmod.c,file2alias * * This software may be used and distributed according to the terms * of the GNU General Public License, incorporated herein by reference. * * Usage: modpost vmlinux module1.o module2.o ... */ #include <ctype.h>#include "modpost.h" /* Are we using CONFIG_MODVERSIONS? */int modversions = 0;/* Warn about undefined symbols? (do so if we have vmlinux) */int have_vmlinux = 0;/* Is CONFIG_MODULE_SRCVERSION_ALL set? */static int all_versions = 0; voidfatal(const char *fmt, ...){ va_list arglist; fprintf(stderr, "FATAL: "); va_start(arglist, fmt); vfprintf(stderr, fmt, arglist); va_end(arglist); exit(1);} voidwarn(const char *fmt, ...){ va_list arglist; fprintf(stderr, "WARNING: "); va_start(arglist, fmt); vfprintf(stderr, fmt, arglist); va_end(arglist);} void *do_nofail(void *ptr, const char *file, int line, const char *expr){ if (!ptr) { fatal("Memory allocation failure %s line %d: %s.\n", file, line, expr); } return ptr;} /* A list of all modules we processed */ static struct module *modules; struct module *find_module(char *modname){ struct module *mod; for (mod = modules; mod; mod = mod->next) if (strcmp(mod->name, modname) == 0) break; return mod;} struct module *new_module(char *modname){ struct module *mod; char *p, *s; mod = NOFAIL(malloc(sizeof(*mod))); memset(mod, 0, sizeof(*mod)); p = NOFAIL(strdup(modname)); /* strip trailing .o */ if ((s = strrchr(p, '.')) != NULL) if (strcmp(s, ".o") == 0) *s = '\0'; /* add to list */ mod->name = p; mod->next = modules; modules = mod; return mod;} /* A hash of all exported symbols, * struct symbol is also used for lists of unresolved symbols */ #define SYMBOL_HASH_SIZE 1024 struct symbol { struct symbol *next; struct module *module; unsigned int crc; int crc_valid; unsigned int weak:1; char name[0];}; static struct symbol *symbolhash[SYMBOL_HASH_SIZE]; /* This is based on the hash agorithm from gdbm, via tdb */static inline unsigned int tdb_hash(const char *name){ unsigned value; /* Used to compute the hash value. */ unsigned i; /* Used to cycle through random values. */ /* Set the initial value from the key size. */ for (value = 0x238F13AF * strlen(name), i=0; name[i]; i++) value = (value + (((unsigned char *)name)[i] << (i*5 % 24))); return (1103515243 * value + 12345);} /* Allocate a new symbols for use in the hash of exported symbols or * the list of unresolved symbols per module */ struct symbol *alloc_symbol(const char *name, unsigned int weak, struct symbol *next){ struct symbol *s = NOFAIL(malloc(sizeof(*s) + strlen(name) + 1)); memset(s, 0, sizeof(*s)); strcpy(s->name, name); s->weak = weak; s->next = next; return s;} /* For the hash of exported symbols */ voidnew_symbol(const char *name, struct module *module, unsigned int *crc){ unsigned int hash; struct symbol *new; hash = tdb_hash(name) % SYMBOL_HASH_SIZE; new = symbolhash[hash] = alloc_symbol(name, 0, symbolhash[hash]); new->module = module; if (crc) { new->crc = *crc; new->crc_valid = 1; }} struct symbol *find_symbol(const char *name){ struct symbol *s; /* For our purposes, .foo matches foo. PPC64 needs this. */ if (name[0] == '.') name++; for (s = symbolhash[tdb_hash(name) % SYMBOL_HASH_SIZE]; s; s=s->next) { if (strcmp(s->name, name) == 0) return s; } return NULL;} /* Add an exported symbol - it may have already been added without a * CRC, in this case just update the CRC */voidadd_exported_symbol(const char *name, struct module *module, unsigned int *crc){ struct symbol *s = find_symbol(name); if (!s) { new_symbol(name, module, crc); return; } if (crc) { s->crc = *crc; s->crc_valid = 1; }} void *grab_file(const char *filename, unsigned long *size){ struct stat st; void *map; int fd; fd = open(filename, O_RDONLY); if (fd < 0 || fstat(fd, &st) != 0) return NULL; *size = st.st_size; map = mmap(NULL, *size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0); close(fd); if (map == MAP_FAILED) return NULL; return map;} /* Return a copy of the next line in a mmap'ed file. spaces in the beginning of the line is trimmed away. Return a pointer to a static buffer.*/char*get_next_line(unsigned long *pos, void *file, unsigned long size){ static char line[4096]; int skip = 1; size_t len = 0; signed char *p = (signed char *)file + *pos; char *s = line; for (; *pos < size ; (*pos)++) { if (skip && isspace(*p)) { p++; continue; } skip = 0; if (*p != '\n' && (*pos < size)) { len++; *s++ = *p++; if (len > 4095) break; /* Too long, stop */ } else { /* End of string */ *s = '\0'; return line; } } /* End of buffer */ return NULL;} voidrelease_file(void *file, unsigned long size){ munmap(file, size);} voidparse_elf(struct elf_info *info, const char *filename){ unsigned int i; Elf_Ehdr *hdr = info->hdr; Elf_Shdr *sechdrs; Elf_Sym *sym; hdr = grab_file(filename, &info->size); if (!hdr) { perror(filename); abort(); } info->hdr = hdr; if (info->size < sizeof(*hdr)) goto truncated; /* Fix endianness in ELF header */ hdr->e_shoff = TO_NATIVE(hdr->e_shoff); hdr->e_shstrndx = TO_NATIVE(hdr->e_shstrndx); hdr->e_shnum = TO_NATIVE(hdr->e_shnum); hdr->e_machine = TO_NATIVE(hdr->e_machine); sechdrs = (void *)hdr + hdr->e_shoff; info->sechdrs = sechdrs; /* Fix endianness in section headers */ for (i = 0; i < hdr->e_shnum; i++) { sechdrs[i].sh_type = TO_NATIVE(sechdrs[i].sh_type); sechdrs[i].sh_offset = TO_NATIVE(sechdrs[i].sh_offset); sechdrs[i].sh_size = TO_NATIVE(sechdrs[i].sh_size); sechdrs[i].sh_link = TO_NATIVE(sechdrs[i].sh_link); sechdrs[i].sh_name = TO_NATIVE(sechdrs[i].sh_name); } /* Find symbol table. */ for (i = 1; i < hdr->e_shnum; i++) { const char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset; if (sechdrs[i].sh_offset > info->size) goto truncated; if (strcmp(secstrings+sechdrs[i].sh_name, ".modinfo") == 0) { info->modinfo = (void *)hdr + sechdrs[i].sh_offset; info->modinfo_len = sechdrs[i].sh_size; } if (sechdrs[i].sh_type != SHT_SYMTAB) continue; info->symtab_start = (void *)hdr + sechdrs[i].sh_offset; info->symtab_stop = (void *)hdr + sechdrs[i].sh_offset + sechdrs[i].sh_size; info->strtab = (void *)hdr + sechdrs[sechdrs[i].sh_link].sh_offset; } if (!info->symtab_start) { fprintf(stderr, "modpost: %s no symtab?\n", filename); abort(); } /* Fix endianness in symbols */ for (sym = info->symtab_start; sym < info->symtab_stop; sym++) { sym->st_shndx = TO_NATIVE(sym->st_shndx); sym->st_name = TO_NATIVE(sym->st_name); sym->st_value = TO_NATIVE(sym->st_value); sym->st_size = TO_NATIVE(sym->st_size); } return; truncated: fprintf(stderr, "modpost: %s is truncated.\n", filename); abort();} voidparse_elf_finish(struct elf_info *info){ release_file(info->hdr, info->size);} #define CRC_PFX MODULE_SYMBOL_PREFIX "__crc_"#define KSYMTAB_PFX MODULE_SYMBOL_PREFIX "__ksymtab_" voidhandle_modversions(struct module *mod, struct elf_info *info, Elf_Sym *sym, const char *symname){ unsigned int crc; switch (sym->st_shndx) { case SHN_COMMON: fprintf(stderr, "*** Warning: \"%s\" [%s] is COMMON symbol\n", symname, mod->name); break; case SHN_ABS: /* CRC'd symbol */ if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) { crc = (unsigned int) sym->st_value; add_exported_symbol(symname + strlen(CRC_PFX), mod, &crc); } break; case SHN_UNDEF: /* undefined symbol */ if (ELF_ST_BIND(sym->st_info) != STB_GLOBAL && ELF_ST_BIND(sym->st_info) != STB_WEAK) break; /* ignore global offset table */ if (strcmp(symname, "_GLOBAL_OFFSET_TABLE_") == 0) break; /* ignore __this_module, it will be resolved shortly */ if (strcmp(symname, MODULE_SYMBOL_PREFIX "__this_module") == 0) break;#ifdef STT_REGISTER if (info->hdr->e_machine == EM_SPARC || info->hdr->e_machine == EM_SPARCV9) { /* Ignore register directives. */ if (ELF_ST_TYPE(sym->st_info) == STT_REGISTER) break; }#endif if (memcmp(symname, MODULE_SYMBOL_PREFIX, strlen(MODULE_SYMBOL_PREFIX)) == 0) mod->unres = alloc_symbol(symname + strlen(MODULE_SYMBOL_PREFIX), ELF_ST_BIND(sym->st_info) == STB_WEAK, mod->unres); break; default: /* All exported symbols */ if (memcmp(symname, KSYMTAB_PFX, strlen(KSYMTAB_PFX)) == 0) { add_exported_symbol(symname + strlen(KSYMTAB_PFX), mod, NULL); } if (strcmp(symname, MODULE_SYMBOL_PREFIX "init_module") == 0) mod->has_init = 1; if (strcmp(symname, MODULE_SYMBOL_PREFIX "cleanup_module") == 0) mod->has_cleanup = 1; break; }} intis_vmlinux(const char *modname){ const char *myname; if ((myname = strrchr(modname, '/'))) myname++; else myname = modname; return strcmp(myname, "vmlinux") == 0;} /* Parse tag=value strings from .modinfo section */static char *next_string(char *string, unsigned long *secsize){ /* Skip non-zero chars */ while (string[0]) { string++; if ((*secsize)-- <= 1) return NULL; } /* Skip any zero padding. */ while (!string[0]) { string++; if ((*secsize)-- <= 1) return NULL; } return string;} static char *get_modinfo(void *modinfo, unsigned long modinfo_len, const char *tag){ char *p; unsigned int taglen = strlen(tag); unsigned long size = modinfo_len; for (p = modinfo; p; p = next_string(p, &size)) { if (strncmp(p, tag, taglen) == 0 && p[taglen] == '=') return p + taglen + 1; } return NULL;} voidread_symbols(char *modname){ const char *symname; char *version; struct module *mod; struct elf_info info = { }; Elf_Sym *sym; parse_elf(&info, modname); mod = new_module(modname); /* When there's no vmlinux, don't print warnings about * unresolved symbols (since there'll be too many ;) */ if (is_vmlinux(modname)) { unsigned int fake_crc = 0; have_vmlinux = 1; add_exported_symbol("struct_module", mod, &fake_crc); mod->skip = 1; } for (sym = info.symtab_start; sym < info.symtab_stop; sym++) { symname = info.strtab + sym->st_name; handle_modversions(mod, &info, sym, symname); handle_moddevtable(mod, &info, sym, symname); } version = get_modinfo(info.modinfo, info.modinfo_len, "version"); if (version) maybe_frob_rcs_version(modname, version, info.modinfo, version - (char *)info.hdr); if (version || (all_versions && !is_vmlinux(modname))) get_src_version(modname, mod->srcversion, sizeof(mod->srcversion)-1); parse_elf_finish(&info); /* Our trick to get versioning for struct_module - it's * never passed as an argument to an exported function, so * the automatic versioning doesn't pick it up, but it's really * important anyhow */ if (modversions) mod->unres = alloc_symbol("struct_module", 0, mod->unres);} #define SZ 500 /* We first write the generated file into memory using the * following helper, then compare to the file on disk and * only update the later if anything changed */ void __attribute__((format(printf, 2, 3)))buf_printf(struct buffer *buf, const char *fmt, ...){ char tmp[SZ]; int len; va_list ap; va_start(ap, fmt); len = vsnprintf(tmp, SZ, fmt, ap);Lines 1–500 of 798. 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/* Postprocess module symbol versions * * Copyright 2003 Kai Germaschewski * Copyright 2002-2004 Rusty Russell, IBM Corporation * * Based in part on module-init-tools/depmod.c,file2alias * * This software may be used and distributed according to the terms * of the GNU General Public License, incorporated herein by reference. * * Usage: modpost vmlinux module1.o module2.o ... */ #include <ctype.h> #include "modpost.h" /* Are we using CONFIG_MODVERSIONS? */ int modversions = 0; /* Warn about undefined symbols? (do so if we have vmlinux) */ int have_vmlinux = 0; /* Is CONFIG_MODULE_SRCVERSION_ALL set? */ static int all_versions = 0; void fatal(const char *fmt, ...) { va_list arglist; fprintf(stderr, "FATAL: "); va_start(arglist, fmt); vfprintf(stderr, fmt, arglist); va_end(arglist); exit(1); } void warn(const char *fmt, ...) { va_list arglist; fprintf(stderr, "WARNING: "); va_start(arglist, fmt); vfprintf(stderr, fmt, arglist); va_end(arglist); } void *do_nofail(void *ptr, const char *file, int line, const char *expr) { if (!ptr) { fatal("Memory allocation failure %s line %d: %s.\n", file, line, expr); } return ptr; } /* A list of all modules we processed */ static struct module *modules; struct module * find_module(char *modname) { struct module *mod; for (mod = modules; mod; mod = mod->next) if (strcmp(mod->name, modname) == 0) break; return mod; } struct module * new_module(char *modname) { struct module *mod; char *p, *s; mod = NOFAIL(malloc(sizeof(*mod))); memset(mod, 0, sizeof(*mod)); p = NOFAIL(strdup(modname)); /* strip trailing .o */ if ((s = strrchr(p, '.')) != NULL) if (strcmp(s, ".o") == 0) *s = '\0'; /* add to list */ mod->name = p; mod->next = modules; modules = mod; return mod; } /* A hash of all exported symbols, * struct symbol is also used for lists of unresolved symbols */ #define SYMBOL_HASH_SIZE 1024 struct symbol { struct symbol *next; struct module *module; unsigned int crc; int crc_valid; unsigned int weak:1; char name[0]; }; static struct symbol *symbolhash[SYMBOL_HASH_SIZE]; /* This is based on the hash agorithm from gdbm, via tdb */ static inline unsigned int tdb_hash(const char *name) { unsigned value; /* Used to compute the hash value. */ unsigned i; /* Used to cycle through random values. */ /* Set the initial value from the key size. */ for (value = 0x238F13AF * strlen(name), i=0; name[i]; i++) value = (value + (((unsigned char *)name)[i] << (i*5 % 24))); return (1103515243 * value + 12345); } /* Allocate a new symbols for use in the hash of exported symbols or * the list of unresolved symbols per module */ struct symbol * alloc_symbol(const char *name, unsigned int weak, struct symbol *next) { struct symbol *s = NOFAIL(malloc(sizeof(*s) + strlen(name) + 1)); memset(s, 0, sizeof(*s)); strcpy(s->name, name); s->weak = weak; s->next = next; return s; } /* For the hash of exported symbols */ void new_symbol(const char *name, struct module *module, unsigned int *crc) { unsigned int hash; struct symbol *new; hash = tdb_hash(name) % SYMBOL_HASH_SIZE; new = symbolhash[hash] = alloc_symbol(name, 0, symbolhash[hash]); new->module = module; if (crc) { new->crc = *crc; new->crc_valid = 1; } } struct symbol * find_symbol(const char *name) { struct symbol *s; /* For our purposes, .foo matches foo. PPC64 needs this. */ if (name[0] == '.') name++; for (s = symbolhash[tdb_hash(name) % SYMBOL_HASH_SIZE]; s; s=s->next) { if (strcmp(s->name, name) == 0) return s; } return NULL; } /* Add an exported symbol - it may have already been added without a * CRC, in this case just update the CRC */ void add_exported_symbol(const char *name, struct module *module, unsigned int *crc) { struct symbol *s = find_symbol(name); if (!s) { new_symbol(name, module, crc); return; } if (crc) { s->crc = *crc; s->crc_valid = 1; } } void * grab_file(const char *filename, unsigned long *size) { struct stat st; void *map; int fd; fd = open(filename, O_RDONLY); if (fd < 0 || fstat(fd, &st) != 0) return NULL; *size = st.st_size; map = mmap(NULL, *size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0); close(fd); if (map == MAP_FAILED) return NULL; return map; } /* Return a copy of the next line in a mmap'ed file. spaces in the beginning of the line is trimmed away. Return a pointer to a static buffer. */ char* get_next_line(unsigned long *pos, void *file, unsigned long size) { static char line[4096]; int skip = 1; size_t len = 0; signed char *p = (signed char *)file + *pos; char *s = line; for (; *pos < size ; (*pos)++) { if (skip && isspace(*p)) { p++; continue; } skip = 0; if (*p != '\n' && (*pos < size)) { len++; *s++ = *p++; if (len > 4095) break; /* Too long, stop */ } else { /* End of string */ *s = '\0'; return line; } } /* End of buffer */ return NULL; } void release_file(void *file, unsigned long size) { munmap(file, size); } void parse_elf(struct elf_info *info, const char *filename) { unsigned int i; Elf_Ehdr *hdr = info->hdr; Elf_Shdr *sechdrs; Elf_Sym *sym; hdr = grab_file(filename, &info->size); if (!hdr) { perror(filename); abort(); } info->hdr = hdr; if (info->size < sizeof(*hdr)) goto truncated; /* Fix endianness in ELF header */ hdr->e_shoff = TO_NATIVE(hdr->e_shoff); hdr->e_shstrndx = TO_NATIVE(hdr->e_shstrndx); hdr->e_shnum = TO_NATIVE(hdr->e_shnum); hdr->e_machine = TO_NATIVE(hdr->e_machine); sechdrs = (void *)hdr + hdr->e_shoff; info->sechdrs = sechdrs; /* Fix endianness in section headers */ for (i = 0; i < hdr->e_shnum; i++) { sechdrs[i].sh_type = TO_NATIVE(sechdrs[i].sh_type); sechdrs[i].sh_offset = TO_NATIVE(sechdrs[i].sh_offset); sechdrs[i].sh_size = TO_NATIVE(sechdrs[i].sh_size); sechdrs[i].sh_link = TO_NATIVE(sechdrs[i].sh_link); sechdrs[i].sh_name = TO_NATIVE(sechdrs[i].sh_name); } /* Find symbol table. */ for (i = 1; i < hdr->e_shnum; i++) { const char *secstrings = (void *)hdr + sechdrs[hdr->e_shstrndx].sh_offset; if (sechdrs[i].sh_offset > info->size) goto truncated; if (strcmp(secstrings+sechdrs[i].sh_name, ".modinfo") == 0) { info->modinfo = (void *)hdr + sechdrs[i].sh_offset; info->modinfo_len = sechdrs[i].sh_size; } if (sechdrs[i].sh_type != SHT_SYMTAB) continue; info->symtab_start = (void *)hdr + sechdrs[i].sh_offset; info->symtab_stop = (void *)hdr + sechdrs[i].sh_offset + sechdrs[i].sh_size; info->strtab = (void *)hdr + sechdrs[sechdrs[i].sh_link].sh_offset; } if (!info->symtab_start) { fprintf(stderr, "modpost: %s no symtab?\n", filename); abort(); } /* Fix endianness in symbols */ for (sym = info->symtab_start; sym < info->symtab_stop; sym++) { sym->st_shndx = TO_NATIVE(sym->st_shndx); sym->st_name = TO_NATIVE(sym->st_name); sym->st_value = TO_NATIVE(sym->st_value); sym->st_size = TO_NATIVE(sym->st_size); } return; truncated: fprintf(stderr, "modpost: %s is truncated.\n", filename); abort(); } void parse_elf_finish(struct elf_info *info) { release_file(info->hdr, info->size); } #define CRC_PFX MODULE_SYMBOL_PREFIX "__crc_" #define KSYMTAB_PFX MODULE_SYMBOL_PREFIX "__ksymtab_" void handle_modversions(struct module *mod, struct elf_info *info, Elf_Sym *sym, const char *symname) { unsigned int crc; switch (sym->st_shndx) { case SHN_COMMON: fprintf(stderr, "*** Warning: \"%s\" [%s] is COMMON symbol\n", symname, mod->name); break; case SHN_ABS: /* CRC'd symbol */ if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) { crc = (unsigned int) sym->st_value; add_exported_symbol(symname + strlen(CRC_PFX), mod, &crc); } break; case SHN_UNDEF: /* undefined symbol */ if (ELF_ST_BIND(sym->st_info) != STB_GLOBAL && ELF_ST_BIND(sym->st_info) != STB_WEAK) break; /* ignore global offset table */ if (strcmp(symname, "_GLOBAL_OFFSET_TABLE_") == 0) break; /* ignore __this_module, it will be resolved shortly */ if (strcmp(symname, MODULE_SYMBOL_PREFIX "__this_module") == 0) break; #ifdef STT_REGISTER if (info->hdr->e_machine == EM_SPARC || info->hdr->e_machine == EM_SPARCV9) { /* Ignore register directives. */ if (ELF_ST_TYPE(sym->st_info) == STT_REGISTER) break; } #endif if (memcmp(symname, MODULE_SYMBOL_PREFIX, strlen(MODULE_SYMBOL_PREFIX)) == 0) mod->unres = alloc_symbol(symname + strlen(MODULE_SYMBOL_PREFIX), ELF_ST_BIND(sym->st_info) == STB_WEAK, mod->unres); break; default: /* All exported symbols */ if (memcmp(symname, KSYMTAB_PFX, strlen(KSYMTAB_PFX)) == 0) { add_exported_symbol(symname + strlen(KSYMTAB_PFX), mod, NULL); } if (strcmp(symname, MODULE_SYMBOL_PREFIX "init_module") == 0) mod->has_init = 1; if (strcmp(symname, MODULE_SYMBOL_PREFIX "cleanup_module") == 0) mod->has_cleanup = 1; break; } } int is_vmlinux(const char *modname) { const char *myname; if ((myname = strrchr(modname, '/'))) myname++; else myname = modname; return strcmp(myname, "vmlinux") == 0; } /* Parse tag=value strings from .modinfo section */ static char *next_string(char *string, unsigned long *secsize) { /* Skip non-zero chars */ while (string[0]) { string++; if ((*secsize)-- <= 1) return NULL; } /* Skip any zero padding. */ while (!string[0]) { string++; if ((*secsize)-- <= 1) return NULL; } return string; } static char *get_modinfo(void *modinfo, unsigned long modinfo_len, const char *tag) { char *p; unsigned int taglen = strlen(tag); unsigned long size = modinfo_len; for (p = modinfo; p; p = next_string(p, &size)) { if (strncmp(p, tag, taglen) == 0 && p[taglen] == '=') return p + taglen + 1; } return NULL; } void read_symbols(char *modname) { const char *symname; char *version; struct module *mod; struct elf_info info = { }; Elf_Sym *sym; parse_elf(&info, modname); mod = new_module(modname); /* When there's no vmlinux, don't print warnings about * unresolved symbols (since there'll be too many ;) */ if (is_vmlinux(modname)) { unsigned int fake_crc = 0; have_vmlinux = 1; add_exported_symbol("struct_module", mod, &fake_crc); mod->skip = 1; } for (sym = info.symtab_start; sym < info.symtab_stop; sym++) { symname = info.strtab + sym->st_name; handle_modversions(mod, &info, sym, symname); handle_moddevtable(mod, &info, sym, symname); } version = get_modinfo(info.modinfo, info.modinfo_len, "version"); if (version) maybe_frob_rcs_version(modname, version, info.modinfo, version - (char *)info.hdr); if (version || (all_versions && !is_vmlinux(modname))) get_src_version(modname, mod->srcversion, sizeof(mod->srcversion)-1); parse_elf_finish(&info); /* Our trick to get versioning for struct_module - it's * never passed as an argument to an exported function, so * the automatic versioning doesn't pick it up, but it's really * important anyhow */ if (modversions) mod->unres = alloc_symbol("struct_module", 0, mod->unres); } #define SZ 500 /* We first write the generated file into memory using the * following helper, then compare to the file on disk and * only update the later if anything changed */ void __attribute__((format(printf, 2, 3))) buf_printf(struct buffer *buf, const char *fmt, ...) { char tmp[SZ]; int len; va_list ap; va_start(ap, fmt); len = vsnprintf(tmp, SZ, fmt, ap);